9. Visibility Lesson

Visibility

Visibility

To see sample solutions, click on the "Configure activity" button and select "Show solutions" from the menu that appears.

Sample solutions not included, because "Show solutions" was not selected before printing.

Learning objectives

After this activity, you should be able to:

• Determine whether a class, instance variable, static variable, method, or constructor is accessible from a given location in a program.

• Explain how packages and inheritance affect access, including the restrictions on protected instance members outside their package.

• Distinguish an access violation from a missing method or constructor and from an attempt to instantiate an abstract class.

• Correct invalid code without unnecessarily widening access.

Why Visibility?

Developers of Java incorporated visibility into the language "to prevent the users of a package or class from depending on unnecessary details of the implementation of that package or class."

How to Analyze an Access

Use these questions for each candidate statement:

1. Which declaration is being accessed? Identify its declaring class and package. For a method call, use the qualifying expression's compile-time type to find the available declaration; the object's runtime class does not make missing methods available to the compiler.

2. Where does the access occur? Identify the caller's class and package. The caller's location is distinct from the object's class.

3. Is access permitted? Check the declaring type's accessibility and the declaration's visibility: private, package private, protected, or public. For a protected instance member used outside its declaring package, identify the subclass containing the access and check the qualifying expression's compile-time type.

4. Is another rule responsible? A missing constructor or method, or an attempt to instantiate an abstract class, is not a visibility violation.

5. What happens if it compiles? Trace only the stated setup and candidate. Identify copied reference values, shared objects, changes to instance variables, and output. If it does not compile, explain the diagnostic's meaning and propose a correction without unnecessarily widening access.

Judge candidates independently unless the question explicitly asks you to execute statements in sequence. Exact compiler wording is not required. Each scenario specifies its own exit-ticket task and memory-map stopping point.

Part 1: Public and Private Visibility

Introduction

Access control helps a class expose operations that callers can depend on while keeping implementation details internal. It supports encapsulation; it is not a substitute for authorization checks or a security boundary for untrusted code. It helps preserve an author's intentions concerning what other pieces of code can access the entities they declare under normal circumstances.

Read the First UML View

Fig. 3 Book-package view of the new example [svg] [png]

Part 1 uses a simplified Book class whose direct superclass is Object. Parts 2 and 3 introduce a version that extends Product. Within each version, a diagram may omit details to focus on the current question. UML uses + for public, - for private, ~ for package private, and # for protected. Underlining identifies static members.

UML Details for Book.java

• Book is a public class in cs1302.books.

• BookDriver is a public class in cs1302.books.

• Book.title is a private String instance variable.

• Book(String) is a public constructor.

• Book(Book) is a public constructor.

• Book.getTitle is a public instance method.

• Book.setTitle is a private instance method.

• BookDriver.main is a public static method.

• BookDriver uses Book.

Code for Book.java

Listing 84 Book.java (simplified version for Part 1)

package cs1302.books;

/** A book used to demonstrate private access and reference values. */
public class Book {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

} // Book

Scenario 1: Classify Declarations

Here, top-level, member-level, and local-level describe declaration locations for this exercise. Member-level includes constructor declarations in a class body; constructors are not class members in Java's terminology and are not inherited. Local-level includes parameters, which are distinct from local variables declared in a body. See JLS 8.2: Class Members.

SVG PNG

On your Exit Ticket: In Book.java, classify each of the following entity declarations as top-level, member-level, or local-level:

• Book (the class);

• title (the instance variable);

• Book and title in Book(String);

• Book and other in Book(Book);

• getTitle();

• setTitle and title in setTitle(String).

On your Exit Ticket: Is this.title = other.title; a declaration?

Sample Solution

• Book (the class): top-level (class declaration).

• title (the instance variable): member-level (instance variable declaration).

• In Book(String), Book is member-level (constructor declaration), and title is local-level (parameter variable declaration).

• In Book(Book), Book is member-level (constructor declaration), and other is local-level (parameter variable declaration).

• getTitle(): member-level (method declaration).

• In setTitle(String), setTitle is member-level (method declaration), and title is local-level (parameter variable declaration).

Here, local-level includes parameter declarations; a parameter variable is distinct from a local variable declared in a body. The assignment statement declares nothing: instead, it copies a reference value into the destination instance variable.

Scenario 2: Book Copy Constructor

SVG PNG

ACTIVITY: For each potential copy constructor shown below, indicate if it is valid (will compile) or invalid (will not compile) – if it is invalid, then write down why:

Listing 85 Book(Book) Version 1

public Book(Book other) {
    this.setTitle(other.title);
} // Book

Listing 86 Book(Book) Version 2

public Book(Book other) {
    this.title = other.title;
} // Book

Listing 87 Book(Book) Version 3

public Book(Book other) {
    super();
    this.title = other.title;
} // Book

Solution

1. Version 1: Will not compile because the Product constructor does not take any arguments. When we pass an argument to super, the compiler will not be able to find that constructor.

2. Version 2: The first two lines will work as expected. However, the serial instance variable in Product cannot be accessed directly from within the Book class because it is package private.

3. Version 3: This compiles and properly initializes both instance variables in the Book object.

Scenario 3: Accessing Private Entities (1)

SVG PNG

This example uses a Book class whose direct superclass is Object. Each super() invokes the no-argument constructor of Object.

Listing 88 Book(String) in Book.java

public Book(String title) {
    super();
    this.setTitle(title);
} // Book

Listing 89 Book(Book) in Book.java

public Book(Book other) {
    super();
    this.title = other.title;
} // Book

Listing 90 Example of main method in BookDriver.java

Book book0 = new Book("LOTR");
Book book1 = new Book(book0);
String title0 = book0.getTitle();

On your Exit Ticket:

• Why do you think the author used setTitle(title) instead of this.title = title in the Book(String) constructor? What might be the benefit?

• Does the current setter validate its argument? If validation were added later, what would need to be true before the copy constructor could safely bypass it using this.title = other.title?

• Draw a memory map immediately after the declaration of title0 finishes, before main returns. Include its three local reference variables, both book objects, and the shared title string.

Sample Solution

• Centralize an assignment: The current setTitle method only assigns the supplied reference value. It accepts null, empty strings, and strings containing only whitespace. Using it gives the class one place where validation could be added later; private visibility does not itself validate data.

• Copy a reference value: The copy constructor can access other.title because the access occurs inside Book. It copies the stored reference value without copying the string. Bypassing a future validation check is justified only if the class maintains an invariant that every existing book has a valid title. Every construction and update path would need to preserve that invariant. The current implementation provides no such guarantee. The copy constructor separately requires a non-null other.

• Distinguish the objects: book0 and book1 refer to two distinct book objects. Their title instance variables and title0 refer to the same "LOTR" string object.

|< < Prev

Step 1 of 13: BookDriver.java:6

Next > >|

12345678910111213

BookDriver.java

package cs1302.books;

public class BookDriver {

    public static void main(String[] args) {
        Book book0 = new Book("LOTR");
        Book book1 = new Book(book0);
        String title0 = book0.getTitle();
    } // main

} // BookDriver

Book.java

package cs1302.books;

/** A book used to demonstrate private access and reference values. */
public class Book {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

} // Book

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book book0 = new Book("LOTR");
        Book book1 = new Book(book0);
        String title0 = book0.getTitle();
    } // main

Memory state at the breakpoint on line 6 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 14 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 15 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 40 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 41 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 16 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book book0 = new Book("LOTR");
        Book book1 = new Book(book0);
        String title0 = book0.getTitle();
    } // main

Memory state at the breakpoint on line 7 in BookDriver.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
    } // Book

Memory state at the breakpoint on line 23 in Book.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
    } // Book

Memory state at the breakpoint on line 24 in Book.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
    } // Book

Memory state at the breakpoint on line 25 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book book0 = new Book("LOTR");
        Book book1 = new Book(book0);
        String title0 = book0.getTitle();
    } // main

Memory state at the breakpoint on line 8 in BookDriver.java. [code listing] [download] [open]

    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

Memory state at the breakpoint on line 32 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book book0 = new Book("LOTR");
        Book book1 = new Book(book0);
        String title0 = book0.getTitle();
    } // main

Memory state at the breakpoint on line 9 in BookDriver.java. [code listing] [download] [open]

Scenario 4: Accessing Private Entities (2)

SVG PNG

This example uses a Book class whose direct superclass is Object. Each super() invokes the no-argument constructor of Object.

Listing 91 Book(String) in Book.java

public Book(String title) {
    super();
    this.setTitle(title);
} // Book

Listing 92 Book(Book) in Book.java

public Book(Book other) {
    super();
    this.title = other.title;
} // Book

Listing 93 Example of main method in BookDriver.java

Book book0 = new Book("LOTR");
Book book1 = new Book(book0);
book1.setTitle("The Lord of the Rings");

On your Exit Ticket: There is an issue with the code in the main method. Is it a run-time issue or compile-time issue? Identify the declaration being accessed, its visibility, and the caller's location. Explain the meaning of the compiler diagnostic; exact wording is not required.

Sample Solution

Listing 94 Example of main method in BookDriver.java

Book book0 = new Book("LOTR");
Book book1 = new Book(book0);
book1.setTitle("The Lord of the Rings");

Compiler error (expected)

cs1302/books/BookDriver.java:8: error: setTitle(String) has private access in Book
        book1.setTitle("The Lord of the Rings");
             ^
1 error

Scenario 5: More Access from BookDriver

SVG PNG

On your Exit Ticket: Judge these candidate statements independently, note your group's thoughts, and how to address any issues (if you think there are any):

Listing 95 Inside the main method in BookDriver.java

Book book = new Book("1984");

System.out.println(book.getTitle()); // A
book.setTitle("Dune");               // B
System.out.println(book.title);      // C

Sample Solution A

A compiles: getTitle is public in the public class Book. The call reads the stored title reference value, and the print statement outputs 1984.

Listing 96 Example of main method in BookDriver.java

Book book = new Book("1984");
System.out.println(book.getTitle()); // A

Example of main method in BookDriver.java [code listing] [download] [open]

Sample Solution B

B does not compile: setTitle is private to Book. Sharing a package does not grant private access. Supply the desired title when constructing a book: book = new Book("Dune");. This replaces the reference value in book; it does not rename the previous object.

Listing 97 Example of main method in BookDriver.java

Book book = new Book("1984");
book.setTitle("Dune");               // B

Compiler error (expected)

cs1302/books/BookDriver.java:7: error: setTitle(String) has private access in Book
        book.setTitle("Dune");               // B
            ^
1 error

Sample Solution C

C does not compile: title is a private instance variable of Book. Use book.getTitle() as in A.

Listing 98 Example of main method in BookDriver.java

Book book = new Book("1984");
System.out.println(book.title);      // C

Compiler error (expected)

cs1302/books/BookDriver.java:7: error: title has private access in Book
        System.out.println(book.title);      // C
                               ^
1 error

Part 2: Package Private and Accessible Classes

Introduction

From this part onward, use the extended Book class in cs1302-visibility: it extends Product and adds serial-number behavior. This replaces Part 1's simplified class; do not combine both versions in the same package. Part 3 examines the inheritance rules.

A top-level class can be public or package private (no access modifier). It cannot be declared private or protected. A public member does not make its enclosing top-level class accessible from another package. For these class members and constructors, omitting an access modifier makes the declaration package private, but only in the Java source code. Use ~ to denote package private declarations in UML diagrams.

Reveal the Second Package

Fig. 4 Two-package view of the new example [svg] [png]

UML Details for the Two Packages

• Product is a public abstract class in cs1302.inventory.

• Book extends Product from cs1302.books.

• InventoryManager uses Product.

• InventoryManager is a public class in cs1302.inventory.

• InventoryDriver is a public class in cs1302.inventory.

• Utility is a package private class in cs1302.inventory.

• InventoryManager.inventory is a private Product[] instance variable.

• InventoryManager() is a private constructor.

• InventoryManager.getManager is a public static method.

• InventoryManager.addItem is a package private instance method.

• InventoryManager.requestPurchase is a public instance method.

• InventoryManager.nullCheck is a private static method.

• Utility() is a private constructor.

• Utility.min is a public static method.

• BookDriver uses InventoryManager.

• InventoryDriver uses InventoryManager.

• InventoryDriver uses Book.

Book.java: Extended Version

Listing 99 Book.java (used in Parts 2 and 3 and additional practice)

package cs1302.books;

import cs1302.inventory.Product;

/** A product with a title. */
public class Book extends Product {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value and serial number.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

    /**
     * Demonstrate permitted protected access from this subclass.
     * @param other non-null reference to a book object
     */
    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

} // Book

InventoryManager.java

package cs1302.inventory;

/** Small manager used to demonstrate package and private access. */
public class InventoryManager {

    /** Reference to this manager's array of product references. */
    private Product[] inventory;

    /** Prevent callers outside this class from invoking this constructor. */
    private InventoryManager() {
        this.inventory = new Product[0]; // Start with an empty array object.
    } // InventoryManager

    /**
     * Create a manager object.
     * @return a reference to a new manager object
     */
    public static InventoryManager getManager() {
        return new InventoryManager(); // return with the new reference value
    } // getManager

    /**
     * Assign a serial number from within the declaring package.
     * This teaching example leaves the inventory array empty.
     * @param product non-null reference to a product object
     */
    void addItem(Product product) {
        InventoryManager.nullCheck(product);
        product.serial = 14; // serial is package private
        product.setSerial(15); // setSerial is protected; this caller shares its package
    } // addItem

    /**
     * Check a product supplied by a caller.
     * @param product non-null reference to a product object
     */
    public void requestPurchase(Product product) {
        InventoryManager.nullCheck(product);
    } // requestPurchase

    /**
     * Reject a null argument value.
     * @param object reference value to check
     * @throws IllegalArgumentException if the value is null
     */
    private static void nullCheck(Object object) {
        if (object == null) {
            throw new IllegalArgumentException("A product reference is required.");
        } // if
    } // nullCheck

} // InventoryManager

Utility.java

package cs1302.inventory;

/** Package private class with a public static helper method. */
class Utility {

    /** Prevent construction outside this class. */
    private Utility() {
    } // Utility

    /**
     * Select the smaller of two values.
     * @param first the first value
     * @param second the second value
     * @return the smaller value
     */
    public static long min(long first, long second) {
        return Math.min(first, second); // return with the smaller value
    } // min

} // Utility

This manager demonstrates access rules, not a complete inventory system. Each manager's private inventory instance variable stores a reference to a distinct Product[] array object of length 0. Creating this empty array does not create any product objects. Include the array and its reference from the manager in memory maps. addItem assigns a serial number without storing products; neither it nor requestPurchase changes the inventory array. getManager returns with a reference to a new manager object on each call. Its call to the private constructor is permitted because it occurs inside InventoryManager.

Visibility Practice: Change the Caller's Package

Scenario 1: Calls from BookDriver

SVG PNG

Import InventoryManager from cs1302.inventory in BookDriver.java. Inside BookDriver.main, use this setup.

Book book = new Book("1984");
InventoryManager manager = InventoryManager.getManager();

On your Exit Ticket: Judge each candidate independently. Predict any compile-time error; for valid code, draw the memory map just before main completes. Explain the visibility rule and suggest a correction for each invalid candidate.

manager.addItem(book); // A
manager.requestPurchase(book); // B
InventoryManager.nullCheck(book); // C
manager = new InventoryManager(); // D
System.out.println(Utility.min(3, 8)); // E

For candidate E only, add this import above the class declaration:

Listing 100 BookDriver.java

import cs1302.inventory.Utility;

Sample Solution A

A does not compile: addItem is package private in cs1302.inventory. Place this operation inside an appropriate inventory-package method.

package cs1302.books;

import cs1302.inventory.InventoryManager;

/** Demonstrate one access from outside the inventory package. */
public class BookDriver {

    /**
     * Run this independent visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager.addItem(book);
    } // main

} // BookDriver

Compiler error (expected)

cs1302/books/BookDriver.java:16: error: addItem(Product) is not public in InventoryManager; cannot be accessed from outside package
        manager.addItem(book);
               ^
1 error

Sample Solution B

B compiles: requestPurchase is public in the public class InventoryManager. It checks the supplied reference value without changing the book object.

package cs1302.books;

import cs1302.inventory.InventoryManager;

/** Demonstrate one access from outside the inventory package. */
public class BookDriver {

    /**
     * Run this independent visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager.requestPurchase(book);
    } // main

} // BookDriver

Final captured memory state. [code listing] [download] [open]

Sample Solution C

C does not compile: nullCheck is private to InventoryManager. Use its public requestPurchase method, which performs the check internally.

package cs1302.books;

import cs1302.inventory.InventoryManager;

/** Demonstrate one access from outside the inventory package. */
public class BookDriver {

    /**
     * Run this independent visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        InventoryManager.nullCheck(book);
    } // main

} // BookDriver

Compiler error (expected)

cs1302/books/BookDriver.java:16: error: nullCheck(Object) has private access in InventoryManager
        InventoryManager.nullCheck(book);
                        ^
1 error

Sample Solution D

D does not compile: the constructor is private to InventoryManager. Call InventoryManager.getManager() instead; it returns with a reference to a new manager object.

package cs1302.books;

import cs1302.inventory.InventoryManager;

/** Demonstrate one access from outside the inventory package. */
public class BookDriver {

    /**
     * Run this independent visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager = new InventoryManager();
    } // main

} // BookDriver

Compiler error (expected)

cs1302/books/BookDriver.java:16: error: InventoryManager() has private access in InventoryManager
        manager = new InventoryManager();
                  ^
1 error

Sample Solution E

E does not compile: Utility is a package private class in another package. Both the import and the use of Utility produce compile-time errors. Its public min method does not make the class accessible. Use Math.min(3, 8) instead.

package cs1302.books;

import cs1302.inventory.InventoryManager;
import cs1302.inventory.Utility;

/** Demonstrate one access from outside the inventory package. */
public class BookDriver {

    /**
     * Run this independent visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        System.out.println(Utility.min(3, 8));
    } // main

} // BookDriver

Compiler error (expected)

cs1302/books/BookDriver.java:4: error: Utility is not public in cs1302.inventory; cannot be accessed from outside package
import cs1302.inventory.Utility;
                       ^
cs1302/books/BookDriver.java:17: error: Utility is not public in cs1302.inventory; cannot be accessed from outside package
        System.out.println(Utility.min(3, 8));
                           ^
2 errors

Scenario 2: Calls from InventoryDriver

SVG PNG

Import Book from cs1302.books in InventoryDriver.java. Inside InventoryDriver.main, use this setup.

Book book = new Book("1984");
InventoryManager manager = InventoryManager.getManager();

On your Exit Ticket: Judge each candidate independently. Predict any compile-time error; for valid code, draw the memory map just before main completes. Explain the visibility rule and suggest a correction for each invalid candidate.

manager.addItem(book); // A
manager.requestPurchase(book); // B
InventoryManager.nullCheck(book); // C
manager = new InventoryManager(); // D
System.out.println(Utility.min(3, 8)); // E

Sample Solution A

A compiles because addItem is package private and the caller shares its package. The method changes the book object's serial number to 14 and then 15.

|< < Prev

Step 1 of 24: InventoryDriver.java:13

Next > >|

123456789101112131415161718192021222324

InventoryDriver.java

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager.addItem(book);
    } // main

} // InventoryDriver

Book.java

package cs1302.books;

import cs1302.inventory.Product;

/** A product with a title. */
public class Book extends Product {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value and serial number.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

    /**
     * Demonstrate permitted protected access from this subclass.
     * @param other non-null reference to a book object
     */
    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

} // Book

InventoryManager.java

package cs1302.inventory;

/** Small manager used to demonstrate package and private access. */
public class InventoryManager {

    /** Reference to this manager's array of product references. */
    private Product[] inventory;

    /** Prevent callers outside this class from invoking this constructor. */
    private InventoryManager() {
        this.inventory = new Product[0]; // Start with an empty array object.
    } // InventoryManager

    /**
     * Create a manager object.
     * @return a reference to a new manager object
     */
    public static InventoryManager getManager() {
        return new InventoryManager(); // return with the new reference value
    } // getManager

    /**
     * Assign a serial number from within the declaring package.
     * This teaching example leaves the inventory array empty.
     * @param product non-null reference to a product object
     */
    void addItem(Product product) {
        InventoryManager.nullCheck(product);
        product.serial = 14; // serial is package private
        product.setSerial(15); // setSerial is protected; this caller shares its package
    } // addItem

    /**
     * Check a product supplied by a caller.
     * @param product non-null reference to a product object
     */
    public void requestPurchase(Product product) {
        InventoryManager.nullCheck(product);
    } // requestPurchase

    /**
     * Reject a null argument value.
     * @param object reference value to check
     * @throws IllegalArgumentException if the value is null
     */
    private static void nullCheck(Object object) {
        if (object == null) {
            throw new IllegalArgumentException("A product reference is required.");
        } // if
    } // nullCheck

} // InventoryManager

Product.java

package cs1302.inventory;

/** Base class for products used in the visibility lesson. */
public abstract class Product {

    /** Serial number; accessible within this package. */
    long serial;

    /** Initialize the serial number to zero. */
    public Product() {
        this.serial = 0;
    } // Product

    /**
     * Obtain the serial number.
     * @return the serial number
     */
    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

    /**
     * Change the serial number.
     * @param serial the new serial number
     */
    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

} // Product

Utility.java

package cs1302.inventory;

/** Package private class with a public static helper method. */
class Utility {

    /** Prevent construction outside this class. */
    private Utility() {
    } // Utility

    /**
     * Select the smaller of two values.
     * @param first the first value
     * @param second the second value
     * @return the smaller value
     */
    public static long min(long first, long second) {
        return Math.min(first, second); // return with the smaller value
    } // min

} // Utility

    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager.addItem(book);
    } // main

Memory state at the breakpoint on line 13 in InventoryDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 43 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 44 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 18 in Book.java. [code listing] [download] [open]

    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager.addItem(book);
    } // main

Memory state at the breakpoint on line 14 in InventoryDriver.java. [code listing] [download] [open]

    public static InventoryManager getManager() {
        return new InventoryManager(); // return with the new reference value
    } // getManager

Memory state at the breakpoint on line 19 in InventoryManager.java. [code listing] [download] [open]

    private InventoryManager() {
        this.inventory = new Product[0]; // Start with an empty array object.
    } // InventoryManager

Memory state at the breakpoint on line 10 in InventoryManager.java. [code listing] [download] [open]

    private InventoryManager() {
        this.inventory = new Product[0]; // Start with an empty array object.
    } // InventoryManager

Memory state at the breakpoint on line 11 in InventoryManager.java. [code listing] [download] [open]

    private InventoryManager() {
        this.inventory = new Product[0]; // Start with an empty array object.
    } // InventoryManager

Memory state at the breakpoint on line 12 in InventoryManager.java. [code listing] [download] [open]

    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager.addItem(book);
    } // main

Memory state at the breakpoint on line 16 in InventoryDriver.java. [code listing] [download] [open]

    void addItem(Product product) {
        InventoryManager.nullCheck(product);
        product.serial = 14; // serial is package private
        product.setSerial(15); // setSerial is protected; this caller shares its package
    } // addItem

Memory state at the breakpoint on line 28 in InventoryManager.java. [code listing] [download] [open]

    private static void nullCheck(Object object) {
        if (object == null) {
            throw new IllegalArgumentException("A product reference is required.");
        } // if
    } // nullCheck

Memory state at the breakpoint on line 47 in InventoryManager.java. [code listing] [download] [open]

    private static void nullCheck(Object object) {
        if (object == null) {
            throw new IllegalArgumentException("A product reference is required.");
        } // if
    } // nullCheck

Memory state at the breakpoint on line 50 in InventoryManager.java. [code listing] [download] [open]

    void addItem(Product product) {
        InventoryManager.nullCheck(product);
        product.serial = 14; // serial is package private
        product.setSerial(15); // setSerial is protected; this caller shares its package
    } // addItem

Memory state at the breakpoint on line 29 in InventoryManager.java. [code listing] [download] [open]

    void addItem(Product product) {
        InventoryManager.nullCheck(product);
        product.serial = 14; // serial is package private
        product.setSerial(15); // setSerial is protected; this caller shares its package
    } // addItem

Memory state at the breakpoint on line 30 in InventoryManager.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Memory state at the breakpoint on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Memory state at the breakpoint on line 28 in Product.java. [code listing] [download] [open]

    void addItem(Product product) {
        InventoryManager.nullCheck(product);
        product.serial = 14; // serial is package private
        product.setSerial(15); // setSerial is protected; this caller shares its package
    } // addItem

Memory state at the breakpoint on line 31 in InventoryManager.java. [code listing] [download] [open]

    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager.addItem(book);
    } // main

Memory state at the breakpoint on line 17 in InventoryDriver.java. [code listing] [download] [open]

Sample Solution B

B compiles: requestPurchase is public in the public class InventoryManager. It checks the supplied reference value without changing the book object.

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager.requestPurchase(book);
    } // main

} // InventoryDriver

Final captured memory state. [code listing] [download] [open]

Sample Solution C

C does not compile: nullCheck is private to InventoryManager. Use its public requestPurchase method, which performs the check internally.

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        InventoryManager.nullCheck(book);
    } // main

} // InventoryDriver

Compiler error (expected)

cs1302/inventory/InventoryDriver.java:16: error: nullCheck(Object) has private access in InventoryManager
        InventoryManager.nullCheck(book);
                        ^
1 error

Sample Solution D

D does not compile: the constructor is private to InventoryManager. Call InventoryManager.getManager() instead; it returns with a reference to a new manager object.

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        manager = new InventoryManager();
    } // main

} // InventoryDriver

Compiler error (expected)

cs1302/inventory/InventoryDriver.java:16: error: InventoryManager() has private access in InventoryManager
        manager = new InventoryManager();
                  ^
1 error

Sample Solution E

E compiles: Utility is accessible in its own package, and min is public. The method returns with the long value 3, which is printed.

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        InventoryManager manager = InventoryManager.getManager();
        // the next line is different for each candidate
        System.out.println(Utility.min(3, 8));
    } // main

} // InventoryDriver

Final captured memory state. [code listing] [download] [open]

Sharing a package does not grant private access. Package names are exact: a subpackage does not gain access to declarations in its parent package.

Part 3: Protected Access and Other Compilation Rules

Reveal the Superclass

Fig. 5 Complete structure of the new example [svg] [png]

UML Details for the Inheritance View

• Product is a public abstract class in cs1302.inventory.

• Product.serial is a package private long instance variable.

• Product() is a public no-argument constructor.

• Product.getSerial is a public instance method.

• Product.setSerial is a protected instance method.

• Book extends Product from cs1302.books.

• InventoryManager uses Product.

Product.java

package cs1302.inventory;

/** Base class for products used in the visibility lesson. */
public abstract class Product {

    /** Serial number; accessible within this package. */
    long serial;

    /** Initialize the serial number to zero. */
    public Product() {
        this.serial = 0;
    } // Product

    /**
     * Obtain the serial number.
     * @return the serial number
     */
    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

    /**
     * Change the serial number.
     * @param serial the new serial number
     */
    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

} // Product

Protected Access Across Packages

Within cs1302.inventory, setSerial is accessible without requiring a subclass. Outside that package, access to this protected instance method must occur within a subclass body. When the access uses a qualifying expression, that expression's compile-time type must be that subclass or one of its subclasses.

For example, inside Book, other.setSerial(20) is permitted when other has type Book. A variable of type Product does not meet that condition, even when its value refers to a Book object. See JLS 6.6.2.1: protected members.

Visibility Reference Table

Table 1 Access guide for members declared in these top-level classes

Access level	Declaring class	Other class in same package	Different package
public	Yes	Yes	Yes, if the class is accessible
protected	Yes	Yes	Subclass access, with the restriction described in Protected Access Across Packages
package private	Yes	Yes	No
private	Yes, including through another instance	No	No

This guide assumes the lesson's top-level classes. Nested classes and protected constructors require additional rules; the Product constructor here is public.

Visibility Practice: Classify the Failure

Scenario 1: Constructor Alternatives

SVG PNG

Listing 101 Inside main in cs1302/books/BookDriver.java

Book original = new Book("1984");
original.updateSerials(original); // the serial number becomes 20
Book copy = new Book(original);

On your Exit Ticket: Consider each alternative declaration of Book(Book other) independently. Predict whether it compiles, identify the governing rule, and correct any invalid alternative. For the valid alternative, draw the memory map after the declaration of copy finishes, before main returns. Include both local reference variables, both book objects, and their title references and serial numbers.

Alternative A

Listing 102 Alternative A — Book(Book) in cs1302/books/Book.java

/**
 * Copy another book's title reference value and serial number.
 * @param other non-null reference to a book object
 */
public Book(Book other) {
    super(42L);
    this.title = other.title;
} // Book

Alternative B

Listing 103 Alternative B — Book(Book) in cs1302/books/Book.java

/**
 * Copy another book's title reference value and serial number.
 * @param other non-null reference to a book object
 */
public Book(Book other) {
    super();
    this.title = other.title;
    this.serial = other.getSerial();
} // Book

Alternative C

Listing 104 Alternative C — Book(Book) in cs1302/books/Book.java

/**
 * Copy another book's title reference value and serial number.
 * @param other non-null reference to a book object
 */
public Book(Book other) {
    super();
    this.title = other.title;
    this.setSerial(other.getSerial());
} // Book

Sample Solution A

A does not compile: Product has no constructor accepting a long argument. Use super(); this failure is not a visibility violation.

Listing 105 Inside main in cs1302/books/BookDriver.java

Book original = new Book("1984");
original.updateSerials(original); // the serial number becomes 20
Book copy = new Book(original);

Compiler error (expected)

cs1302/books/Book.java:25: error: constructor Product in class Product cannot be applied to given types;
        super(42L);
        ^
  required: no arguments
  found:    long
  reason: actual and formal argument lists differ in length
1 error

Sample Solution B

B does not compile: serial is package private in cs1302.inventory. Use the inherited protected setSerial method as in C.

Listing 106 Inside main in cs1302/books/BookDriver.java

Book original = new Book("1984");
original.updateSerials(original); // the serial number becomes 20
Book copy = new Book(original);

Compiler error (expected)

cs1302/books/Book.java:27: error: serial is not public in Product; cannot be accessed from outside package
        this.serial = other.getSerial();
            ^
1 error

Sample Solution C

C compiles. super() invokes the public no-argument constructor. Private access to other.title occurs inside Book, and this.setSerial is permitted subclass access. The new book has its own instance variables, with a copy of the title reference value and serial number.

|< < Prev

Step 1 of 29: BookDriver.java:6

Next > >|

1234567891011121314151617181920212223242526272829

BookDriver.java

package cs1302.books;

public class BookDriver {

    public static void main(String[] args) {
        Book original = new Book("1984");
        original.updateSerials(original); // the serial number becomes 20
        Book copy = new Book(original);
    } // main

} // BookDriver

Book.java

package cs1302.books;

import cs1302.inventory.Product;

/** A product with a title. */
public class Book extends Product {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value and serial number.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

    /**
     * Demonstrate permitted protected access from this subclass.
     * @param other non-null reference to a book object
     */
    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

} // Book

Product.java

package cs1302.inventory;

/** Base class for products used in the visibility lesson. */
public abstract class Product {

    /** Serial number; accessible within this package. */
    long serial;

    /** Initialize the serial number to zero. */
    public Product() {
        this.serial = 0;
    } // Product

    /**
     * Obtain the serial number.
     * @return the serial number
     */
    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

    /**
     * Change the serial number.
     * @param serial the new serial number
     */
    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

} // Product

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book original = new Book("1984");
        original.updateSerials(original); // the serial number becomes 20
        Book copy = new Book(original);
    } // main

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 6 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 43 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 44 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 18 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book original = new Book("1984");
        original.updateSerials(original); // the serial number becomes 20
        Book copy = new Book(original);
    } // main

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 7 in BookDriver.java. [code listing] [download] [open]

    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 51 in Book.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 28 in Product.java. [code listing] [download] [open]

    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 52 in Book.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 28 in Product.java. [code listing] [download] [open]

    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 53 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book original = new Book("1984");
        original.updateSerials(original); // the serial number becomes 20
        Book copy = new Book(original);
    } // main

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 8 in BookDriver.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 25 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 12 in Product.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 26 in Book.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 27 in Book.java. [code listing] [download] [open]

    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 19 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 28 in Product.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 28 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book original = new Book("1984");
        original.updateSerials(original); // the serial number becomes 20
        Book copy = new Book(original);
    } // main

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 9 in BookDriver.java. [code listing] [download] [open]

Scenario 2: Compile-Time Type versus Object Class

SVG PNG

Import Book from cs1302.books in InventoryDriver.java. Inside InventoryDriver.main, use this setup:

Listing 107 Inside main in cs1302/inventory/InventoryDriver.java

Book book = new Book("1984");
Product product = book; // copies the reference value; both refer to one object

On your Exit Ticket: Judge each candidate independently. Predict compile-time errors and explain their causes. For each valid candidate, draw the memory map just before main completes.

Listing 108 Inside main in cs1302/inventory/InventoryDriver.java

System.out.println(product.getTitle()); // A
product.setSerial(14); // B
product.serial = 14; // C
Product another = new Product(); // D

Sample Solution A

A does not compile: the compile-time type Product does not declare or inherit getTitle. This is not a visibility violation. Use book.getTitle() instead.

Listing 109 cs1302/inventory/InventoryDriver.java

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        Product product = book; // copies the reference value; both refer to one object

        System.out.println(product.getTitle());
    } // main

} // InventoryDriver

Compiler error (expected)

cs1302/inventory/InventoryDriver.java:16: error: cannot find symbol
        System.out.println(product.getTitle());
                                  ^
  symbol:   method getTitle()
  location: variable product of type Product
1 error

Sample Solution B

B compiles: setSerial is protected, and the caller shares its declaring package. Both reference variables still refer to the same book object, whose serial number is now 14.

Listing 110 cs1302/inventory/InventoryDriver.java

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        Product product = book; // copies the reference value; both refer to one object

        product.setSerial(14);
    } // main

} // InventoryDriver

cs1302/inventory/InventoryDriver.java [code listing] [download] [open]

Sample Solution C

C compiles: serial is package private, and the caller is in cs1302.inventory. The assignment changes the same book object observed through both reference variables.

Listing 111 cs1302/inventory/InventoryDriver.java

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        Product product = book; // copies the reference value; both refer to one object

        product.serial = 14;
    } // main

} // InventoryDriver

cs1302/inventory/InventoryDriver.java [code listing] [download] [open]

Sample Solution D

D does not compile because Product is abstract, even though its constructor is public. Use Product another = new Book("Dune");. Constructing a book invokes a superclass constructor to initialize state in that same object; it does not construct a separate Product object.

Listing 112 cs1302/inventory/InventoryDriver.java

package cs1302.inventory;

import cs1302.books.Book;

/** Demonstrate one independent inventory-package access. */
public class InventoryDriver {

    /**
     * Run this visibility candidate.
     * @param args unused command-line arguments
     */
    public static void main(String[] args) {
        Book book = new Book("1984");
        Product product = book; // copies the reference value; both refer to one object

        Product another = new Product();
    } // main

} // InventoryDriver

Compiler error (expected)

cs1302/inventory/InventoryDriver.java:16: error: Product is abstract; cannot be instantiated
        Product another = new Product();
                          ^
1 error

Scenario 3: Protected Access through a Parameter

SVG PNG

Listing 113 Inside main in cs1302/books/BookDriver.java

Book first = new Book("1984");
Book second = new Book("Dune");
first.updateSerials(second);

On your Exit Ticket: Compare these alternatives inside Book. Each replaces updateSerials independently and uses the caller setup above, which creates two distinct book objects. Does each method compile? Does the referred-to object's class change the access rule? For the valid alternative, draw the memory map just before updateSerials returns. Include first and second in main, the parameter other, and the object referred to by this in the active method call.

Alternative A

Listing 114 Replacement method in cs1302/books/Book.java

public void updateSerials(Book other) {
    this.setSerial(10);
    other.setSerial(20);
} // updateSerials

Alternative B

Listing 115 Replacement method in cs1302/books/Book.java

public void updateSerials(Product other) {
    this.setSerial(10);
    other.setSerial(20);
} // updateSerials

Sample Solution A

A compiles: access occurs inside the subclass Book, and the qualifying expression other has compile-time type Book. The first book's serial number becomes 10 and the second's becomes 20. Java passes arguments by value: the reference value is copied into parameter variable other; no book object is copied.

|< < Prev

Step 1 of 27: BookDriver.java:6

Next > >|

123456789101112131415161718192021222324252627

BookDriver.java

package cs1302.books;

public class BookDriver {

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.updateSerials(second);
    } // main

} // BookDriver

Book.java

package cs1302.books;

import cs1302.inventory.Product;

/** A product with a title. */
public class Book extends Product {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value and serial number.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

    /**
     * Demonstrate permitted protected access from this subclass.
     * @param other non-null reference to a book object
     */
    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

} // Book

Product.java

package cs1302.inventory;

/** Base class for products used in the visibility lesson. */
public abstract class Product {

    /** Serial number; accessible within this package. */
    long serial;

    /** Initialize the serial number to zero. */
    public Product() {
        this.serial = 0;
    } // Product

    /**
     * Obtain the serial number.
     * @return the serial number
     */
    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

    /**
     * Change the serial number.
     * @param serial the new serial number
     */
    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

} // Product

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.updateSerials(second);
    } // main

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 6 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 43 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 44 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 18 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.updateSerials(second);
    } // main

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 7 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 43 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 44 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 18 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.updateSerials(second);
    } // main

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 8 in BookDriver.java. [code listing] [download] [open]

    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 51 in Book.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 28 in Product.java. [code listing] [download] [open]

    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 52 in Book.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 28 in Product.java. [code listing] [download] [open]

    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 53 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.updateSerials(second);
    } // main

Inside main in cs1302/books/BookDriver.java Note: The debugger breakpoint is on line 9 in BookDriver.java. [code listing] [download] [open]

Sample Solution B

B does not compile: other has compile-time type Product. Outside Product's package, this access inside Book requires a qualifying expression whose type is Book or a subclass of Book. The class of the referred-to object does not change that rule. Use the Book parameter type as in A when that matches the operation's contract. The access through this remains permitted in both alternatives.

Listing 116 Inside main in cs1302/books/BookDriver.java

Book first = new Book("1984");
Book second = new Book("Dune");
first.updateSerials(second);

Compiler error (expected)

cs1302/books/Book.java:52: error: setSerial(long) has protected access in Product
        other.setSerial(20); // the qualifying expression has type Product
             ^
1 error

Additional Practice: Apply the Visibility Rules

Scenario 1: Two Objects, One Declaring Class

SVG PNG

Inside Book(Book other), immediately after super(), consider these statements in order. The original book's title is "1984".

this.title = other.title; // A
other.setTitle("Dune");   // B

On your Exit Ticket: Does each statement compile inside Book? Explain which object's instance variable each statement changes. Draw a memory map immediately after statement B executes inside the copy constructor. Include its parameter other and the object referred to by this. Would a driver be permitted to perform the same accesses through book references? Explain the rule and suggest corrections without changing visibility.

Sample Solution A and B: Inside Book

Both statements compile because the accesses occur inside Book. A copies the title reference value into the new object's instance variable. B changes the original object's title reference value; it does not modify a String object. The new book retains the reference to "1984" while the original book now refers to "Dune" through its title variable.

|< < Prev

Step 1 of 24: BookDriver.java:6

Next > >|

123456789101112131415161718192021222324

BookDriver.java

package cs1302.books;

public class BookDriver {

    public static void main(String[] args) {
        Book original = new Book("1984");
        Book copy = new Book(original);
    } // main

} // BookDriver

Book.java

package cs1302.books;

import cs1302.inventory.Product;

/** A product with a title. */
public class Book extends Product {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value and serial number.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
        other.setTitle("Dune");
        this.setSerial(other.getSerial());
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

    /**
     * Demonstrate permitted protected access from this subclass.
     * @param other non-null reference to a book object
     */
    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials

} // Book

Product.java

package cs1302.inventory;

/** Base class for products used in the visibility lesson. */
public abstract class Product {

    /** Serial number; accessible within this package. */
    long serial;

    /** Initialize the serial number to zero. */
    public Product() {
        this.serial = 0;
    } // Product

    /**
     * Obtain the serial number.
     * @return the serial number
     */
    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

    /**
     * Change the serial number.
     * @param serial the new serial number
     */
    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

} // Product

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book original = new Book("1984");
        Book copy = new Book(original);
    } // main

Memory state at the breakpoint on line 6 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 44 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 45 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 18 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book original = new Book("1984");
        Book copy = new Book(original);
    } // main

Memory state at the breakpoint on line 7 in BookDriver.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        other.setTitle("Dune");
        this.setSerial(other.getSerial());
    } // Book

Memory state at the breakpoint on line 25 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 12 in Product.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        other.setTitle("Dune");
        this.setSerial(other.getSerial());
    } // Book

Memory state at the breakpoint on line 26 in Book.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        other.setTitle("Dune");
        this.setSerial(other.getSerial());
    } // Book

Memory state at the breakpoint on line 27 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 44 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 45 in Book.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        other.setTitle("Dune");
        this.setSerial(other.getSerial());
    } // Book

Memory state at the breakpoint on line 28 in Book.java. [code listing] [download] [open]

    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

Memory state at the breakpoint on line 19 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Memory state at the breakpoint on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Memory state at the breakpoint on line 28 in Product.java. [code listing] [download] [open]

    public Book(Book other) {
        super();
        this.title = other.title;
        other.setTitle("Dune");
        this.setSerial(other.getSerial());
    } // Book

Memory state at the breakpoint on line 29 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book original = new Book("1984");
        Book copy = new Book(original);
    } // main

Memory state at the breakpoint on line 8 in BookDriver.java. [code listing] [download] [open]

Sample Solution A: Access from BookDriver

This driver cannot access either private title variable. Use the public copy constructor to create a book with the same title. The driver uses copy instead of this because static main has no current object.

Book original = new Book("1984");
Book copy = new Book(original);
copy.title = original.title;

Compiler error (expected)

cs1302/books/BookDriver.java:8: error: title has private access in Book
        copy.title = original.title;
            ^
cs1302/books/BookDriver.java:8: error: title has private access in Book
        copy.title = original.title;
                             ^
2 errors

Sample Solution B: Access from BookDriver

setTitle is private to Book and cannot be called here. The driver can read the title using getTitle or construct a new book with the desired title. The class has no public operation for changing an existing book's title.

Book original = new Book("1984");
Book copy = new Book(original);
original.setTitle("Dune");

Compiler error (expected)

cs1302/books/BookDriver.java:8: error: setTitle(String) has private access in Book
        original.setTitle("Dune");
                ^
1 error

Scenario 2: Protected Access through Two Reference Variables

SVG PNG

Suppose this method is added inside Book:

public void revise(Book book, Product product) {
    this.setSerial(1);    // A
    book.setSerial(2);    // B
    product.setSerial(3); // C
    book.serial = 4;      // D
} // revise

On your Exit Ticket: Judge each labeled statement independently, with the other three omitted. Predict whether it compiles and explain the access rule. Correct invalid statements without changing visibility. For valid statements, draw a memory map just before revise returns to its caller. Include the caller's variables, both parameter variables, and the object referred to by this:

Book first = new Book("1984");
Book second = new Book("Dune");
first.revise(second, second);

The call supplies one argument for each parameter. Each argument evaluates to a reference value that is copied into its parameter variable; Java passes by value and does not copy either book object.

Sample Solution A

A compiles: access occurs inside subclass Book, and this has type Book. The first book's serial number becomes 1; the second book's serial number remains 0.

|< < Prev

Step 1 of 24: BookDriver.java:6

Next > >|

123456789101112131415161718192021222324

BookDriver.java

package cs1302.books;

public class BookDriver {

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

} // BookDriver

Product.java

package cs1302.inventory;

/** Base class for products used in the visibility lesson. */
public abstract class Product {

    /** Serial number; accessible within this package. */
    long serial;

    /** Initialize the serial number to zero. */
    public Product() {
        this.serial = 0;
    } // Product

    /**
     * Obtain the serial number.
     * @return the serial number
     */
    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

    /**
     * Change the serial number.
     * @param serial the new serial number
     */
    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

} // Product

Book.java

package cs1302.books;

import cs1302.inventory.Product;

/** A product with a title. */
public class Book extends Product {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value and serial number.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

    /**
     * Demonstrate permitted protected access from this subclass.
     * @param other non-null reference to a book object
     */
    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials
    /**
     * Demonstrate one candidate access from inside the subclass.
     * @param book non-null reference to a book object
     * @param product non-null reference to a book object
     */
    public void revise(Book book, Product product) {
        this.setSerial(1);
    } // revise

} // Book

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

Memory state at the breakpoint on line 6 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 43 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 44 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 18 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

Memory state at the breakpoint on line 7 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 43 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 44 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 18 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

Memory state at the breakpoint on line 8 in BookDriver.java. [code listing] [download] [open]

    public void revise(Book book, Product product) {
        this.setSerial(1);
    } // revise

Memory state at the breakpoint on line 60 in Book.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Memory state at the breakpoint on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Memory state at the breakpoint on line 28 in Product.java. [code listing] [download] [open]

    public void revise(Book book, Product product) {
        this.setSerial(1);
    } // revise

Memory state at the breakpoint on line 61 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

Memory state at the breakpoint on line 9 in BookDriver.java. [code listing] [download] [open]

Sample Solution B

B compiles: the qualifying expression book has type Book. The second book's serial number becomes 2; the first remains 0. Both parameter variables refer to the second book object.

|< < Prev

Step 1 of 24: BookDriver.java:6

Next > >|

123456789101112131415161718192021222324

BookDriver.java

package cs1302.books;

public class BookDriver {

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

} // BookDriver

Product.java

package cs1302.inventory;

/** Base class for products used in the visibility lesson. */
public abstract class Product {

    /** Serial number; accessible within this package. */
    long serial;

    /** Initialize the serial number to zero. */
    public Product() {
        this.serial = 0;
    } // Product

    /**
     * Obtain the serial number.
     * @return the serial number
     */
    public long getSerial() {
        return this.serial; // return with the serial number
    } // getSerial

    /**
     * Change the serial number.
     * @param serial the new serial number
     */
    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

} // Product

Book.java

package cs1302.books;

import cs1302.inventory.Product;

/** A product with a title. */
public class Book extends Product {

    /** Reference to this book's title string. */
    private String title;

    /**
     * Initialize a book with the supplied title reference value.
     * @param title reference to the title string
     */
    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

    /**
     * Copy another book's title reference value and serial number.
     * @param other non-null reference to a book object
     */
    public Book(Book other) {
        super();
        this.title = other.title;
        this.setSerial(other.getSerial());
    } // Book

    /**
     * Obtain the stored title reference value.
     * @return the title reference value
     */
    public String getTitle() {
        return this.title; // return with the title reference value
    } // getTitle

    /**
     * Replace the stored title reference value.
     * @param title the new title reference value
     */
    private void setTitle(String title) {
        this.title = title;
    } // setTitle

    /**
     * Demonstrate permitted protected access from this subclass.
     * @param other non-null reference to a book object
     */
    public void updateSerials(Book other) {
        this.setSerial(10);
        other.setSerial(20); // the qualifying expression has type Book
    } // updateSerials
    /**
     * Demonstrate one candidate access from inside the subclass.
     * @param book non-null reference to a book object
     * @param product non-null reference to a book object
     */
    public void revise(Book book, Product product) {
        book.setSerial(2);
    } // revise

} // Book

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

Memory state at the breakpoint on line 6 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 43 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 44 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 18 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

Memory state at the breakpoint on line 7 in BookDriver.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 16 in Book.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 10 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 11 in Product.java. [code listing] [download] [open]

    public Product() {
        this.serial = 0;
    } // Product

Memory state at the breakpoint on line 12 in Product.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 17 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 43 in Book.java. [code listing] [download] [open]

    private void setTitle(String title) {
        this.title = title;
    } // setTitle

Memory state at the breakpoint on line 44 in Book.java. [code listing] [download] [open]

    public Book(String title) {
        super();
        this.setTitle(title);
    } // Book

Memory state at the breakpoint on line 18 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

Memory state at the breakpoint on line 8 in BookDriver.java. [code listing] [download] [open]

    public void revise(Book book, Product product) {
        book.setSerial(2);
    } // revise

Memory state at the breakpoint on line 60 in Book.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Memory state at the breakpoint on line 27 in Product.java. [code listing] [download] [open]

    protected void setSerial(long serial) {
        this.serial = serial;
    } // setSerial

Memory state at the breakpoint on line 28 in Product.java. [code listing] [download] [open]

    public void revise(Book book, Product product) {
        book.setSerial(2);
    } // revise

Memory state at the breakpoint on line 61 in Book.java. [code listing] [download] [open]

cs1302/books/BookDriver.java

    public static void main(String[] args) {
        Book first = new Book("1984");
        Book second = new Book("Dune");
        first.revise(second, second);
    } // main

Memory state at the breakpoint on line 9 in BookDriver.java. [code listing] [download] [open]

Sample Solution C

C does not compile: product has compile-time type Product. For this access inside Book outside the declaring package, its type must be Book or a subclass of Book. Referring to a book object does not change that rule. If the operation is intended for books, change the parameter's declared type to Book.

Book first = new Book("1984");
Book second = new Book("Dune");
first.revise(second, second);

Compiler error (expected)

cs1302/books/Book.java:60: error: setSerial(long) has protected access in Product
        product.setSerial(3);
               ^
1 error

Sample Solution D

D does not compile: serial is package private in another package. Replace the statement with book.setSerial(4) to use permitted protected access inside Book. A driver in the book package cannot make that protected call merely because it shares a package with Book.

Book first = new Book("1984");
Book second = new Book("Dune");
first.revise(second, second);

Compiler error (expected)

cs1302/books/Book.java:60: error: serial is not public in Product; cannot be accessed from outside package
        book.serial = 4;
            ^
1 error