7. Inheritance Lesson

Lesson Objectives

Show the benefits of inheritance by expanding on our Drawable interface.

Expanding our Code Base

Expanding our Code Base

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We have decided to expand our code base and build a game of life simulation. In doing so, we added different types of employees to model the various types of pay people receive. We want our employees to be drawn in the simulation, so we added them as implementing classes to the Drawable interface.

Part 1: An Interface Solution

Objectives

This part allows us to review interfaces and highlight the benefits of using them. It also allows us to point out a scenario where an interface is good, but inheritance is better. Doing interfaces first lets the students compare an interface solution to an inheritance solution side by side.

Adding a Utility Method

We want to write a method to loop over all relevant objects and count the number of humans (not just objects of type Person) with a given hair color. An incomplete method signature for this can be seen below.

With your groups, write the method signature without the blank. Note: The blank may represent multiple parameters, if needed.

Listing 34 in Utility.java

/**
 * Returns the number of people with the specified hair color.
 * This method considers all humans in the simulation (not
 * just objects of type {@code Person}.
 *
 * @return the number of people with the specified hair color.
 * @param hairColor the hair color to count
 */
public static int numWithHairColor(Color hairColor, _________) {
    // ...
} // numWithHairColor

Solution

Listing 35 in Utility.java

public static int numWithHairColor(Color hairColor, Person[] people,
                                   HourlyEmployee[] hourlyEmployees,
                                   SalariedEmployee[] salariedEmployees) {
    // ...
} // numWithHairColor

Adding a Utility Method (Code)

With our current UML diagram, this is the required method signature:

Listing 36 in Utility.java

public static int numWithHairColor(Color hairColor, Person[] people,
                                   HourlyEmployee[] hourlyEmployees,
                                   SalariedEmployee[] salariedEmployees) {
    // ...
} // numWithHairColor

Take a minute with your group to think about the code would look for this method. Then, answer the following questions on your exit ticket:

1. Would there be redundant code within numWithHairColor?

2. If we add a class called ContractEmployee to our system, would numWithHairColor have to change?

3. Can we replace the three arrays with a single array of type Drawable?

Working Code

Listing 37 in Utility.java

public static int numWithHairColor(Color hairColor, Person[] people,
                                   HourlyEmployee[] hourlyEmployees,
                                   SalariedEmployee[] salariedEmployees) {
    int count = 0;

    // Count Person objects with the desired hair color
    for (Person p: people) {
        if (p.getHairColor() == hairColor) {
            count++;
        } // if
    } // for

    // Count HourlyEmployee objects with the desired hair color
    for (HourlyEmployee he: hourlyEmployees) {
        if (he.getHairColor() == hairColor) {
            count++;
        } // if
    } // for

    // Count SalariedEmployee objects with the desired hair color
    for (SalariedEmployee se: salariedEmployees) {
        if (se.getHairColor() == hairColor) {
            count++;
        } // if
    } // for

    return count;
} // numWithHairColor

Solution

Yes, the method would have to change if we added another class. We would need another loop and another array parameter.

We cannot use type Drawable for the array since we cannot get the hair color if we use a Drawable reference.

Adding a Utility Method (UML)

Adding numWithHairColor to Utility.java means that the Utility class is now dependent on Person, HourlyEmployee, and SalariedEmployee.

This is the exact thing we’ve been trying to avoid!

Answer the following question with your groups:

Question

Is it possible to avoid adding these dependencies while still having a working numWithHairColor method?

Avoid using inheritance for now.

Solution

1. Yes, and we need to avoid this if possible. A change in one place in the code should not cause us to have to change other parts of the code (when possible).

2. Yes, we could use an interface or inheritance (coming soon)!

Updated UML

We could solve our problem with an interface, let’s call it HairColorGettable:

In your groups, write the updated method signature for numWithHairColor on your exit ticket.

Does the new method leverage polymorphism? Explain.

Solution

Listing 38 in Utility.java

public static int numWithHairColor(Color hairColor, HairColorGettable[] people) {
    // ...
} // numWithHairColor

This method now leverages polymorphism and allows all implementing class types to be passed in using the array.

Implementing the Improved Method

On your exit ticket, implement the method below:

Listing 39 in Utility.java

/**
 * Returns the number of people with the specified hair color.
 * This method considers all humans in the simulation (not
 * just objects of type {@code Person}.
 *
 * @return the number of people with the specified hair color.
 * @param hairColor the hair color to count
 */
public static int numWithHairColor(Color hairColor, HairColorGettable[] people) {
    // ...
} // numWithHairColor

Solution

Listing 40 in Utility.java

public static int numWithHairColor(Color hairColor, HairColorGettable[] people) {
    int count = 0;
    for (HairColorGettable person: people) {
        if (person.getHairColor().equals(hairColor)) {
            count++;
        } // if
    } // if
    return count;
} // numWithHairColor

Eliminating Redundancy

Our solution improves the numWithHairColor method by using polymorphism, but is this a good scenario for using an interface?

Review the UML diagram below and answer the following on your exit tickets:

1. Which methods/instance variables are likely the exact same across multiple classes?

2. Which methods have the same signature, but are likely implemented differently?

3. Can inheritance help? How could we add inheritance without adding classes to our diagram?

Part 2: An Initial Inheritance Solution

Objectives

Help students understand how inheritance / extends relationships can help them promote common instance methods and instance variables to a parent class and enable polymorphism outside of the class.

Inheritance: Initial Attempt

As a first attempt, lets have both employee classes directly extend Person.

No Inheritance

How many total methods (excluding constructors) in Person, SalariedEmployee, and HourlyEmployee do we need to implement?

Inheritance

How many total methods (excluding constructors) in Person, SalariedEmployee, and HourlyEmployee do we need to implement?

What is Inherited?

Using inheritance, we can reduce the redundancy within the classes without changing their functionality.

In the second tab, the UML shows the methods/variables that are inherited as well.

Zoomed In (Standard)

Using the diagram below: answer the following with your groups on your exit tickets:

1. How many instance variables are declared in SalariedEmployee.java?

2. How many instance variables are in an object of type SalariedEmployee?

3. How many methods are declared in SalariedEmployee.java?

4. How many methods can be called using a reference of type SalariedEmployee?

Zoomed In (Detailed)

Note

Showing this level of detail is not standard/common in a UML diagram. We do it for educational purposes. Once we get used to seeing the detailed diagrams, the standard ones will be sufficient.

Compatibility

Using inheritance has reduced redundancy in our code base. To make sure it hasn’t also limited the functionality of our classes, lets explore compatibility.

Assignments (Code)

Imagine the code below is found in the main method of a Driver class.

For each numbered block of code, write on your exit tickets whether or not the code will compile. Explain your answer for each.

Drawable bob = new Person("Bob", Color.BROWN);
Drawable lee = new HourlyEmployee("Lee", 4187, 24.0, 12, Color.GREY);
Utility lee = new HourlyEmployee("Lee", 4187, 24.0, 12, Color.GREY);
Airplane a = new Person("Bob", Color.BROWN);
SalariedEmployee s = new Utility();
SalariedEmployee s2 = new Person("Susan", Color.BROWN);
Drawable a = null; // or, e.g., new Person(...);
Drawable b = a;
Drawable a = new Drawable();
Drawable a = getDrawable();     // assume return type of method is Drawable

Solutions

1. Yes. A Person reference can be assigned to a Drawable variable since Person implements Drawable.

2. Yes. An HourlyEmployee reference can be assigned to a Drawable variable since HourlyEmployee implements Drawable (it inherits the methods in the interface from its parent).

3. No. These are not compatible types.

4. No. These are not compatible types.

5. No. These are not compatible types.

6. No. These are not compatible types. A Person reference cannot be stored in a SalariedEmployee variable.

7. Yes. Reference variables can store null.

8. Yes. The types are compatible.

9. No. You cannot create an instance of an interface type.

10. Yes. The types are compatible.

Assignments Discussion

Given a variable of each type, write all compatible expression types such that an assignment of the form type name = expression will compile. Write your answers on your exit ticket.

We have completed the first few to help get you started.

1. Person: Person, SalariedEmployee, HourlyEmployee

2. SalariedEmployee[]: SalariedEmployee[]

3. Person[]:

4. Tree:

5. Airplane:

6. Drawable:

7. Flower:

8. SalariedEmployee:

9. HourlyEmployee:

10. Drawable[]:

Solutions

1. Person: Person, SalariedEmployee, HourlyEmployee

2. SalariedEmployee[]: SalariedEmployee[]

3. Person[]: Person[], SalariedEmployee[], HourlyEmployee[]

4. Tree: Tree

5. Airplane: Airplane

6. Drawable: Drawable, Flower, Tree, Airplane, Person, SalariedEmployee, HourlyEmployee

7. Flower: Flower

8. SalariedEmployee: SalariedEmployee

9. HourlyEmployee: HourlyEmployee

10. Drawable[]: Drawable[], Flower[], Tree[], Airplane[], Person[], SalariedEmployee[], HourlyEmployee[]

Available Methods (Code)

Question 1

Which of the following are valid method calls?

public static void doSomething(Drawable d) {
    d.draw();          // a
    d.getHairColor();  // b
    d.calculatePay();  // c
    Utility.drawIt(d); // d
} // doSomething

Question 2

Which of the following are valid method calls?

Person p = new SalariedEmployee("Bob", 4819, 45000.00, Color.BROWN);

p.getHairColor();  // a
p.calculatePay();  // b
p.draw();          // c
Utility.drawIt(p); // d

Question 3

Which of the following are valid method calls?

public static void doStuff(Drawable[] d) {

    d.draw();             // a
    d[0].draw();          // b
    d[0].calculatePay();  // c
    d[1].getHairColor();  // d
    Utility.drawIt(d);    // e
    Utility.drawIt(d[0]); // f
    Utility.drawAll(d);   // g
} // doStuff

Question 4

Which of the following are valid method calls?

Drawable[] d = new Person[2];
d[0] = new SalariedEmployee("Bob", 4819, 45000.00, Color.BROWN);
d[1] = new Person("Susan", Color.GREY);

d.draw();             // a
d[0].draw();          // b
d[0].calculatePay();  // c
d[1].getHairColor();  // d
Utility.drawIt(d);    // e
Utility.drawIt(d[0]); // f
Utility.drawAll(d);   // g

Question 5

Which of the following are valid method calls?

Person[] people = new Person[2];
people[0] = new SalariedEmployee("Bob", 4819, 45000.00, Color.BROWN);
people[1] = new Person("Susan", Color.GREY);

people.draw();             // a
people[0].draw();          // b
people[0].calculatePay();  // c
people[1].getHairColor();  // d
Utility.drawIt(people);    // e
Utility.drawIt(people[0]); // f
Utility.drawAll(people);   // g

Solutions

1a. Yes
1b. No
1c. No
1d. Yes

2a. Yes
2b. No
2c. Yes
2d. Yes

3a. No
3b. Yes
3c. No
3d. No
3e. No
3f. Yes
3g. Yes

4a. No
4b. Yes
4c. No
4d. No
4e. No
4f. Yes
4g. Yes

5a. No
5b. Yes
5c. No
5d. Yes
5e. No
5f. Yes
5g. Yes

Available Methods Discussion (Table)

Imagine you are writing code in a separate driver class. Given a variable of each type in the driver, write all methods/variables that are available. In other words, if we use the . operator after the variable in the form varName._____, what can follow the .? Write your answers on your exit ticket.

We have completed the first few to help get you started.

1. Drawable d: d.draw()

2. Person[] people: people.length, people[0], people[1]

3. Person p:

4. SalariedEmployee se:

5. HourlyEmployee he:

6. Drawable[] dArray:

Solutions

1. Drawable d: d.draw

2. Person[] people: people.length, people[0], people[1]

3. Person p: getName, getHairColor, toString, draw

4. SalariedEmployee se: getName, getHairColor, toString, draw, getEmployeeID, calculatePay

5. HourlyEmployee he: getName, getHairColor, toString, draw, getEmployeeID, calculatePay

6. Drawable[] dArray: dArray.length, dArray[0], dArray[1]

Deep Dive: HourlyEmployee

Assume we have a variable, sue, of type HourlyEmployee.

• methods directly declared in HourlyEmployee:

  • sue.getEmployeeID()

  • sue.calculatePay()

• method that HourlyEmployee inherits from Person that have not yet been overriden:

  • sue.getName()

  • sue.getHairColor()

  • sue.draw()

  • sue.toString()

• method that HourlyEmployee inherits from Object that have not yet been overriden:

  • sue.equals(Object)

  • etc.

Javadoc Example

On the Javadoc page, you can see the inheritance hierarchy for any class along with the inherited methods and which parent they came from.

As an example, take a look at the JavaFX 17 Button Class.

Answer the following questions with your groups:

1. How many parent (ancestor) classes does Button have?

2. Which parent class declares (creates) the setOnAction method?

Constructing Objects

Questions

• What does the constructor look like in the parent class?

• What does the constructor look like in a child class?

Code (1)

public Person(String name, Color hairColor) {
    this.name = name;
    this.hairColor = hairColor;
} // Person

public SalariedEmployee(String name, int employeeID, double annualSalary, Color hairColor) {
    this.employeeID = employeeID;
    this.annualSalarary = annualSalary;
    //
    // What about the other parameters?
    //
} // SalariedEmployee

Code (2)

public Person(String name, Color hairColor) {
    this.name = name;
    this.hairColor = hairColor;
} // Person

public SalariedEmployee(String name, int employeeID, double annualSalary, Color hairColor) {
    super(name, hairColor); // What is this calling?
    this.employeeID = employeeID;
    this.annualSalarary = annualSalary;
} // SalariedEmployee

Adding a Utility Method

Let’s see how our new system design helps us write numWithHairColor.

With your groups, write the method signature without the blank. Note: The blank may represent multiple parameters, if needed.

Listing 41 in Utility.java

/**
 * Returns the number of people with the specified hair color.
 * This method considers all humans in the simulation (not
 * just objects of type {@code Person}.
 *
 * @return the number of people with the specified hair color.
 * @param hairColor the hair color to count
 */
public static int numWithHairColor(Color hairColor, _________) {
    // ...
} // numWithHairColor

Solution

Listing 42 in Utility.java

public static int numWithHairColor(Color hairColor, Person[] people) {
    // ...
} // numWithHairColor

Other Utility Methods

How many different kinds of arrays are needed to complete the tasks below? Write your answers on your exit ticket.

Also note: is there redundancy in the code? Don’t write the code out, just try to imagine.

1. Listing 43 In Utility.java

    // Determine the length of the longest name
    public static int getLongestNameLength(_________) {
        // ...
    } // getLongestNameLength
   

2. Listing 44 In Utility.java

   // Determine Largest Employee ID
   public static int findLargestEmployeeID(________) {
       // ...
   } // findLargestEmployeeID
   

Practice at Home

Fully implement the two methods above (method signature and body) for extra practice.

Solution

1. We just need one array of type Person.

2. We need two arrays - one of type SalariedEmployee and the other of type HourlyEmployee for this task.

Discussion: Initial Attempt

We still have a little bit of redundancy. Discuss possible solutions with your group. Write your thoughts on your exit ticket.

Solution

We still have redundancy with employeeID being declared in multiple classes.

Part 3: An Improved Inheritance Solution

Objectives

Help students understand how inheritance can enable them to promote some common methods and instance variables to a parent class and enable polymorphism outside of the class.

Updated UML (Full)

No Inheritance

Updated Inheritance

Updated UML (Inheritance Focused)

Here is the same content focused on the inheritance relationship.

In the second tab, the UML shows the methods/variables that are inherited as well. Showing this level of detail is not common in a UML diagram. We do it for educational purposes. Once we get used to seeing the detailed diagrams, the standard ones will be sufficient.

Zoomed In (Standard)

Using the diagram below: answer the following with your groups on your exit tickets:

1. How many instance variables are declared in SalariedEmployee.java?

2. How many instance variables are in an object of type SalariedEmployee?

3. How many methods are declared in SalariedEmployee.java?

4. How many methods can be called using a reference of type SalariedEmployee?

Zoomed In (Detailed)

More Utility Methods

With our old design, findLargestEmployeeID required an array parameter for each type of Employee. It also contained redundant code (a nearly identical for loop for each employee type).

Old Design and Code

Working Code

Listing 45 in Utility.java

public static int findLargestEmployeeID(HourlyEmployee[] hourlyEmployees,
                                   SalariedEmployee[] salariedEmployees) {

    // Assume there is at least one item in the array
    // and set that employee ID to max to get us started.
    int max = hourlyEmployees[0].getEmployeeID();

    // Check hourly employees for a larger ID.
    for (HourlyEmployee he: hourlyEmployees) {
        if (he.getEmployeeID() > max) {
            max = he.getEmployeeID();
        } // if
    } // for

    // Check salaried employees for a larger ID.
    for (SalariedEmployee se: salariedEmployees) {
        if (se.getEmployeeID() > max) {
            max = se.getEmployeeID();
        } // if
    } // for

    return max;
} // findLargestEmployeeID

How many different kinds of arrays are needed to complete the tasks below now? Write your answers on your exit ticket. Is there redundancy within the code?

Listing 46 in Utility.java

// Determine Largest Employee ID
public static int findLargestEmployeeID(________) {
    // ...
} // findLargestEmployeeID

Listing 47 in Utility.java

public static double getAveragePay(________) {
    // ...
} // getAveragePay

Solution

1. We just need one array of type Employee now and one for loop! There is no more redundancy.

2. We need two arrays: one of type SalariedEmployee and another of type HourlyEmployee. The code in the method will be redundant.

getAveragePay Discussion

Listing 48 in Utility.java

public static double getAveragePay(SalariedEmployee[] salariedEmployees,
                                   HourlyEmployee[] hourlyEmployees) {
    // ...
} // getAveragePay

How can we eliminate this redundancy? Write your ideas on your exit ticket with your groups.

Fixing getAveragePay

Solution 1: Use an interface

Could we add an interface called Payable?

Payable UML

Now we can do this!

Listing 49 in Utility.java

public static double getAveragePay(Payable[] employees) {

    // ...
} // getAveragePay

What is the downside?

Solution 2: Use Employee

Can we move calculatePay into Employee?

Listing 50 in Employee.java

public class Employee extends Person {

     //...

     public double calculatePay() {
         // What goes here?
     } // calculatePay

 } // Employee

• We need the Employee class to contain a calculatePay method so we can use polymorphism to calculate the pay of any type of employee.

• We should not be able to call the calculatePay method on an Employee object since that method does not have a reasonable implementation.

What do we do?? Discuss with your groups.

Solution

There is really no way to calculate pay for an arbitrary employee. The most elegant solution to our problem is to use an abstract class as seen in the next section.

Part 4: Abstract Classes

Objectives

Help students understand how abstract classes can enable them to prevent object creation and setup abstract methods to enable even more polymorphism outside of the class without having to introduce a separate interface.

Abstract Employee UML

More Utility Methods

How many different kinds of arrays are needed to complete the tasks below? Write your answers on your exit ticket.

Also note: is there redundancy in the code? Don’t write the code out, just try to imagine.

drawAll

Listing 51 in Utility.java

public static void drawAll(________) {
    // ...
} // drawAll

printAllEmployeeIDs

Listing 52 in Utility.java

public static void printAllEmployeeIDs(________) {
    // ...
} // printAllEmployeeIDs

getAveragePay

Listing 53 in Utility.java

public static double getAveragePay(________) {
    // ...
} // getAveragePay

Solution

1. We can use Drawable[] as the type to draw all objects.

2. We can use Employee[] as the type to print all IDs.

3. We can use Employee[] as the type to get the average pay.

There is no redundancy in the solutions now!

Constructing Objects

Questions (1)

• What does the constructor look like for Person?

• What does the constructor look like for Employee?

• What does the constructor look like for SalariedEmployee?

• What does the constructor look like for HourlyEmployee?

• Why does Employee have a constructor? Is the following allowed?

Employee e1 = new Employee(~, ~, ~);

Code (1)

public Person(String name, Color hairColor) {
    this.name = name;
    this.hairColor = hairColor;
} // Person

public Employee(String name, int employeeID, Color hairColor) {
    this.employeeID = employeeID;
    //
    // What about the other parameters?
    //
} // Employee

Code (2)

public Person(String name, Color hairColor) {
    this.name = name;
    this.hairColor = hairColor;
} // Person

public Employee(String name, int employeeID, Color hairColor) {
    super(name, hairColor); // What is this doing?
    this.employeeID = employeeID;
} // Employee

Code (3)

public SalariedEmployee(String name, int employeeID, double annualSalary, Color hairColor) {
    //
    // How do you write this?
    //
} // SalariedEmployee

public HourlyEmployee(String name, int employeeID, double hourlyRate, int hoursWorked, Color hairColor) {
    //
    // How do you write this?
    //
} // HourlyEmployee

The Object Class

Objectives

Help students understand where methods like toString and equals come from. If time permits, discuss the final keyword to inhibit inheritance and/or overrides.

Test Yourself

Note

Explicit means the extends/implements relationship of the class can be seen in its source code.

Implicit means the extends/implements relationship of the class exists but cannot be seen in its source code.

Answer the following questions on your exit ticket with your groups:

1. Which class(es) does HourlyEmployee explicitly extend?

2. Which class(es) does HourlyEmployee implicitly extend?

3. Does HourlyEmployee implement Drawable? If so, is it implicit or explicit?

Scenarios

Child	Explicitly Extends	Implicitly Extends	Implements Drawable?
Airplane		Object	Explicitly
Employee	Person	Object	Implicitly
HourlyEmployee	Employee	Person, Object	Implicitly
SalariedEmployee	Employee	Person, Object	Implicitly
Person		Object	Explicitly
Tree		Object	Explicitly
Utility		Object	No

Printing a Reference

In the code below, we create a Circle object. For the purposes of this discussion, assume the Circle class exists and that its constructor requires a parameter of type double representing the radius of the circle.

public static void main(String[] args) {

    // Create a Circle and assign it a radius
    Circle c = new Circle(4.2);

    System.out.println(c);
} // main

If we ran the code above, we would see something similar to:

Circle@251a69d7

Why do we see this output? What does it mean? Discuss with your groups and write your thoughts on your exit ticket.

Understanding toString

Override toString to customize the string representation for objects of the class.

/**
 * Returns a string representation of the object.
 *
 * @return a string representation of the object.
 */
@Override
public String toString() {
    // ...
} // toString

Assuming obj is not null, the following are equivalent:

System.out.println(obj);

System.out.println(obj.toString());

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character '@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

this.getClass().getName() + '@' + Integer.toHexString(this.hashCode())

equals

Override equals to customize how objects of a class are compared to other objects for equality.

/**
 * Indicates whether some other object is "equal to" this one.
 *
 * @param obj the reference object with which to compare.
 * @return {@code true} if this object is the same as the {@code obj} argument;
 *     {@code false} otherwise.
 */
@Override
public boolean equals(Object obj) {
    ...
} // equals

Overriding toString

We want to override toString in the Employee class so that all subclasses inherit the method and print their calculated pay when printed. After we finish, the UML will look like the diagram above.

Listing 54 in Employee.java

public abstract class Employee extends Person {

     //...

     public abstract double calculatePay();

     @Override
     public String toString() {
         // Implement me!
         // Return a string representation of the employee's ID and pay.
     } // toString

 } // Employee

Using Overridden toString

Now, the boss says he wants to print the ID and payment information for all types of employees. We should be able to use our new toString method!

Implement the method below:

Listing 55 in Utility.java

public static void printInfo(________) {
    // ...
} // printInfo

Part 5: Adding a Manager Class

Objectives

Help students understand how to use super in overrides to “adjust” an implementation and not just replace it.

Let’s add a class called Manager that extends SalariedEmployee. Every time a manager’s pay is calculated, it’s some bonus dollar amount + 110% of what their pay would be if they were only a salaried employee.

New Members

Below, we zoom in on the relevant aspects of the UML diagram seen above.

public Manager(String name, int employeeID, double annualSalary, Color hairColor, double bonus) {
    super(name, employeeID, annualSalary, hairColor);
    this.bonus = bonus;
} // Manager

public double getBonus() {
    return this.bonus;
} // getBonus

Overrides

In your groups, write the code to implement calculatePay within the Manager class on your exit ticket:

Listing 56 in Manager.java

@Override
public double calculatePay() {
    //
    // How can we do this?
    //
} // calculatePay

Sample

Listing 57 in Manager.java

@Override
public double calculatePay() {
    return this.bonus + 1.10 * super.calculatePay();
} // calculatePay

Part 6: Compatibility, Available Methods, and Dispatch

Compatibility (Example 1)

Given a variable of each type, write all compatible expression types such that an assignment of the form type name = expression will compile.

1. Person:

2. Employee:

3. SalariedEmployee:

4. HourlyEmployee:

5. Manager:

6. Drawable:

Solutions

1. Person: Person, Employee, SalariedEmployee, HourlyEmployee, Manager

2. Employee: Employee, SalariedEmployee, HourlyEmployee, Manager

3. SalariedEmployee: SalariedEmployee, Manager

4. HourlyEmployee: HourlyEmployee

5. Manager: Manager

6. Drawable: Drawable, Flower, Tree, Airplane, Person, SalariedEmployee, HourlyEmployee

Dispatch

Dispatch (Code)

Which class contains the code that will be run for each method below?

public class Driver {

    public static void main(String[] args) {

        Drawable[] refs = new Drawable[2];
        refs[0] = new Person("Joe", Color.BROWN, Color.RED);
        refs[1] = new SalariedEmployee("Carolyn", 85_000.00, Color.BROWN, Color.GREEN);

        HourlyEmployee sue = new HourlyEmployee("Sue", 45.00, 24, Color.BROWN, Color.BROWN);
        Drawable test2 = new HourlyEmployee("Bill", 9.00, 2, Color.BROWN, Color.BROWN);

        Person test = sue;
        test2 = sue;

        Object oJoe = refs[0];

        String name = test.getName();        // 1
        refs[1].draw();                      // 2
        String s = oJoe.toString();          // 3
        test2.draw();                        // 4

    } // main
} // Driver

Available Methods

What methods can be called using the variable in each scenario below? You may safely assume that all expressions are valid Java code and that all assignments are compatible.

variable name = expression;

Q1

Variable Type	Expression Type
Manager	Manager

S1

Variable Type	Expression Type
Manager	Manager

• Methods declared in Manager:

  • getBenefits()

  • calculatePay()

• Methods inherited from SalariedEmployee:

  • calculatePay()

• Methods inherited from Employee:

  • getEmployeeID()

• Methods inherited from Person:

  • getName()

  • getHairColor()

  • draw()

  • toString()

• Methods inherited from Object:

  • equals(Object)

  • toString()

  • etc.

Q2

Variable Type	Expression Type
Manager	SalariedEmployee

S2

Variable Type	Expression Type
Manager	SalariedEmployee

• same as before!

Q3

Variable Type	Expression Type
Drawable	Person

S3

Variable Type	Expression Type
Drawable	Person

• Methods declared in Drawable:

  • draw()

• Methods inherited from Object:

  • equals(Ovject)

  • toString()

  • etc.

Dispatch

What method body is used in each call scenario below? You may safely assume that each reference variable is not null; that is, it refers to some object.

Q1

Variable Type	Variable Name	Object Type	Call
Drawable	d	Person	d.draw()

S1

Variable Type	Variable Name	Object Type	Call
Drawable	d	Person	d.draw()

• draw() from Person

Q2

Variable Type	Variable Name	Object Type	Call
Drawable	m	Manager	m.draw()

S2

Variable Type	Variable Name	Object Type	Call
Drawable	m	Manager	m.draw()

• draw() from Person

Q3

Variable Type	Variable Name	Object Type	Call
Employee	e	SalariedEmployee	e.calculatePay()

S3

Variable Type	Variable Name	Object Type	Call
Employee	e	SalariedEmployee	e.calculatePay()

• calculatePay() from SalariedEmployee

Q4

Variable Type	Variable Name	Object Type	Call
Employee	m	Manager	m.calculatePay()
Employee	m	Manager	m.getEmployeeID()

S4

Variable Type	Variable Name	Object Type	Call
Employee	m	Manager	m.calculatePay()
Employee	m	Manager	m.getEmployeeID()

• calculatePay() from SalariedEmployee

• getEmployeeID() from Employee

Q5

Variable Type	Variable Name	Object Type	Call
Drawable	h	HourlyEmployee	m.toString()
Drawable	t	Tree	t.toString()

S5

Variable Type	Variable Name	Object Type	Call
Drawable	h	HourlyEmployee	m.toString()
Drawable	t	Tree	t.toString()

• m.toString() uses toString() from Person

• t.toString() uses toString() from Object

Test Yourself

Which of the following are considered to be compatible assignment scenarios, assumming expression is valid Java code?

variable name = expression;

Q1

Variable Type	Expression Type
Manager[]	Manager[]
Manager	Manager
Manager	SalariedEmployee
SalariedEmployee	Manager
Object	Manager
Manager	Object

S1

Variable Type	Expression Type	Compatible?
Manager[]	Manager[]	Yes
Manager	Manager	Yes
Manager	SalariedEmployee	Yes
SalariedEmployee	Manager	No
Object	Manager	Yes
Manager	Object	No

Q2

Variable Type	Expression Type
Employee[]	Employee[]
Employee	Employee
Employee	Manager
Employee	Person
Employee	Drawable
Object	Employee
Employee	Object

S2

Variable Type	Expression Type	Compatible?
Employee[]	Employee[]	Yes
Employee	Employee	Yes
Employee	Manager	Yes
Employee	Person	No
Employee	Drawable	No
Object	Employee	Yes
Employee	Object	No

Q3

Variable Type	Expression Type
Drawable[]	Drawable[]
Drawable	Drawable
Drawable	Utility
Tree	Drawable
Object	Drawable
Drawable	Object

S3

Variable Type	Expression Type	Compatible?
Drawable[]	Drawable[]	Yes
Drawable	Drawable	Yes
Drawable	Utility	NO
Tree	Drawable	No
Object	Drawable	Yes
Drawable	Object	No

Note

Write out quick examples in Java, as needed, to provide clarification.

Part 7: Summary and Additional Practice

As we evolved our design, we realized the following benefits:

• Reduced redundancy in the classes used in the hierarchy (Person, SalariedEmployee, HourlyEmployee, and Manager). These classes no longer have repeated instance variables or instance methods.

• Reduced unecessary parameters in the methods of Utility.java. By the end, each of the methods associated with our tasks were able to be completed with a single parameter and contained no redundancy within the methods themselves. Here are the tasks we added and improved (although more could be added depending on need):

  Listing 58 in Utility.java

  public static void drawAll(________) {
      // ...
  } // drawAll
  
  public static void printAllEmployeeIDs(________) {
      // ...
  } // printAllEmployeeIDs
  
  // Determine Largest Employee ID
  public static int findLargestEmployeeID(________) {
      // ...
  } // findLargestEmployeeID
  
  public static double getAveragePay(________) {
      // ...
  } // getAveragePay
  
  public static int numWithHairColor(Color hairColor, _________) {
      // ...
  } // numWithHairColor
  

For extra practice, we highly recommend implementing each of these methods and focusing on the method body and the best replacement for the blank.

Inheritance Activity (Shapes Hierarchy)

Inheritance Activity (Shapes Hierarchy)

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Note

Some parts of this activity contain solutions. To show the solutions, you will need to toggle “Show Solutions” above.

Information

This activity explores how to utilize inheritance in Java to create new classes that are directly based on existing ones, with an emphasis on code reuse and design implications. It also revisits how to commit and view changes to a local Git repository.

Prerequisite Knowledge

• CSCI 1302 Inheritance Chapter

• CSCI 1302 UML Chapter

Course-Specific Learning Outcomes

• LO3.b: Create class, interface, method, and inline documentation that satisfies a set of requirements.

• LO3.c: Generate user-facing API documentation for a software solution.

• LO4.c: (Partial) Design, create and use inheritance relationships in a software solution.

• LO4.d: Utilize inheritance-based polymorphism in a software solution.

Getting Started

Download Source Code (Instructors)

Execute the command below to download and extract the files:

sh -c "$(curl -fsSL https://cs1302book.com/_bundle/cs1302-inheritance-shapes.sh)"

- downloading cs1302-inheritance-shapes bundle...
- verifying integrity of downloaded files using sha256sum...
- extracting downloaded archive...
- removing intermediate files...
subdirectory cs1302-inheritance-shapes successfully created

Download Source Code (Students)

Listing 59 Getting the Starter Code

shapes1302

- downloading cs1302-inheritance-shapes bundle...
- verifying integrity of downloaded files using sha256sum...
- extracting downloaded archive...
- removing intermediate files...
subdirectory cs1302-inheritance-shapes successfully created

Change into the cs1302-inheritance-shapes directory that was just created and look around. There should be multiple Java files contained within the directory structure. To see a listing of all of the files under the src subdirectory, use the tree command.

You should see output similar to the following:

Listing 60 The current directory structure

.
├── compile_and_run.sh
└── src
    └── cs1302
        └── shapes
            ├── Circle.java
            ├── Driver.java
            ├── Ellipse.java
            ├── Rectangle.java
            └── Shape.java

Practice Opportunity (UML)

If you would like extra practice drawing UML, take this opportunity to draw a complete, proper UML diagram for Shape and its child classes contained in the starter code.

Refer to the CSCI 1302 UML Chapter if needed. Specifically, each individual class diagram should contain:

• Class name;

• Variables;

• Constructors and methods;

• If needed, a solid generalization arrow (extends) to a parent class; and

• If needed, a dashed generalization arrow (implements) to an interface.

In a class diagram, do not list inherited members (methods or attributes) unless they are explicitly overridden. Be sure to include visibility modifiers (e.g., +, #, ~, -) and type/return type information where needed.

Solution

The UML solution is given later in the activity.

Version Control with Git

Make sure that you are directly inside the cs1302-inheritance-shapes directory, then initialize a new local Git repository:

git init

Initialized empty Git repository in ./.git/

Note

When you execute the git clone command to download the starter code for a project, you will not need to run the git init command. In that scenario, you are making a copy of an existing Git repository. The other commands in this section would still apply.

Next, add all the starter file to that repository

git add .

Verify that the files will be committed:

git status

On branch main

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)
    new file:   compile_and_run.sh
    new file:   src/cs1302/shapes/Circle.java
    new file:   src/cs1302/shapes/Driver.java
    new file:   src/cs1302/shapes/Ellipse.java
    new file:   src/cs1302/shapes/Rectangle.java
    new file:   src/cs1302/shapes/Shape.java

Commit those changes to the local repository with a nice log message:

git commit -m "initial commit"

[main (root-commit) ???????] initial commit
 6 files changed, 342 insertions(+)
 create mode 100755 compile_and_run.sh
 create mode 100644 src/cs1302/shapes/Circle.java
 create mode 100644 src/cs1302/shapes/Driver.java
 create mode 100644 src/cs1302/shapes/Ellipse.java
 create mode 100644 src/cs1302/shapes/Rectangle.java
 create mode 100644 src/cs1302/shapes/Shape.java

Check the log:

Listing 61 The adog alias is setup on Odin. It is short for git log with --all, --decorate, --oneline, and --graph.

git adog

* ??????? (HEAD -> main) initial commit

Starter Code UML

Adding a Square Class

Planning for a Square Class

In this section, we want to add a Square class to our hierarchy, but first let’s think about how it fits in. Answer the following questions with your group:

1. Should Square have a parent class? If so, what would it be?

2. What instance variables need to be declared in Square?

3. What methods will we need in Square?

Solution

1. Yes! It is best to place it as a child of Rectangle to minimize redundancy.

2. None! It will inherit length and width from Rectangle.

3. None! It will inherit the methods from Rectangle and Shape.

Coding a Square Class

1. Create and document a Square class in the cs1302.shapes package based on the UML diagram above.

   Solution

   /**
    * A {@code Square} is a rectangle where the sides
    * are the same length.
    */
   public class Square extends Rectangle {
   
       /**
        * Constructs a {@link Square} object with the specified length
        * and width.
        *
        * @param sideLength the length of the sides of the square.
        */
       public Square (double sideLength) {
           super(sideLength, sideLength);
           this.setName("Square");
           // What's name?
       } // square
   } // Square
   

2. Compile your Square class and fix any errors.

   Note

   Don’t forget to ensure your new file passes the check1302 audit!

3. Since we’ve added a new class to our project, it’s a good idea to save our work using Git. Check the status of your local copy of the repository using the following command:

   git status
   

   To add the new file to our repository so we can save the changes, use the git add command:

   git add src/cs1302/shapes/Square.java
   

   Now, use Git to commit the changes that you made to your source code:

   git commit -m "Added a Square class to the Shapes hierarchy"
   

Javadoc Practice Opportunity

Make sure all classes have proper Javadoc comments and run the check1302 utility to check your code style. Then, run the javadoc command and make sure your website is up-to-date with your new Square class.

Square Discussion

Using the diagram above: answer the following with your groups on your exit ticket:

1. How many instance variables are declared in Square.java?

2. How many instance variables are in an object of type Square?

3. How many methods are declared in Square.java?

4. How many methods can be called using a reference of type Square?

Solution

1. 0

2. 3 (2 from Rectangle and 1 from Shape.

3. 0

4. 6 plus the methods inherited from Object.

Overriding the toString method

Planning for a toString method

We want to override the toString method inherited from the object class so that it returns the name, area, and perimeter of a shape instead of the default behavior provide by Object.

To accomplish our goal, which class(es) will need to override the toString method and why? Discuss with your groups and write your answer on your exit ticket.

Solution

We can place the toString method in Shape because we can call all of the necessary methods from that context.

Implementing a toString method

1. Implement the toString method as described in the previous section in the appropriate place(s).

2. Compile your code and ensure that it still passes the check1302 audit.

3. Tell Git to track changes made to your Driver.java file, then commit.

Solution

@Override
public String toString() {
    // returns the shape's name, area, and perimeter.
    // The exact formatting isn't important.

    String returnVal = this.getName() + " ";
    returnVal += "has an area of: " + this.getArea();
    returnVal += " and a perimeter of: " + this.getPerimeter();

    return returnVal;
} // toString

Modifying the Driver Class

1. Open the Driver class in the cs1302.shapes package. Inside of the main method do the following:

2. Finish populating the provided Shape array with at least one object of each compatible type.

3. Uncomment and complete the printInfo method near the bottom of Driver.java. This will require filling in the blanks and writing the method body.

4. Call the printInfo method in the main method below the comment.

5. Compile and run the Driver class. You should see info printed about all of your shapes!

6. Uncomment and complete the getLargestByArea method near the bottom of Driver.java. This will require filling in the blanks and writing the method body.

7. Call the getLargestByArea method in the main method below the comment and print the result.

8. Compile and run the Driver class. You should see output related to the largest shape!

9. Tell Git to track changes made to your Driver.java file, then commit.

10. To view all of the commits you made in this activity assignment, run:

    git adog
    

Sample Solution

Here is one possible solution for the Driver class:

/**
 * The driver program for our shapes example. This class
 * contains the main method and can be executed.
 */
public class Driver {

    public static void main(String[] args) {

        // Add two shapes of each type
        // Circle, Ellipse, Rectangle, and Square
        Shape[] shapes = new Shape[] {
            new Ellipse(1.1, 2.5),
            new Circle(1.5),
            new Square(4.2),
            new Rectangle(1.2, 3.4)
        };

        // Call the printInfo method below:
        Driver.printInfo(shapes);
        // Call the getLargestByArea method and print the result below:
        Shape largest = Driver.getLargestByArea(shapes);
        System.out.println(largest); // calls our new toString method!
    } // main

    /**
     * Prints information about every shape in the specified array.
     * Loops over the array, and, for each element, prints the name of
     * the shape, the area, and the perimeter.
     *
     * @param shapes the array of shapes
     */
    public static void printInfo(Shape[] shapes) {
        for (Shape s: shapes) {
            System.out.println(s); // calls our new toString method!
        } // for
    } // printInfo

    /**
     * Returns the largest shape by area. Compares all of
     * the provided shapes and returns the one with the largest area.
     *
     * @param shapes the array of shapes
     * @return the largest shape
     * @throws NullPointerException if {@code shapes} has length 0
     */
    public static Shape getLargestByArea(Shape[] shapes) {
        // Assume the first shape is the largest
        Shape largest = shapes[0];

        for (Shape s: shapes) {
            if (s.getArea() > largest.getArea()) {
                largest = s;
            } // if
        } // for

        return largest;
    } // getLargestByArea

Compatibility: Test Yourself

1. Answer the following short answer questions in your notes:

   1. How many methods does Circle inherit from Ellipse? Why isn’t getPerimeter listed as being inherited from Ellipse?

   2. How many methods does Circle inherit from Shape? Why aren’t all methods in Circle listed as being inherited from Shape?

   3. How many instance variables are contained in an object of type Ellipse? How many are directly accessible?

   4. How many instance variables are contained in an object of type Circle? How many are directly accessible?

   5. The setName method in Shape is declared to be protected. Explain how the protected access modifier differs from public and private.

2. In your notes, recreate the following table:

   Variable Type	Object Type	Valid? (Guess)	Valid? (Actual)	Explain
   Shape	Shape
   Shape	Circle
   Shape	Ellipse
   Circle	Shape
   Circle	Ellipse
   Circle	Circle
   Ellipse	Shape
   Ellipse	Circle
   Ellipse	Ellipse

   Create a ShapeTest.java program in the cs1302.shapes package. In the main method of ShapeTest.java, do the following steps for each row in the table:

   1. Declare a reference variable called obj with the type denoted in the Variable Type column.

   2. On the same line, create an object of the type denoted in the Object Type column using new and an appropriate constructor call and assign it to obj. The first declaration and instantiation would look like this: Shape obj = new Shape();

   3. Make an educated guess as to whether or not the line will successfully compile. Note this in the Valid? (Guess) column of your table.

   4. Save the file, then attempt to compile ShapeTest.java. Note whether it compiled in the Valid? (Actual) column of your table.

   5. Now for the most important part! In the Explain column of your table, explain why the statement is valid or invalid. If the statement is invalid, do not simply copy the error message provided by the compiler–do your best to explain it in your own words.

   6. Delete the line of code before proceeding to the next row.

3. In your notes, recreate the following table:

   Variable Type	Object Type	Method Call	Valid? (Guess)	Valid? (Actual)	Explain
   Shape	Ellipse	getPerimeter()
   Ellipse	Circle	getName()
   Ellipse	Circle	getSemiMajorAxisLength()
   Shape	Ellipse	getSemiMinorAxisLength()
   Shape	Ellipse	setName()
   Ellipse	Ellipse	getDiameter()
   Shape	Circle	getRadius()
   Ellipse	Circle	getCircumference()
   Circle	Circle	getPerimeter()

   In the main method of ShapeTest.java, do the following steps for each row in the table:

   1. Declare a reference variable called obj with the type denoted in the Variable Type column.

   2. On the same line, create an object of the type denoted in the Object Type column using new and an appropriate constructor call and assign it to obj.

   3. On the next line, call the method listed in the Method Call column using obj as the calling object. If the method has a return type other than void, then simply assign it to a new variable of the appropriate type.

   4. Make an educated guess as to whether or not the two lines of code will successfully compile. Note this in the Valid? (Guess) column of your table.

   5. Save the file, then attempt to compile ShapeTest.java. Note whether it compiled in the Valid? (Actual) column of your table.

   6. Now for the most important part! In the Explain column of your table, explain why the statement is valid or invalid. If the statement is valid, give the fully qualified name (FQN) of the class containing the implementation for the method.

   7. Delete the two lines of code before proceeding to the next row.