1.3. Reference Type Examples¶

For the examples in this section, you should assume access to a class called Person (which defines the Person reference type) that has the following declaration:

public class Person {

    // instance variables

    private String name;
    private int age;

    // constructor

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    } // Person

    // assume getter and setter methods exist...

} // Person

In Java, the reference types ([4.3]) are class types, interface types, and array types. Of these three, you are likely familiar with declaring variables of both class and array types. Interfaces will be covered later in this course.

Below are two example declarations of reference type variables:

Person ada;      // A class type variable (valid because we have a class called Person)
double[] array;  // An array type variable

An object is really just a collection of variables and methods that are defined by a class. It is not uncommon to describe Java objects as dynamically constructed instances of a class. This means that an object is constructed using the class as a blueprint when the keyword new is encountered (while the program is running).

When an object is constructed, it is stored on the heap along with its instance variables, and the expression that constructed it evaluates to a reference: a value that refers to the new object. In Java, the possible values of a reference type are references to compatible objects or null.

Note

A reference variable either stores:

null - when the variable does not refer to an object.
A reference that refers to an object.

Below we describe various scenarios in which you may encounter/use reference variables in your code.

Test Yourself

Consider the following initializations:

Person sue = new Person("Sue", 19);
Person sally = sue;

Before moving on, take a moment to draw a diagram on a sheet of paper that shows how this looks under the hood (sometimes called a memory map or a memory diagram). Use the diagram above as an example. Draw an arrow from each variable to the object it refers to.

Do the best you can before looking at the solution.

Test Yourself

Consider the following code:

Person joe = new Person("Joe", 37);
Person chris = joe;

chris.setName("Chris"); // assume the setName method exists and changes the name of the calling object.
System.out.println(joe.getName());

Take a moment to draw the memory map diagram for the code above. If a line of code changes an instance variable, make sure your diagram reflects that change. Then, write the output from the program in your notes.

Do the best you can before looking at the solution.