What exactly is a reference variable in Java? How does it differ from other variables?

Question

I have been studying Inheritance in Java and the writer makes the statement "it is the type of object being referred to (not the type of reference variable) that determines which version of an overridden method will be executed." This statement is awfully confusing.

Magenmagena

Answer 1

What the book is referring to is polymorphism, more specifically through dynamic dispatch.

In a nutshell, imagine the following classes:

public class Person {

    public Person() {
    }

    public void introduceYourself() {
    } 

}

public class Texan extends Person {

    public Texan() {
    }

    public void introduceYourself() {
        System.out.printLn("Howdy y'all!");
    } 

}

public class NewYorker extends Person {

    public NewYorker() {
    }

    public void introduceYourself() {
        System.out.printLn("Yo. You got a problem with that?");
    } 

}

Now, let's create a reference variable of type Person.

Person myBFF;

Let's instantiate him

myBFF = new NewYorker();

Let's ask him to introduce himself

myBFF.introduceYourself();

This prints:

Yo. You got a problem with that?

Now, let's change your BFF to a Texan.

myBFF = new Texan();

Let's call that same line again and ask our BFF to introduce himself.

myBFF.introduceYourself();

This prints:

Howdy y'all!

In each case, the reference variable you were using was of type Person. The instance of the variable, in each case, was NewYorker and Texan respectively. That instance type determines which verson of introduceYourself() is called.

Jagir

You have a real knack for explaining! Really grateful but I do have one question; is the reference variable always of the type superclass no matter how many generations of sub classes? Eg. If I extended Texan to SouthernTexan, would that reference variable still be of type Person? – Magenmagena

Yes, but you can declare a reference variable to be of a type closer to the instance type. For example, you could do Texan bob = new SouthernTexan(); or SouthernTexan sue = new SouthernTexan();, but you could not do NewYorker clyde = new SouthernTexan();, nor could you do SouthernTexan henry = new Person();. – Jagir

Answer 2

A reference variable is the type you specify on the left hand side (a variable that holds a reference type). What the author is referring to is when the right hand side then differs. Consider

Object a = new Foo(); 
System.out.println(a.toString());

if Foo overrides the Object.toString() (that is, if Foo provides a public String toString()) method then it is Foo's toString that is invoked (not Object's). See also Overriding and Hiding Methods in the Java tutorials.

Detoxicate

Answer 3

A reference variable looks like this :

Coordinate cords; //Cords is the ref. var

Inside of that reference variable is an address inside of your computer's RAM which stores the attributes of that object. Since we did not instantiate (actually make an object of) the address of the aforementioned cords object is null

Reference variables hold addresses to reserved parts of memory.

cords = new Coordinate(0.0,0.0,0.0);

Now inside of the RAM of the computer is a reserved space that holds three floating type variables. Upon instantiation, a reference variable holds the address. So what can we do with addresses in Java?

Nothing of use. Java memory address are useless, and you can not see them(Though they look something like 0xFFFFFFF)

For a visual representation click here

Diaphony

Answer 4

Reference variable is a term to indicate/explain that a variable is only referring to a type and yet not instantiated. Also after instantiation it's type will be something that would depend on how or by which class a reference variable will be instantiated.

Say from the above example by Robert Columbia saying

   Person myBFF;

Or by the example of Brenann

   Coordinate cords; //Cords is the ref. var

In both these cases myBFF and cords are only referring to Person or Coordinate classes respectively. Note that they are not instantiated yet!

Everyplace