(Java) Difference ways of instantiating subclasses

Question

I have these two classes:

public class A {}
public class B extends A {}

In the main class, I would declare:

A a = new B();
B b = new B();

What is the difference between a and b ? Is this what we called a subtyping in java?

Answer 1

The difference between a and b is that with a you can only use the public API that the A class provides even though its really a B type object, but with b, you can use both the A's public API as well as B's public API.

For example:

Suppose A and B are defined as follows:

// A.java
public class A
{
  public void x()
  {
    z("X");
  }

  public void y()
  {
    z("Y");
  }

  protected void z(String message)
  {
    System.out.println(message);
  }
}

// B.java
public class B extends A
{
  public void a()
  {
    z("A");
  }

  public void b()
  {
    z("B");
  }
}

And here's a demo:

// Demo.java
public class Demo
{
  public static void main(String[] args)
  {
    A a = new B();
    B b = new B();

    // Can only call A's public methods
    a.x();
    a.y();
    // a.a(); Can't use
    // a.b(); Can't use

    // Can call all public methods
    b.a();
    b.b();
    b.x();
    b.y();
  }
}