changing reference types

[Neha Mittal]

What does this mean -> "Although a reference variable's type can't be changed, it can be used to refer to an object whose type is a subtype of its own" ?

[Campbell Ritchie]

You can have a reference to a Vehicle. You can use that reference to point to a Car object or a Bus object.

[Aum Tao]

This is referring to inheritance. Consider the following example:

 class Animal {
// some code
}
 

public class Dog extends Animal {
// Some code
 
public static void main(String[] args) {
Animal dog = new Dog(); // Works fine, since Dog IS An Animal
Animal animal = new Shape();  // ERROR
}   
 
class Shape {
 
}

Taking your case, the Animal reference can be assigned to the Dog object, since it satisfies the IS A relationship. However, this fails for the second assignment since Shape doesn't extend Animal. Moreover, it shouldn't since they are not logically related.