Java - Reference Variables

Question

" It is important to understand that it is the type of reference variable - not the type of object that it refers to - that determines what members can be accessed. "

What do you exactly mean by that statement ? Is this restricted to concept of Inheritance ? How does JVM handles it ?

Answer 1

It means that suppose you have:

Object x = "hello";

The type of the variable is Object, but the type of the object it refers to is String. it's the variable type which determines what you can do though - so you can't call

// Invalid
String y = x.toUpperCase();

The compiler only knows that you're calling a method on Object, which doesn't include toUpperCase. Similarly, overloaded methods are only resolved against the ones you know about:

public class Superclass
{
    public void foo(Object x) {}
}

public class Subclass extends Superclass
{
    public void foo(String y) {}
}
...
Subclass x = new Subclass();
Superclass y = x;

x.foo("hello"); // Calls Subclass.foo(String)
y.foo("hello"); // Calls Superclass.foo(Object)