Explain polymorphism in Java?

Given the statement

A m = new B开发者_开发问答();

and knowing that class B is a subclass of class A and A is a subclass of Object, explain what happens when the statement is evaluated.

Many thanks in advance!

The simplest answer is that since B is a subclass of A, this evaluation can occur normally.

A pointer to a base class can point to a derived class just fine, since the derived class is an instance of the base class.

Let's expand the example a bit more:

 A m;
 B n = new B();
 m = n; // Valid statement

m = n is a valid statement because we can only assign an A object to m. The compiler sees that B derives from A, so it is in actuality an A object. If it were an unrelated type, the compiler would yell at you.

• A variable called m is created, of type A
• An object of type B is constructed
• The variable m is assigned a reference to the created object

You can refer to m as if it is of type A without a cast. This means that you can call any methods that are defined in A; if they are overridden, B's version of the method is called instead. You cannot call any methods that are specific to B, however, unless you use a cast.

RTTI is the word. Run-Time Type Identifications.
I recommend you "Thinking in Java" by Bruce Eckel