related to abstract class reference holding object of its derived class

Question

class A is abstract and class B extends class A now class A reference can hold object of class B,that is

A aObj = new B();

and assume class B has some extra methods.... like

class A
{
public show();
}
class B extends A
{
public show(){}
public method1(){}
private method2(){}
}

now tell me what things variable aObj can access from class B can it access everything?

Answer 1

aObj can only use show() as the compiler thinks aObj is of type A, and the only known method of A is show().

If you know that you actually have a B you can cast that object to a B:

if (aObj instanceof B.class) {
  B bObj = (B) aObj;
  bObj.method1(); //OK
} else {
  log.debug("This is an A, but not a B");
}
aObj.show();

Answer 2

aObj only sees the public show() method. If you cast aObj to B, you can then access public method1(). public method2() is only accessible to the implementation of B.

Answer 3

For reference and completeness, here's a list of the possibilities:

A aObj = new B();
aObj.show(); // Works
aObj.method1(); // Error
aObj.method2(); // Error

And with casting to B:

B bObj = (B)aObj; bObj
bObj.show(); // Works
bObj.method1(); // Works
bObj.method2(); // Works inside bObj, but error otherwise