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();