ClassCastException

i have two classes in java as:

class A {

 int a=10;

 public void sayhello() {
 System.out.println("class A");
 }
}

class B extends A {

 int a=20;

 public void sa开发者_Python百科yhello() {
 System.out.println("class B");
 }

}

public class HelloWorld {
    public static void main(String[] args) throws IOException {

 B b = (B) new A();
     System.out.println(b.a);
    }
}

at compile time it does not give error, but at runtime it displays an error : Exception in thread "main" java.lang.ClassCastException: A cannot be cast to B

This happens because the compile-time expression type of new A() is A - which could be a reference to an instance of B, so the cast is allowed.

At execution time, however, the reference is just to an instance of A - so it fails the cast. An instance of just A isn't an instance of B. The cast only works if the reference really does refer to an instance of B or a subclass.

B extends A and therefore B can be cast as A. However the reverse is not true. An instance of A cannot be cast as B.

If you are coming from the Javascript world you may be expecting this to work, but Java does not have "duck typing".

First do it like this :

  A aClass = new B(); 

Now do your Explicit casting, it will work:

   B b = (B) aClass;

That mean's Explicit casting must need implicit casting. elsewise Explicit casting is not allowed.

Once you create the object of a child class you cannot typecast it into a superClass. Just look into the below examples

Assumptions: Dog is the child class which inherits from Animal(SuperClass)

Normal Typecast:

Dog dog = new Dog();
Animal animal = (Animal) dog;  //works

Wrong Typecast:

Animal animal = new Animal();
Dog dog = (Dog) animal;  //Doesn't work throws class cast exception

The below Typecast really works:

Dog dog = new Dog();
Animal animal = (Animal) dog;
dog = (Dog) animal;   //This works

A compiler checks the syntax it's during the run time contents are actually verified