Downcasting objects in Java

Question

class A{

}

public class Demo 
{
  public static void   main(String s[])
  {         
      Object o=(Object) new Demo();
      if (((A)(o)) instanceof Object) 
      {
           System.out.println("true");
      }
  }
}

I am getting Exception while running the class Demo.java:

java.lang.ClassCastException: Demo cannot be cast to A

How to downcast o reference to class A?

Answer 1

Let's start from the beginning: This is terrible code. That being said:

• You are casting Demo to Object (for whatever reason, since in Java everything is Object, no need to cast).
• You are then casting o, that you know it's of type Demo, to A (why would this work?).
• You are checking if Object o is of type Object (why would this fail?)

Some notes:

• o should not be viewed as a reference, it is an instance of Object, as you declared it. Forget how things worked in C.
• Consider interfaces and if you want A to be an interface that Demo implements.
• You can only cast instances to a class that they already extend.

Downcast example:

    public class A {
    int variable = 0; 
}

public class Demo extends A{

}

public void testDowncast(){
    Demo myClass = new Demo();
    myClass.variable = 2;
    A morphingTime = myClass;
    System.out.println("And now Power Ranger Demo has turned into Mighty A:");
    System.out.println("I am: "+morphingTime.getClass() + " and my variable is: " + morphingTime.variable);
}