Java Object Casting.

Java Object Casting

• A cast, instructs the compiler to change the existing type of an object reference to another type.
• In Java, all casting will be checked both during compilation and during execution to ensure that they are legitimate.
• An attempt to cast an object to an incompatible object at runtime will results in a ClassCastException.
• A cast can be used to narrow or downcast the type of a reference to make it more specific.
  Casting example:

  class Animal {
  }
  
  class Cow extends Animal {
  }
  
  public class ObjectCasting {
    public static void main(String args[]) {
      Animal creature;
      Cow daisy = new Cow();
      creature = daisy;        // OK
      creature = new Animal();
      //daisy = creature;     // Compile-time error, incompatible type
      daisy = (Cow) creature;   // OK at Compile-time, but it throws exceptions at run time.
    }
  }

  You can check this with instanceof before you perform the cast:

  ...
      if ( creature instanceof Cow ) {
          Cow cow = (Cow)creature;
      }
  ...

  Casting example with the instanceof test :

  class Animal {
    @Override
    public String toString() {
      return "I am an Animal"; 
    }
  }
  
  class Cow extends Animal {
      @Override
    public String toString() {
      return "I am a Cow"; 
    }
  }
  
  public class ObjectCasting {
    public static void main(String args[]) {
      Animal creature;
      Cow daisy = new Cow();
      System.out.println( daisy); // prints: I am a Cow
      creature = daisy;        // OK
      System.out.println(creature); // prints: I am a Cow
      creature = new Animal();
      System.out.println(creature); // prints: I am a Animal
  //    daisy = creature;     // Compile-time error, incompatible type
      if (creature instanceof Cow) {
        daisy = (Cow) creature;   // OK but not an instance of Cow
        System.out.println( daisy);
      }
    }
  }

  The result of this is:

  I am a Cow
  I am a Cow
  I am an Animal

  You can download this example here (needed tools can be found in the right menu on this page).