Object i = Integer.valueOf(42);
String s = (String)i;            // ClassCastException thrown here.

class A {...}
class B extends A {...}
class C extends A {...}

Animal a = new Dog();
Dog d = (Dog) a; // no problem, the type animal can be casted to a dog, because its a dog
Cat c = (Dog) a; // raises class cast exception, you cant cast a dog to a cat

public void manipulate(Object o) {
     Dog d = (Dog) o;
 }

Animal a = new Animal();
 manipulate(a);

Dog d;
 if(o instanceof Dog) {
     d = (Dog) o;
 } else {
     //what you need to do if not
 }