A ClassCastException ocurrs when you try to cast an instance of an Object to a type that it is not. Casting only works when the casted object follows an "is a" relationship to the type you are trying to cast to. For Example
Apple myApple = new Apple();
Fruit myFruit = (Fruit)myApple;
This works because an apple 'is a' fruit. However if we reverse this.
Fruit myFruit = new Fruit();
Apple myApple = (Apple)myFruit;
This will throw a ClasCastException because a Fruit is not (always) an Apple.
It is good practice to guard any explicit casts with an instanceof check first:
if (myApple instanceof Fruit) {
Fruit myFruit = (Fruit)myApple;
}