Reference variable casting doubt

[saloni jhanwar]

class Animal
{
	void makeNoise()
	{
		System.out.println("Generic noise");
	}
}
class Dog extends Animal
{
	void makeNoise()
	{
		System.out.println("bark");
	}
	void playDeath()
	{
		System.out.println("roll over");
	}
}
class CastTest2 
{
	public static void main(String[] args) 
	{
		Animal a1[]={new Animal(),new Dog(),new Animal()};
		for(Animal a2:a1)
		{
			a2.makeNoise();
			if(a2 instanceof Dog)
			{
				Dog d=(Dog)a2;
				d.playDeath();
			}
		}
	}
}

If above program can compile and run successfully then what's problem in given below thanks

class Animal
{
}
class Dog extends Animal
{
}
class DogTest 
{
	public static void main(String[] args) 
	{
		Animal a=new Animal();
		Dog d=(Dog)a;

	}
}

[Matthew Brown]

In your second case, you're casting an Animal reference to a Dog reference. Now, that might work, so it will compile OK. But at run-time the JVM will look at the actual type of the object, which is Animal, and will try and cast that to a Dog. That's not possible, because an Animal is-not-a Dog, so you get a ClassCastException.

In the first example you're preceding the cast with an instanceof check. Which means that for those cases where you would get a ClassCastException (the first and last elements in the array), it doesn't try and cast. It only tries it for the case where the cast is safe - when the object actually is a Dog.

You should always check instanceof before casting an object like that unless you know you can guarantee (from your understanding of the rest of the code) that it is safe.

[saloni jhanwar]

Matthew Brown wrote:In your second case, you're casting an Animal reference to a Dog reference. Now, that might work, so it will compile OK. But at run-time the JVM will look at the actual type of the object, which is Animal, and will try and cast that to a Dog. That's not possible, because an Animal is-not-a Dog, so you get a ClassCastException.

In the first example you're preceding the cast with an instanceof check. Which means that for those cases where you would get a ClassCastException (the first and last elements in the array), it doesn't try and cast. It only tries it for the case where the cast is safe - when the object actually is a Dog.

You should always check instanceof before casting an object like that unless you know you can guarantee (from your understanding of the rest of the code) that it is safe.

Thanks Matthew:)