Upcasting and Downcasting

Question

I have studied that upcasting happens automatically in JAVA but am not sill cleared with the concept of downcasting. Whether is it possible to do downcasting without any instanceof operator or is necessary to downcastcast each and every object of superclass to typecast in subclass?

Mucharla Venkata Uday Kumar · Answer

Upcasting is casting to a supertype, while downcasting is casting to a subtype. Upcasting is always allowed, but downcasting involves a type check and can throw a ClassCastException. In your case, a cast from from Dog to an Animal is an upcast, because a Dog is-a Animal. In general, you can upcast whenever there is an is-a relationship between two classes. Downcasting would be something like this: Animal animal = new Dog(); Dog castedDog = (Dog) animal; Basically what you're doing is telling the compiler that you know what the runtime type of the object really is. The compiler will allow the conversion, but will still insert a runtime sanity check to make sure that the conversion makes sense. In this case, the cast is possible because at runtime animalis actually a Dog even though the static type of animal is Animal. However, if you were to do this: Animal animal = new Animal(); Dog notADog = (Dog) animal; You'd get a ClassCastException. The reason why is because animal's runtime type is Animal, and so when you tell the runtime to perform the cast it sees that animal isn't really a Dog and so throws a ClassCastException. To call a superclass's method you can do super.method() or by performing the upcast. To call a subclass's method you have to do a downcast. As shown above, you normally risk a ClassCastException by doing this; however, you can use the instanceof operator to check the runtime type of the object before performing the cast, which allows you to prevent ClassCastExceptions: Animal animal = getAnimal(); // Maybe a Dog? Maybe a Cat? Maybe an Animal? if (animal instanceof Dog) { // Guaranteed to succeed, barring classloader shenanigans Dog castedDog = (Dog) animal; }

Upcasting and Downcasting

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