No ClassCastException is thrown inside Java generics

Question

Below is the first Java generics I've ever written :

public class MyClass {

    public static <T> T castToAnotherType(Object param) {
        T ret = null;
        try {
            ret = (T) param;
        } catch (ClassCastException e) {
            System.out.print("Exception inside castToAnotherType()");
        }
        return ret;
    }

    public static void main(String[] args) {
        try {
            String obj = MyClass.castToAnotherType(new Object());
        } catch (ClassCastException e) {
            System.out.print("Exception outside castToAnotherType()");
        }
    }

}

The result is "Exception outside castToAnotherType()". Why did the exception not occur inside the generic method?

Answer 1

T is effectively erased during compilation. See here:

Generics were introduced to the Java language to provide tighter type checks at compile time and to support generic programming. To implement generics, the Java compiler applies type erasure to:

• Replace all type parameters in generic types with their bounds or Object if the type parameters are unbounded. The produced bytecode, therefore, contains only ordinary classes, interfaces, and methods.
• Insert type casts if necessary to preserve type safety. Generate bridge methods to preserve polymorphism in extended generic types.
• Type erasure ensures that no new classes are created for parameterized types; consequently, generics incur no runtime overhead.

So your castToAnotherType gets T erased into ca. the following:

public static Object castToAnotherType(Object param) {
    Object ret = null;
    try {
        ret = (Object) param;
    } catch (ClassCastException e) {
        System.out.print("Exception inside castToAnotherType()");
    }
    return ret;
}

Which does obviously not produce any ClassCastException.

main(...) is a different story, it results into the following:

public static void main(String[] args) {
    try {
        String obj = (String) MyClass.castToAnotherType(new Object());
    } catch (ClassCastException e) {
        System.out.print("Exception outside castToAnotherType()");
    }
}

Which produces the ClassCastException when trying to cast Object to String.

Please see the Type Erasure part of the Generics tutorial.

Answer 2

Well, since the compiler erases the generic type parameters, the casting inside the method is essentially equivalent to :

    Object ret = null;
    try {
        ret = (Object) param;
    } 
    ...

which is not a problem, regardless of what you pass to your method (since any Object can be cast to Object).

However, when you try to assign that Object to a String, in your main method, the ClassCastException occurs, since Object cannot be cast to a String.