Down casting Object array to array of type parameter

Question

How is the statement in the GenericArrayCreationIsDisallowedInJava constructor working? Could some1 please explain ?

public class GenericArrayCreationIsDisallowedInJava<I> {


    private I[] i;


    public GenericArrayCreationIsDisallowedInJava(int n)
    {
        i = (I[]) new Object[n];//works and prints out "Created". How?
        System.out.println("Created");
    }

    public static void main(String[] args) {

        new GenericArrayCreationIsDisallowedInJava<String>(2);

        String[] strs = (String[])  new Object[2]; // throws ClassCastException. When this statement doesn't work, how is the "(I[]) new Object[n]" statement working? 

    }

}

Answer 1

i = (I[]) new Object[n];

It is not entirely clear what you think the above does, but one thing it most definitely doesn't do is create an array of I . The only part responsible for choosing the type of the array comes after the new keyword: It's an array of Object s, unsafely cast to I[] .

The compiler must allow the line to compile because it doesn't know anything about the type I : it could as well be Object , so the line is legal.

Also note that the erased type of I[] is Object[] : the actual type (in the bytecode) of the i variable is Object[] . Therefore the assignment succeeds at runtime no matter what type the instance is parameterized with.

A ClassCastException will only be thrown if the array is assigned back to a reifiable type. For example:

GenericArrayCreationIsDisallowedInJava o =
        new GenericArrayCreationIsDisallowedInJava<String>(2);

String[] strs = o.i; // ClassCastException thrown here

During the type erasure process, the compiler inserts the cast to String[] in the line assigning strs . This demonstrates the need to keep i from "leaking" outside the class that is using it.

Answer 2

To understand what is going on, simply consider the same code after type erasure (just remove type parameters and insert casts if necessary):

public class GenericArrayCreationIsDisallowedInJava {


    private Object[] i;


    public GenericArrayCreationIsDisallowedInJava(int n)
    {
        i = (Object[]) new Object[n];//works and prints out "Created". How?
        System.out.println("Created");
    }

    public static void main(String[] args) {

        new GenericArrayCreationIsDisallowedInJava(2);

        String[] strs = (String[])  new Object[2]; // throws ClassCastException. When this statement doesn't work, how is the "(I[]) new Object[n]" statement working? 

    }

}