Returning an Array from a generic class

Question

I get a ClassCastException error when I run this because of the implicit cast of d to a double when I run the code. However, if I change reference to d in to Object[] then it can't be a parameter to the set function. If I change the set function to accept an Object[] then everything works fine, but then the class will fail at runtime if anyone calls set on an object that isn't of type N.

I need a solution that can get() the old array (or a clone thereof) and that can set() data to a new array.

public class Foo<N> {

    public static void main(String[] args) {
        Foo<Double> foo = new Foo<Double>();
        Double[] d = foo.get();

        // do stuff to d ...

        foo.set(d);
    }

    N[] data;

    public Foo() {
        data = (N[]) new Object[2];
    }

    public N[] get() {
        return (N[]) data;
    }

    public void set(N[] data) {
        this.data = data;
    }

}

Answer 1

To create an array with the right runtime type requires some kind of runtime representation of that type. A generic class like Foo<N> has no runtime representation of N.

There are two solutions:

1. Use a List<N> instead. This is best if it is possible!
2. Manually add a runtime representation of N by passing in a Class<N> to Foo, use java.lang.reflect.Array.newInstance to create the array.

Code for the second solution:

public Foo(Class<N> newDataClass) {
    data = (N[]) Array.newInstance(newDataClass, 2);
}

Foo<Double> foo = new Foo<>(Double.class);

---

EDIT:

If what you want to do is instead to get a copy of an existing Double array you can do that with (N[]) data.clone(). Setting it will not be a problem.

Answer 2

There's a trick to this.

class Foo<N> {

    // Deliberately no parameters.
    N[] myArray = makeArray();

    private N[] makeArray(N... ns) {
        return ns;
    }

    public N[] get() {
        return myArray;
    }
}

This may look like there is no casting (which is a good thing) but actually there is, it is just being done by the varargs system.