Cast from generic array to my class?

Question

I have a generic class, SimpleStack<T>, which contains these methods:

private T[] stack;

public T[] getStack() {
    return this.stack;
}

public boolean add(T item) {
    boolean filled = isFull();
    if (!filled) {
        stack[size()] = item;
        return true;
    }
    else {
        return false;
    }
}

public int size() {
    int count = 0;
    for (T item : stack) {
        if (item != null) {
            count++;
        }
    }
    return count;
}

I'm trying to use this class to act as a Profile class, and I've filled the array stack with Profile objects. However, when I try to return this array, it says that it can't cast from Object to Profile.

Here's a snippet from where I fill the array, inside my main method:

SimpleStack<Profile> profiles = new SimpleStack<Profile>(50);
Profile profile = new Profile();
profiles.add(profile);

I try to call the getter to return the array, and cast it to a Profile[]. This is what fails:

Profile[] tempStack = Arrays.copyOf(profiles.getStack(), (profiles.getStack()).length, Profile[].class);

I've tried it several ways, but the ultimately all do the same thing (attempt to cast the generic array to Profile[]).

Does anyone know what I've done wrong? I can provide more code if necessary. I tried to take the relevant bits.

@Codebender I create stack like this: `stack = (T[]) new Object[size];` — Cory, Sep 24 '15 at 02:37
Please include the stack trace or compiler error you're getting. In general, arrays and generics don't mix; you'd be much better off replacing this with a `List`. — dimo414, Sep 24 '15 at 02:40
Make `getStack` return `Object[]`. (Also you may or may not want to stop pretending it's a `T[]`, since that *only* works as a result of a "bug" in the generics system) — user253751, Sep 24 '15 at 02:41
Don't return the array. Don't expose implementation details. — Sotirios Delimanolis, Sep 24 '15 at 02:59

Amir Raminfar · Answer 1 · 2015-09-24T02:55:33.213

It all depends how you initialize it. See How to create a generic array in Java? for more details.

public class SimpleStack<T> {
    private final T[] stack;

    public SimpleStack(Class<T[]> clazz, int size) {
        this.stack = clazz.cast(Array.newInstance(clazz.getComponentType(), size));
    }

    public T[] getStack() {
        return stack;
    }

    public static void main(String... args) {
        SimpleStack<Profile> profiles = new SimpleStack<>(Profile[].class, 50);
        Profile profile = new Profile();
        Profile[] array = profiles.getStack();
    }

    public static class Profile {

    }
}

Note that I prefer to use clazz.cast() instead of a dirty cast, because it is safer and you don't have to use @SuppressWarnings

score 0 · Answer 2 · answered Sep 24 '15 at 02:50

This, stack = (T[]) new Object[size]; isn't allowed. Because of type-erasure, the type T is Object at run-time. However, you can pass in the class type to your generic class, that will allow you to use Array.newInstance(Class, int).

static <E> E[] makeAnArray(Class<E> cls, int length) {
    return (E[]) Array.newInstance(cls, length);
}

Cast from generic array to my class?

2 Answers2