How would you refactor Arrays.copyOf() method in order to remove the @SuppressWarnings("unchecked") annotation?

Question

I was looking inside Arrays class in java and saw that @SuppressWarnings("unchecked") annotation is being used in the following code.

    public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    @SuppressWarnings("unchecked")
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

How would you refactor the following code in order to remove @SuppressWarnings("unchecked") annotation?

You can't, [because you can't create array of generic type in java](https://stackoverflow.com/questions/2927391/whats-the-reason-i-cant-create-generic-array-types-in-java). So you would have to typecast. — Adwait Kumar, Feb 08 '20 at 16:53
@AdwaitKumar But we do know the runtime type of `T[]` here. The problem is that there isn't an `Array.newInstance` with an appropriate signature. — Tom Hawtin - tackline, Feb 08 '20 at 17:08
@TomHawtin-tackline The problem is not the *signature* of `Array.newInstance(Class>, int)`, but its *return type*. It’s declared to return `Object` to cater for the case where the element type is a primitive type so the array type is not a subtype of `Object[]`. — Ole V.V., Feb 08 '20 at 19:08
Yeah, signature is the wrong term. I mean to include the return type, the type of `componentType` and that it is the `length` version not `dimensions` (which has *another* reason not to return `T[]`). Perhaps *signatures* is better. — Tom Hawtin - tackline, Feb 09 '20 at 01:19

score 1 · Accepted Answer · answered Feb 09 '20 at 06:45

1

Here.

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
T[] copy = newType.cast(
    Array.newInstance(newType.getComponentType(), newLength)
);

But it is not as fast, which is important for this method.

answered Feb 09 '20 at 06:45

Tom Hawtin - tackline

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why it is not so fast? – IonKat Feb 09 '20 at 10:40
1

@IonKat There's a call to `Class.cast` and no special case for `Object[]`. – Tom Hawtin - tackline Feb 09 '20 at 14:46
Sorry, but with my current knowledge level I do not understand what you mean. – IonKat Feb 09 '20 at 16:22
1

but then the source of `Class.cast()` itself has a `@SuppressWarnings("unchecked")`, so in some sense this just moves the annotation from one place to another – newacct Feb 17 '20 at 04:00
@newacct That would be an implementation detail. The important thing is that it's a sound library method. – Tom Hawtin - tackline Feb 17 '20 at 07:01

How would you refactor Arrays.copyOf() method in order to remove the @SuppressWarnings("unchecked") annotation?

1 Answers1