How to use Java 7+ 'Objects.hash()' with arrays?

Question

I really like Java 7+ style of writing hashCode() method:

@Override
public int hashCode() {
    Objects.hash(field1, field2);
}

It doesn't work correctly with arrays though. The following code:

@Override
public int hashCode() {
    Objects.hash(field1, field2, array1, array2);
}

will not work, as for array1 and array2 regular hashCode() instead of Arrays.hashCode() would be invoked.

How can I use Objects.hash() with arrays in a proper way?

Answer 1

You could try:

Objects.hash(field1, field2, Arrays.hashCode(array1), Arrays.hashCode(array2));

This is the same as creating one array that contains field1, field2, the contents of array1 and the contents of array2. Then computing Arrays.hashCode on this array.

Answer 2

Arrays.deepHashCode will discover arrays in the array (via instanceof Object[]) and recursively call itself. See the code.

If the array contains other arrays as elements, the hash code is based on their contents and so on, ad infinitum.

@Override
public int hashCode() {
    Object[] fields = {field1, field2, array1, array2};
    return Arrays.deepHashCode(fields);
}

It would be useful to have a params version like Objects.hash, but you'd have to write your own:

public final class HashUtils {
    private HashUtils(){}

    public static int deepHashCode(Object... fields) {
        return Arrays.deepHashCode(fields);
    }
}

Usage:

@Override
public int hashCode() {
    return HashUtils.deepHashCode(field1, field2, array1, array2);
}