http://www.guardian.co.uk/commentisfree/2010/feb/27/charlie-brooker-forgotten-your-password

So, there you go. Saving the planet, one BitTorrent at a time.

The client could use the contents of your method if he wanted to swap the references but you can’t do that in a method. That’s because the method was passed a copy of the reference the client has. All you can do is change the state of object that reference points to, you can’t change where his reference points.

Set the next puzzle on something nice and OOey.

     @Test
    public void testMismatchedArraySizes() {
        byte[] uno = new byte[] { 1, 1 };
        byte[] due = new byte[] { 2 };

        swapArraysPadAndTrunc(uno, due);

        Assert.assertArrayEquals(new byte[] { 2, 0 }, uno);
        Assert.assertArrayEquals(new byte[] { 1 }, due);
    }

    void swapArraysPadAndTrunc(byte[] a1, byte[] a2) {
        byte[] left = Arrays.copyOf(a1, a2.length);
        byte[] right = Arrays.copyOf(a2, a1.length);
        System.arraycopy(right, 0, a1, 0, right.length);
        System.arraycopy(left, 0, a2, 0, left.length);
    }

    void swapArrays(byte[] a1, byte[] a2) {
        byte[] temp = java.util.Arrays.copyOf(a1, a1.length);
        System.arraycopy(a2, 0, a1, 0, a2.length);
        System.arraycopy(temp, 0, a2, 0, temp.length);
    }

Although I find the following slightly more readable (but uses more memory I suppose):

    void swapArrays2(byte[] a1, byte[] a2) {
        byte[] left = Arrays.copyOf(a1, a1.length);
        byte[] right = Arrays.copyOf(a2, a2.length);
        System.arraycopy(right, 0, a1, 0, right.length);
        System.arraycopy(left, 0, a2, 0, left.length);
    }

arraycopy will throw ArrayIndexOutOfBounds if there is a length mismatch.

I have no idea why arraycopy is in System. Horrible method anyway. I say horrible because it mutates the given objects rather than returning something (like the methods in java.util.Arrays). But hey, that was the spec of the desired method for this puzzle.

As you already pointed out, arrays and primitives suck. The real problem is that someone wanted this method. I’d personally implement it using collections and force them to convert their arrays if they want to use it. (they could use Arrays.asList(T… a))

(As there’s no preview function for commenters, I can only assume this will render as unreadable garbage. Well tough, I’m not sorry about that, fix it.)