Using a byte array as Map key

We can use ByteBuffer for this (This is basically the byte[] wrapper with a comparator)

HashMap<ByteBuffer, byte[]> kvs = new HashMap<ByteBuffer, byte[]>();
byte[] k1 = new byte[]{1,2 ,3};
byte[] k2 = new byte[]{1,2 ,3};
byte[] val = new byte[]{12,23,43,4};

kvs.put(ByteBuffer.wrap(k1), val);
System.out.println(kvs.containsKey(ByteBuffer.wrap(k2)));

will print

true

It's okay so long as you only want reference equality for your key - arrays don't implement "value equality" in the way that you'd probably want. For example:

byte[] array1 = new byte[1];
byte[] array2 = new byte[1];

System.out.println(array1.equals(array2));
System.out.println(array1.hashCode());
System.out.println(array2.hashCode());

prints something like:

false
1671711
11394033

(The actual numbers are irrelevant; the fact that they're different is important.)

Assuming you actually want equality, I suggest you create your own wrapper which contains a byte[] and implements equality and hash code generation appropriately:

public final class ByteArrayWrapper
{
    private final byte[] data;

    public ByteArrayWrapper(byte[] data)
    {
        if (data == null)
        {
            throw new NullPointerException();
        }
        this.data = data;
    }

    @Override
    public boolean equals(Object other)
    {
        if (!(other instanceof ByteArrayWrapper))
        {
            return false;
        }
        return Arrays.equals(data, ((ByteArrayWrapper)other).data);
    }

    @Override
    public int hashCode()
    {
        return Arrays.hashCode(data);
    }
}

Note that if you change the values within the byte array after using the ByteArrayWrapper, as a key in a HashMap (etc) you'll have problems looking up the key again... you could take a copy of the data in the ByteArrayWrapper constructor if you want, but obviously that will be a waste of performance if you know you won't be changing the contents of the byte array.

EDIT: As mentioned in the comments, you could also use ByteBuffer for this (in particular, its ByteBuffer#wrap(byte[]) method). I don't know whether it's really the right thing, given all the extra abilities that ByteBuffers have which you don't need, but it's an option.

The problem is that byte[] uses object identity for equals and hashCode, so that

byte[] b1 = {1, 2, 3}
byte[] b2 = {1, 2, 3}

will not match in a HashMap. I see three options:

1. Wrapping in a String, but then you have to be careful about encoding issues (you need to make certain that the byte -> String -> byte gives you the same bytes).
2. Use List<Byte> (can be expensive in memory).
3. Do your own wrapping class, writing hashCode and equals to use the contents of the byte array.