ConcurrentHashMap doesn't work as expected when use byte array as key in Xamarin

Question

Here is code block I use in Xamarin Android.

Java.Util.Concurrent.ConcurrentHashMap map = new Java.Util.Concurrent.ConcurrentHashMap();
var key = new byte[] { 1, 2, 3 };
var data = new byte[] { 1, 2, 3, 4, 5 };

map.Put(key, data);
var val = map.Get(key); // null, WHY?

IEnumeration keys = map.Keys();
while (keys.HasMoreElements)
{
    var k = keys.NextElement();
    var val2 = map.Get(k); // NOT null, val2 is byte array of {1,2,3,4,5}
}

I expected val is byte array (data), but val is null, while val2 is not null.

But this Java code works very well.

java.util.concurrent.ConcurrentHashMap map = new java.util.concurrent.ConcurrentHashMap();

byte[] key = {1, 2, 3};
byte[] data = {1, 2, 3, 4, 5};

map.put(key, data);
Object o = map.get(key); // byte array of {1,2,3,4,5}

What is the reason?

Answer 1

Using a byte array as a key to a Java map is a really bad idea for a few reasons: arrays are mutable, and arrays don't have reference equality. Both of these things break the general contract of Map. I suggest that you use a different key for the map. See this question and its answers for more information, including some possible ways to convert a byte array into a map key that will work.

Answer 2

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, But List does. For example:

   byte[] key = new byte[] { 1, 2, 3 };
            byte[] data = new byte[] { 1, 2, 3 };
            System.out.println(key.equals(data));
            System.out.println(key.hashCode());
            System.out.println(data.hashCode());

This will give a output :

false
1829164700
2018699554

Example 2 ::

int[] key = new int[] { 1, 2, 3 };
        int[] data = new int[] { 1, 2, 3 };
        List<Integer> list1 = Arrays.asList(1, 2);
        List<Integer> list2 = Arrays.asList(1, 2);

        System.out.println("Comparison between LIST ::" + list1.equals(list2));
        System.out.println("Comparison between arrays ::" + key.equals(data));
        System.out.println(key.hashCode());
        System.out.println(data.hashCode());

The output is ::

Comparison between LIST ::true
Comparison between arrays ::false
1829164700
2018699554

The problem is that byte[] uses object identity for equals and hashCode , so that will not match in a HashMap.

You can use the given options :

1. Use List<Byte> (can be expensive in memory).
2. Do your own wrapping class, writing hashCode and equals to use the contents of the byte array.