java hashmap : enexpected behaviour for containsKey

Here is a code snippet :

public static void main(String[] args) {
    final byte key = 0;
    Map<Integer, Integer> test = new HashMap<>();
    test.put(0, 10);
    System.out.println(test.containsKey(key));
}

and I get false printed on console. Only if I cast key to int I get true, like this

System.out.println(test.containsKey((int)key));

Can someone explain what is going on here?

Boxing

put takes an Integer as the key. So the primitive numeric literal 0 in put(0, 10) gets boxed to an Integer object.

But containsKey takes an Object (see also). So the byte gets boxed to a Byte object. This Byte object does not equals the Integer object that was put into the map since they are completely different classes. Therefore containsKey returns false.

See Auto-boxing, at Wikipedia. And Autoboxing tech note at Oracle.com.