java hashmap : enexpected behaviour for containsKey

November 19, 2024

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?

>Solution :

put takes an Integer as the key, so the 0 gets boxed to an Integer.

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