How does HashMap behave when adding an element with a null value, if the hashCode method always returns the same number?

If you add an element to a Java HashMap with a key whose hashCode() method always returns the same number, then all such keys will fall into the same bucket of the internal HashMap structure.

When adding an element with a null value (meaning the value can be null, but the key is not necessarily), HashMap handles this normally, as values can be null. A key with the same hashCode will be placed in the same bucket, and in case of collisions, HashMap compares keys using the equals() method.

If all keys have the same hashCode(), the performance of insert and search operations will degrade to linear, as all elements will end up in one list or tree within a single bucket.

Example:

class Key {
    private String id;

    public Key(String id) {
        this.id = id;
    }

    @Override
    public int hashCode() {
        return 42; // always the same number
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Key other = (Key) obj;
        return id.equals(other.id);
    }
}

HashMap<Key, String> map = new HashMap<>();
map.put(new Key("a"), null); // null value is allowed
map.put(new Key("b"), "value");

// All keys with hashCode=42 will fall into one bucket, which will degrade performance.

Thus, HashMap works correctly with such keys, but efficiency decreases due to collisions.