HashMap Error.

Question

Hi,
If a key of a HashMap is changed then how can I refresh the hashmap so that the value are rehashed on the newly changed key. The following program has output:
[5]
null
nullI wish to have output as
[5]
20
nullAny clues?
regards,
import java.util.HashMap;
public class Main {
    private static HashMap<KeyClass, Integer> map = new HashMap<KeyClass, Integer>();
    private static class KeyClass {
        private int key = -1;
        public KeyClass(int key) {
            this.key = key;
        public void setKey(int key) {
            this.key = key;
        @Override
        public boolean equals(Object obj) {
            if (obj == null && !(obj instanceof KeyClass)) {
                return false;
            KeyClass input = (KeyClass) obj;
            if (input.key == key) {
                return true;
            return false;
        @Override
        public int hashCode() {
            int hash = 7;
            hash = 53 * hash + this.key;
            return hash;
        public String toString() {
            return key + "";
    public static void main(String args[]) {
        KeyClass key = new KeyClass(3);
        map.put(key, 20);
        key.setKey(5);
        System.out.println(map.keySet());
        System.out.println(map.get(key));  // returns null (How to fix it?)
        System.out.println(map.get(new KeyClass(3))); //returns null too.
}

Answer 1

Per the [Map interface API|http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/api/java/util/Map.html]:
Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. ...I think the best answer is: don't do this.
As an aside: shouldn't this:
      @Override
      public boolean equals(Object obj) {
         if (obj == null && !(obj instanceof KeyClass)) {
            return false;
         KeyClass input = (KeyClass) obj;
         if (input.key == key) {
            return true;
         return false;
      }be this:      @Override
      public boolean equals(Object obj) {
         if (obj == null || !(obj instanceof KeyClass)) {
            return false;
         KeyClass input = (KeyClass) obj;
         if (input.key == key) {
            return true;
         return false;
      }