how using Mutable key in hashamp working in this example?

Question

  HashMap hm = new HashMap();
  StringBuilder sb = new StringBuilder("test");
  hm.put(sb, "second");
  // above code gets hacode for sb and places it corresponding bucket

  sb.append("123");// with appending of "123", sb hascode will change

  System.out.println("element is is" + hm.get(sb));// print "second"

Now hm.get(sb) should search the key in the bucket as per new hashcode. so it should not get any object under that bucket and should get null . So why its getting "second"?

Answer 1

The StringBuilder class does not override hashCode, so it inherits the hashCode method from Object. Therefore, the StringBuilder's contents do not affect the hash code, and even when you change its contents, the HashMap is still able to find the StringBuilder.

Answer 2

By default it looks like StringBuilder doesn't use its contents as part of its hashCode(), it just inherits the default hashCode() from Object which uses the memory address as its seed. So the memory address hasn't changed, thus the key still works.

If you wanted the behavior you expected, override the hashCode method to return

@Override
public int hashCode() {
    return this.toString().hashCode();
}

and don't forget to override equals() as well.