Question

Let V be a class with a single attribute named K and its getter and setters.

What’s supposed to happen if I do:

V v = new V();
v.setK("a");
HashMap<K,V> map = new HashMap<K,V>();
map.put(v.getk(),v);
v.setK("b");

As far as I know, this should cause some kind of problem because a map key is supposed to be invariable. What would happen here?

Edit: Consider the key not to be a String but a mutable object as stated in the coment below.

Answer 1

The title of this question is misleading — you are not changing the map key, as in mutating the object used as map key. When you say map.put(x, y), you are creating a map entry that aggregates two independent values: a key and a value. Where the key originates from is not seen by the map, it’s just two objects. So, you have created a map entry ("a", v) and after that you just changed the state of v — there is no way this could have influenced the map entry’s key “a”. If, on the other hand, you had an object K of your own making, like

public class K { 
  private String s;
  public K(String s) { this.s = s; }
  public void setS(String s) { this.s = s; }
  public boolean equals(Object o) { return ((K)o).s.equals(this.s); }
  public int hashCode() { return s.hashCode(); }

}

and now you do

final K k = new K("a");
map.put(k, v);
k.setS("b");
map.get(k);

then you would face the problem — you mutated the object used as the map key.