java - Changing an object which is used as a Map key

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

Quote from "Map" interface JavaDoc:

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.

You simply shouldn't mutate keys (in the way which changes their "hashCode"/"equals"). You will definitely have very long and awful debugging if you try.

It's like you swap books in the library. Index became unreliable. You search for "Bradbury", but find "Simak".