Java swap two keys in a Map

Question

Welcome. I have a generic void method swap that takes a Map and 2 keys. The method will swap the values associated with the keys in the Map. I already checked that both keys are contained in the Map somewhere else, but in this method, I can't use looping. My method answer:

public static<K, W> swap(Map<K,V m, K key1, K key2>){
  m.put(key2, m.put(?)) // I don't really understand what I would have to
                        // put in this part, so how would i have to remember
                        // the 1st key, would I just set the value to
                        // a new initialized key?
}

Answer 1

This method will swap the values for two keys:

public void swap(Object key1, Object key2, Map map)
{
    Object temp = map.get(key1);
    map.put(key1, map.get(key2));
    map.put(key2, temp);
}

Yes, you do need to save the value for 1 of the keys, otherwise it will be lost when replacing key1's value with key2's.

Answer 2

This will swap the values of two keys:

Map<K, V> aMap;

V temp = aMap.get(key1);
aMap.put(key1, aMap.put(key2, temp);

Answer 3

Use a temporary holder for value:

public static<K, V> swap(Map<K,V> m, K key1, K key2){
  V value = m.get(key1);
  m.put(key1, m.get(key2));
  m.put(key2, value);
}

Answer 4

m.put(key2, m.put(key1, m.get(key2))

...will work. put(..) returns the previous value for that key that was already in the map, which will then become associated with key2. You can do it without using a local variable; however this code is a little hard to understand, a variable might make the intent of your code clearer.