Its astounding how often I find maps being used like this.

for(String name: names.keySet()) {
    mymap.put(mymap.get(name),name);
}

Its inefficient because you have to fetch the keys and perform a lookup in the map with the key.

Instead using the EntrySet you can get all the keys and values in one hit. This saves having to perform the map lookup.

for (Entry<String, String> entry : entrySet) {
    mymap.put(entry.getValue(),entry.getKey());
}