When would I use java Collections singletonMap method?

Question

I don't understand why you would need java Collections singletonMap? Is it useful in multithreaded applications?

Answer 1

Basically, it allows you to do this:

callAPIThatTakesAMap(Collections.singletonMap(key, value)); 

rather than this:

Map<KeyType, ValueType> m = new HashMap<KeyType, ValueType>(); m.put(key, value); callAPIThatTakesAMap(m); 

which is much nicer when you only have a single key/value pair. This situation probably does not arise very often, but singleton() and singletonList() can quite frequently be useful.

Answer 2

It is useful if you need to pass a map to some general code (as a parameter, or as a result from a method) and you know that in this particular case -- but perhaps not in other cases that pass map to the same general code -- the map you want to pass has only a single key. In that case, the SingletonMap is more efficient than a full-blown map implementation, and also more convenient for the programmer because everything you need to say can be said in the constructor.