Question

What is the best way ?

Just looping through and putting the key and zero, or is there another more elegant or existing library method. I am also using Google’s guava java library if that has any useful functionality ?

Wanted to check if there was anything similar to the copy method for lists, or Map’s putAll method, but just for keys.

Answer 1

Don’t think there’s much need for anything fancy here:

Map<String, Boolean> map = ...;
Map<String, Integer> newMap = Maps.newHashMapWithExpectedSize(map.size());
for (String key : map.keySet()) {
  newMap.put(key, 0);
}

If you do want something fancy with Guava, there is this option:

Map<String, Integer> newMap = Maps.newHashMap(
    Maps.transformValues(map, Functions.constant(0)));

// 1-liner with static imports!
Map<String, Integer> newMap = newHashMap(transformValues(map, constant(0)));