HashMap<String, boolean> copy all the keys into HashMap<String, Integer>and initialize values to zero

问题

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.

回答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)));

回答2:

Looping is pretty easy (and not inelegant). Iterate over the keys of the original Map and put it in them in the new copy with a value of zero.

Set<String> keys = original.keySet();
Map<String, Integer> copy = new HashMap<String, Integer>();
for(String key : keys) {
    copy.put(key, 0);
}

Hope that helps.

回答3:

final Integer ZERO = 0;

for(String s : input.keySet()){
   output.put(s, ZERO);
}

来源：https://stackoverflow.com/questions/4156306/hashmapstring-boolean-copy-all-the-keys-into-hashmapstring-integerand-init