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

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.

Solution 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)));

Solution 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.

Solution 3

final Integer ZERO = 0;

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

Comments

[Isac]

Please, rephrase your question, I couldn't understand it.

[Vishy]

I would just loop through the keys, it would only take three lines of code. If you want it shorter you could write a method to do it.

[Kevin Bourrillion]

Note that if you use a Multiset instead of a Map you don't have to initialize all the keys to zero. The first time you add any number of occurrences of a new key it will initialize it for you.

[Isac]

Why create a constant named ZERO? If it doesn't add meaning to the value, why not use 0?

[pgras]

@Isac: Not sure but wont Integer.valueOf(0) be evaluated for every loop ?

[C. K. Young]

Using new Integer(0) is very wasteful of the heap. Consider using 0 and letting the autoboxing work, or if you detest autoboxing, use Integer.valueOf(0).

[ColinD]

@Isac: Pretty sure that'd be to avoid boxing every iteration, which may be slightly better in terms of performance. I don't feel like it's worth the reduced clarity though.

[C. K. Young]

Yes, but so? It gets inlined by the JIT compiler into the cached 0 value every time.

[Todd]

I was just trying to give a simple answer. And autoboxing is the same as new Integer(0), is it not? Yes, Integer.valueOf(0) would use less memory.

[C. K. Young]

Nope, it does Integer.valueOf(0) (which uses cached boxed values for numbers between -128 and 127).