what is the best way to get a sub HashMap based on a list of Keys?

Question

I have a HashMap and I would like to get a new HashMap that contains only the elements from the first HashMap where K belongs to a specific List.

I could look through all the keys and fillup a new HashMap but I was wondering if there is a more efficient way to do it?

thanks

Answer 1

With Java8 streams, there is a functional (elegant) solution. If keys is the list of keys to keep and map is the source Map.

keys.stream()     .filter(map::containsKey)     .collect(Collectors.toMap(Function.identity(), map::get)); 

Complete example:

    List<Integer> keys = new ArrayList<>();     keys.add(2);     keys.add(3);     keys.add(42); // this key is not in the map      Map<Integer, String> map = new HashMap<>();     map.put(1, "foo");     map.put(2, "bar");     map.put(3, "fizz");     map.put(4, "buz");      Map<Integer, String> res = keys.stream()         .filter(map::containsKey)         .collect(Collectors.toMap(Function.identity(), map::get));      System.out.println(res.toString()); 

Prints: {2=bar, 3=fizz}

EDIT add a filter for keys that are absent from the map