CODEWITHSUNDEEP
javacollectionsdictionarysetintersection
dave
dave

Reputation: 1637

Filter the elements of a map based on a subset of its keys without iterating through the entire thing

I have a Map<String, ArrayList> and a Set<String>. Is there a way to "intersect" the keys of the map with the set of strings such that only the pairs with the given key remain, without iterating over the entire map? My main concern is performance and re-inventing the wheel on something that can be done more elegantly.

Upvotes: 63

Views: 33352

Answers (2)

BalusC
BalusC

Reputation: 1108742

Just do:

map.keySet().retainAll(set);

As per the javadoc, the changes in the key set are reflected back in the map.

... The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. ...

Here's a demo:

var map = new HashMap<String, String>();
map.put("1", "one");
map.put("2", "two");
map.put("3", "three");

var set = new HashSet<String>();
set.add("1");
set.add("3");

map.keySet().retainAll(set);

System.out.println(map); // {1=one, 3=three}

Upvotes: 143

Siddhartha
Siddhartha

Reputation: 4454

Elaborating on BalusC's excellent answer, values() supports retainAll() as well:

Map<String, String> map = new HashMap<String, String>();
map.put("1", "one");
map.put("2", "two");
map.put("3", "three");

Set<String> set = new HashSet<String>();
set.add("one");
set.add("two");

map.values().retainAll(set);

System.out.println(map);   // prints {1=one, 2=two}

retailAll retains duplicate values as well, as you would expect:

Map<String, String> map = new HashMap<String, String>();
map.put("1", "one");
map.put("2", "two");
map.put("3", "three");
map.put("4", "two");

Set<String> set = new HashSet<String>();
set.add("one");
set.add("two");

map.values().retainAll(set);

System.out.println(map);  // prints {1=one, 2=two, 4=two}

Upvotes: 14

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