Update map value using java Streams

Question

I need help in making this java code of adding value to map efficient by making use of Java streams to perform this put to map action

  class Pair{
    String key;
    int value;
 }


void addtoMap(List<Pair> list){
    Map<String, Integer> hashMap= new HashMap();
    list.forEach(pair -> {
        if(hashMap.containsKey(pair.key))
        {
            int p= hashMap.get(pair.key);
            hashMap.put(pair.key,p+pair.value)
        }
        else 
            hashMap.put(pair.key,pair.value);
    });
}
 

hashMap.put("a",1); [["a",1]]
hashMap.put("b",2);  [["a",1],["b",2]]
hashMap.put("a',3);  [["a",4],["b",2]]
 

Answer 1

You can use Collectors.toMap:

Map<String, Integer> hashMap =
    list.stream()
        .collect(Collectors.toMap(p -> p.key,
                                  p -> p.value,
                                  (v1,v2)->v1+v2));