Count occurrences by stream

Question

LinkedList<Double> list = new LinkedList<Double>();
list.add(9.5);
list.add(4.9);
list.add(3.2);
list.add(4.9);

I want to count the duplicate element in the list through a stream and put them into a HashMap which represent the occurrence of each number in the list: e.g: (9.5=1, 4.9=2, 3.2=1) Does anybody know how this works?

Answer 1

Using Collections.frequency

Make a list of all the distinct values, and for each of them, count their occurrences using the Collections.frequency method. Then collect into a Map

Map<Double, Integer> result = list.stream()
        .distinct()
        .collect(Collectors.toMap(
            Function.identity(),
            v -> Collections.frequency(list, v))
        );

Using Collectors.groupingBy

I think it is not as nice as the example above.

Map<Double, Integer> result2 = list.stream()
    .collect(Collectors.groupingBy(Function.identity())) // this makes {3.2=[3.2], 9.5=[9.5], 4.9=[4.9, 4.9]}
    .entrySet().stream()
    .collect(Collectors.toMap(
        Map.Entry::getKey,
        e -> e.getValue().size())
    );

Plain for loop

A plain for loop is quite short, you might not need streams and lambdas

Map<Double, Integer> map = new HashMap<>();
for(Double d : list)
  map.put(d, map.containsKey(d) ? map.get(d)+1 : 1);

Using forEach

Even shorter with forEach

Map<Double, Integer> map = new HashMap<>();
list.forEach(d -> map.put(d, map.containsKey(d) ? map.get(d)+1 : 1));