Count frequency (Specialized) in Java 8 using Lambdas

Question

I have a list of strings like this: "na","na","na","uk","uk". I want to count the frequency of each element in such a way that if the existing map's value is even, then I will add 1; otherwise 2.

 List<String> streamer = Arrays.asList("na", "na", "na", "uk", "uk");
 Map<String, Integer> m = new HashMap<>();

 for(String s:streamer) {
     if (m.containsKey(s)) {
         if(m.get(s) % 2 == 0)
             m.put(s, m.get(s) + 1);
         else
             m.put(s,m.get(s)+2);
     }
     else
         m.put(s,1);
  }
  System.out.println("CUSTOM Frequency::::" + m); 

Now, I want to achieve the exact same thing using streams and lambdas.

All I could do is this:

Map<String, Long>map4 = streamer.stream()
    .collect(Collectors.groupingBy(
        Function.identity(),
        Collectors.counting())); // How can I get custom counting instead of Collectors.counting() ?

Answer 1

Besides @Sweeper's answer to get the expected counting result, if your question also implied how to customize a collect operation, you could use the method Collector.of().

The method accepts a supplier, a BiConsumer as the accumulator, and a BinaryOperator as the combiner. The supplier simply provides the container where to store the result, in your case a Map<String, Long>. The accumulator populates the container by putting or updating an entry with the custom frequency logic. Lastly, The combiner simply merges the sub-results of parallel executions in case the stream is executed as a parallel stream.

List<String> streamer = Arrays.asList("na", "na", "na", "uk", "uk");
Map<String, Long> map4 = streamer.stream()
        .collect(Collector.of(
                HashMap::new,
                (Map<String, Long> map, String s) -> {
                    if (!map.containsKey(s)) {
                        map.put(s, 1L);
                    } else {
                        map.computeIfPresent(s, (String key, Long val) -> val % 2 != 0 ? val + 2 : val + 1);
                    }
                },
                (Map<String, Long> map1, Map<String, Long> map2) -> {
                    for (Map.Entry<String, Long> entry : map2.entrySet()) {
                        map1.computeIfPresent(entry.getKey(), (key, val) -> val + entry.getValue() + 1);
                        map1.computeIfAbsent(entry.getKey(), key -> entry.getValue());
                    }
                    return map1;
                }
        ));

Output

{na=5, uk=3}