Counting size of List by element

Question

I'd like to count up List's size by element. For example,

List<String> source = Arrays.asList("USA", "USA", "Japan", "China", "China", "USA", "USA");

I'd like to create object (Map) from this source, such as,

int usa_count = result.get("USA").intValue(); // == 4
int javan_count = result.get("Japan").intValue(); // == 1
int china_count = result.get("China").intValue(); // == 2
int uk_count = result.get("UK").intValue(); // == 0 or NPE (both OK)

Now, I composed below.

Map<String, Integer> result = new HashMap<>();
for (String str : source) {
    Integer i = result.getOrDefault(str, Integer.valueOf(0));
    result.put(str, i + 1);
}

Though, this is enough to my purpose, I think this is not elegant code, and I wanna be a elegant coder. I use Java8. Is there any elegant solution instead of my solution?

I think this question fits well in http://codereview.stackexchange.com/ — NewUser, Dec 02 '16 at 08:19
https://www.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/ — Anton Kolyaev, Dec 02 '16 at 08:23

Chandana Kumara · Answer 1 · 2016-12-02T08:55:07.077

3

You can use frequency method in collection.

int usa_count = Collections.frequency(source, "USA");
int javan_count = Collections.frequency(source, "Japan");
int china_count = Collections.frequency(source, "CHINA"); 
int uk_count = Collections.frequency(source, "UK");

Refer:https://www.tutorialspoint.com/java/util/collections_frequency.htm

in Java 8:-

groupingBy collector allows to group together elements that have the same classification into a map.

Map<String, Long> countContries = source.stream().collect(Collectors.groupingBy(Function.identity() , Collectors.counting()));

Refer: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html

https://www.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/

edited Dec 02 '16 at 08:55

answered Dec 02 '16 at 08:21

Chandana Kumara

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3

This will not construct a map though... – Maurice Perry Dec 02 '16 at 08:24
1

Btw, you can add the javadoc of that function to make the answer more useful – Raskayu Dec 02 '16 at 08:24

score 3 · Answer 2 · answered Dec 02 '16 at 08:27

You can use streams groupingBy as shown below:

List<String> source = Arrays.asList("USA", "USA", "Japan", 
        "China", "China", "USA", "USA");

Map<String, Long> result =
                source.stream().collect(
                        //Use groupingBy
                        Collectors.groupingBy(
                                Function.identity(), Collectors.counting()
                        )
                );


System.out.println(result);

Output: {USA=4, China=2, Japan=1}

score 1 · Answer 3 · answered Dec 02 '16 at 08:29

You can use the Stream-api to do this.

source.stream()
      .collect(Collectors.groupingBy(
                      Function.identity(), 
                      Collectors.counting());

The groupingBy-collector generates a map where the keys are given by the first parameter. The second parameter can be omitted to give a list of each group that has the same key. In your case, that would give this:

Map("USA" -> List("USA", "USA", "USA", "USA"),
    "Japan" -> List("Japan"),
    "Chine" -> List("China", China"))

By giving a collector as a parameter, it will collect each group (in this case, counting the number of elements).

score 0 · Answer 4 · answered Dec 02 '16 at 08:30

IMHO your code is OK, but it could be simplified slightly:

Map<String, Integer> result = new HashMap<>();
for (String str : source) {
    int i = result.getOrDefault(str, 0);
    result.put(str, i + 1);
}

or even:

Map<String, Integer> result = new HashMap<>();
for (String str : source) {
    result.put(str, result.getOrDefault(str, 0) + 1);
}

Counting size of List by element

4 Answers4