Java count occurrence of each item in an array

Is there any method for counting the occurrence of each item on an array?

Lets say I have:

String[] array = {"name1","name2","name3","name4", "name5"};

Here the output will be:

name1 1
name2 1
name3 1
name4 1
name5 1

and if I have:

String[] array = {"name1","name1","name2","name2", "name2"};

The output would be:

name1 2
name2 3

The output here is just to demonstrate the expected result.

List asList = Arrays.asList(array);
Set<String> mySet = new HashSet<String>(asList);

for(String s: mySet){
 System.out.println(s + " " + Collections.frequency(asList,s));
}

With java-8, you can do it like this:

String[] array = {"name1","name2","name3","name4", "name5", "name2"};
Arrays.stream(array)
      .collect(Collectors.groupingBy(s -> s))
      .forEach((k, v) -> System.out.println(k+" "+v.size()));

Output:

name5 1
name4 1
name3 1
name2 2
name1 1

What it does is:

• Create a Stream<String> from the original array
• Group each element by identity, resulting in a Map<String, List<String>>
• For each key value pair, print the key and the size of the list

If you want to get a Map that contains the number of occurences for each word, it can be done doing:

Map<String, Long> map = Arrays.stream(array)
    .collect(Collectors.groupingBy(s -> s, Collectors.counting()));

For more informations:

• Stream
• Collectors

Hope it helps! :)

You could use a MultiSet from Google Collections/Guava or a Bag from Apache Commons.

If you have a collection instead of an array, you can use addAll() to add the entire contents to the above data structure, and then apply the count() method to each value. A SortedMultiSet or SortedBag would give you the items in a defined order.

Google Collections actually has very convenient ways of going from arrays to a SortedMultiset.