Java Collections 

Following example shows how to find how many times each element is repeated (i.e. frequency of each element) in a collection.

Example

Frequency of a particular element

To find frequency of a particular element we can use java.util.Collections#frequency() method:

package com.logicbig.example;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionUtil {
  public static void main(String[] args) {
      List<Integer> list = Arrays.asList(1, 3, 4, 5, 3, 2, 2, 4, 2);
      int frequency = Collections.frequency(list, 2);
      System.out.println(frequency);
  }
}

3

Frequencies of all elements

Following example shows how to find frequency of each element in a collection:

package com.logicbig.example;

import java.util.*;

public class CollectionUtil2 {
  public static void main(String[] args) {
      List<Integer> list = Arrays.asList(1, 3, 4, 5, 3, 2, 2, 4, 2);
      Map<Integer, Integer> frequencyMap = new HashMap<>();
      list.stream()
          .distinct()
          .forEach(element -> frequencyMap.put(element, Collections.frequency(list, element)));
      System.out.println(frequencyMap);
  }
}

{1=1, 2=3, 3=2, 4=2, 5=1}