Count frequency of elements in a List in Java

This post will discuss how to count the frequency of the elements in a list in Java.

1. Using a Set

We know that set stores only distinct entries. The idea is to get distinct elements in the list by inserting all elements in the set & then call static method frequency(Collection<?> c, Object o) provided by the Collections class for each distinct element. frequency() returns the total number of occurrences of the specified element in the list.

import java.util.*;

// Program to count the frequency of the elements in a list

class Main

{

public static void main(String[] args)

{

List<String> list = Arrays.asList("B", "A", "A", "C", "B", "A");

Set<String> distinct = new HashSet<>(list);

for (String s: distinct) {

System.out.println(s + ": " + Collections.frequency(list, s));

}

Download Run Code

Output:

A: 3
B: 2
C: 1

2. Using a Map

Instead of storing the distinct elements in the set and then calling Collections.frequency() for each distinct element, we can construct a map that stores the frequencies of the elements present in a list.

import java.util.Arrays;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

// Program to count the frequency of the elements in a list

class Main

{

public static void main(String[] args)

{

List<String> list = Arrays.asList("B", "A", "A", "C", "B", "A");

Map<String, Integer> frequencyMap = new HashMap<>();

for (String s: list)

{

Integer count = frequencyMap.get(s);

if (count == null) {

count = 0;

}

frequencyMap.put(s, count + 1);

}

for (Map.Entry<String, Integer> entry: frequencyMap.entrySet()) {

System.out.println(entry.getKey() + ": " + entry.getValue());

}

Download Run Code

Output:

A: 3
B: 2
C: 1

We can even simplify things by using streams in Java 8 and above:

import java.util.Arrays;

import java.util.List;

import java.util.Map;

import java.util.function.Function;

import java.util.stream.Collectors;

// Program to count the frequency of the elements in a list

class Main

{

public static void main(String[] args)

{

List<String> list = Arrays.asList("B", "A", "A", "C", "B", "A");

Map<String, Long> frequencyMap = list.stream()

.collect(Collectors.groupingBy(Function.identity(),

Collectors.counting()));

for (Map.Entry<String, Long> entry: frequencyMap.entrySet()) {

System.out.println(entry.getKey() + ": " + entry.getValue());

}

Download Run Code

Output:

A: 3
B: 2
C: 1

Here’s a version without streams (works with Java 8 and above):

import java.util.Arrays;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

class Main

{

public static void main(String[] args)

{

List<String> list = Arrays.asList("B", "A", "A", "C", "B", "A");

Map<String, Long> frequencyMap = new HashMap<>();

for (String s : list) {

frequencyMap.merge(s, 1L, Long::sum);

}

for (Map.Entry<String, Long> entry: frequencyMap.entrySet()) {

System.out.println(entry.getKey() + ": " + entry.getValue());

}

Download Run Code

Output:

A: 3
B: 2
C: 1

That’s all about counting the frequency of elements in a List in Java.

Count frequency of elements in a List in Java

1. Using a Set

2. Using a Map

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