Create a Frequency Map in Java 8 and above

Given a set of words, create a frequency map out of it in Java.

In Java 8, we can convert the given set of words to stream and use a collector to count the occurrences of elements in a stream.

The groupingBy(classifier, downstream) collector converts the collection of elements into a map by grouping elements according to a classification method and then performing a reduction operation on the values associated with a given key using the specified downstream Collector.

import java.util.Map;

import java.util.function.Function;

import java.util.stream.Collectors;

import java.util.stream.Stream;

class Main

{

// Program to create a frequency map in Java 8 and above

public static void main(String[] args)

{

String[] chars = { "A", "B", "C", "A", "C", "A" };

Map<String, Long> freq = Stream.of(chars)

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

Collectors.counting()));

System.out.println(freq);

}

Download Run Code

Output:

{A=3, B=1, C=2}

Here’s a version without using streams:

import java.util.HashMap;

import java.util.Map;

class Main

{

public static void main(String[] args)

{

String[] chars = { "A", "B", "C", "A", "C", "A" };

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

for (String s: chars) {

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

}

System.out.println(freq);

}

Download Run Code

Output:

{A=3, B=1, C=2}

For Java 7 and before, here’s a naive solution to create a frequency map:

import java.util.HashMap;

import java.util.Map;

class Main

{

// A naive solution to create a frequency map in Java

public static void main(String[] args)

{

String[] chars = { "A", "B", "C", "A", "C", "A" };

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

for (String s: chars)

{

int prev = 0;

// get the previous count

if (freq.get(s) != null) {

prev = freq.get(s);

}

freq.put(s, prev + 1);

}

System.out.println(freq);

}

Download Run Code

Output:

{A=3, B=1, C=2}

That’s all about creating a frequency Map in Java.

Create a Frequency Map in Java 8 and above

Share this: