Java 8 - Count Duplicate Characters in a String

In this short article, we will write a Java program to count duplicate characters in a given String.

We will use Java 8 lambda expression and stream API to write this program.

Learn Java programming at https://www.javaguides.net/p/java-tutorial-learn-java-programming.html.

Learn Java 8 at https://www.javaguides.net/p/java-8.html.

Java 8 - Count Duplicate Characters from a String

package com.java.tutorials.programs;

import java.util.Map;

import java.util.stream.Collectors;

/**
 * Java 8 - Count Duplicate Characters from a String
 * 
 * @author RAMESH
 *
 */
public class CountDuplicateCharsJava8 {

    public static void main(String[] args) {

        // given input string
        String input = "JavaJavaEE";

        // convert string into stream
        Map < Character, Long > result = input
            .chars().mapToObj(c -> (char) c)
            .collect(Collectors.groupingBy(c -> c, Collectors.counting()));

        result.forEach((k, v) -> {
            if (v > 1) {
                System.out.println(k + " : " + v);
            }
        });
    }
}

Output:

a : 4
E : 2
v : 2
J : 2

Let's rewrite the above program without using Java 8 features.

Java program that counts duplicate characters from a given string (without Java 8)

package com.java.tutorials.programs;

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

public class CountDuplicateChars {

    public static void main(String[] args) {

        // given input string
        String input = "JavaJavaEE";

        // create a HashMap
        Map < Character, Integer > countDuplicateMap = new HashMap < > ();

        // Converts this string to a new character array.
        char[] charArray = input.toCharArray();

        for (char c: charArray) {

            if (countDuplicateMap.containsKey(c)) {

                // get value by key and increment it's value by a
                countDuplicateMap.put(c, countDuplicateMap.get(c) + 1);
            } else {
                countDuplicateMap.put(c, 1);
            }
        }


        for (Entry < Character, Integer > entry: countDuplicateMap.entrySet()) {

            if (entry.getValue() > 1) {
                System.out.println(entry.getKey() + " : " + entry.getValue());
            }
        }
    }
}

Output:

a : 4
E : 2
v : 2
J : 2

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