A Java program that counts duplicate characters from a given string.

java

import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class CountDuplicates {

    // using classic for loop and Map.merge method
    private static Map<Character, Integer> countChars(String input) {
        var result = new HashMap<Character, Integer>();

        for (char ch : input.toCharArray()) {
            result.merge(ch, 1, (a, b) -> a + b);
        }
        return result;
    }

    // using streams and collectors
    private static Map<Character, Long> countCharsStream(String input) {
        return input.chars()
                .mapToObj(c -> (char) c)
                .collect(Collectors.groupingBy(c -> c, Collectors.counting()));
    }

    public static void main(String[] args) {
        String input = "çekoslavakyalılaştıramadıklarımızdanmısınız";

        var result1 = countChars(input);
        var result2 = countCharsStream(input);

        System.out.println(result1);
        System.out.println(result2);
    }
}