How to count duplicate elements in ArrayList?

Question

I need to separate and count how many values in arraylist are the same and print them according to the number of occurrences.

I\'ve got an arraylist called digits :<

Answer 1

The question is to count how many ones twos and threes are there in an array. In Java 7 solution is:

import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class howMany1 {
public static void main(String[] args) {

    List<Integer> list = Arrays.asList(1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765);

    Map<Integer ,Integer> map = new HashMap<>();

      for(  Integer r  : list) {
          if(  map.containsKey(r)   ) {
                 map.put(r, map.get(r) + 1);
          }//if
          else {
              map.put(r, 1);
          }
      }//for

      //iterate

      Set< Map.Entry<Integer ,Integer> > entrySet = map.entrySet();
      for(    Map.Entry<Integer ,Integer>  entry : entrySet     ) {
          System.out.printf(   "%s : %d %n "    , entry.getKey(),entry.getValue()  );
      }//for

}}

In Java 8, the solution to the problem is :

import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

public class howMany2 {
public static void main(String[] args) {

    List<Integer> list = Arrays.asList(1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765);
     // we can also use Function.identity() instead of c->c
    Map<Integer ,Long > map = list.stream()
            .collect(  Collectors.groupingBy(c ->c , Collectors.counting())         ) ;


    map.forEach(   (k , v ) -> System.out.println( k + " : "+ v )                    );

}}

One another method is to use Collections.frequency. The solution is:

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

public class Duplicates1 {
public static void main(String[] args) {

    List<Integer> list = Arrays.asList(1, 1, 2, 3, 5, 8, 13,13, 21, 34, 55, 89, 144, 233);

    System.out.println("Count all with frequency");
    Set<Integer> set = new HashSet<Integer>(list);
    for (Integer r : set) {
        System.out.println(r + ": " + Collections.frequency(list, r));
    }

}}

Another method is to change the int array to Integer List using method => Arrays.stream(array).boxed().collect(Collectors.toList()) and then get the integer using for loop.

public class t7 {
    public static void main(String[] args) {
        int[] a = { 1, 1, 2, 3, 5, 8, 13, 13 };
        List<Integer> list = Arrays.stream(a).boxed().collect(Collectors.toList());

        for (Integer ch : list) {
            System.out.println(ch + " :  " + Collections.frequency(list, ch));
        }

    }// main
}

Answer 2

List<String> list = new ArrayList<String>();
    list.add("a");
    list.add("b");
    list.add("c");
    list.add("a");
    list.add("a");
    list.add("a");

int countA=Collections.frequency(list, "a");
int countB=Collections.frequency(list, "b");
int countC=Collections.frequency(list, "c");

Answer 3

Use below function for count duplicate elements :

public void countDuplicate(){
        try {
            Set<String> set = new HashSet<>(original_array);
            ArrayList<String> temp_array = new ArrayList<>();
            temp_array.addAll(set);
            for (int i = 0 ; i < temp_array.size() ; i++){
                Log.e(temp_array.get(i),"=>"+Collections.frequency(original_array,temp_array.get(i)));
            }
        }catch (Exception e){
            e.printStackTrace();
        }
    }

Answer 4

Java 8, the solution: 1. Create Map when the Key is the Value of Array and Value is counter. 
2. Check if Map contains the Key increase counter or add a new set.

 private static void calculateDublicateValues(int[] array) {
      //key is value of array, value is counter
      Map<Integer, Integer> map = new HashMap<Integer, Integer>();

      for (Integer element : array) {
        if (map.containsKey(element)) {
          map.put(element, map.get(element) + 1); // increase counter if contains
        } else
          map.put(element, 1);
      }

      map.forEach((k, v) -> {
        if (v > 1)
          System.out.println("The element " + k + " duplicated " + v + " times");
      });

    }