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

    Map 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 > entrySet = map.entrySet();
      for(    Map.Entry  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 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 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 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 set = new HashSet(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 list = Arrays.stream(a).boxed().collect(Collectors.toList());

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

    }// main
}