How to count duplicate enteries/items from List in Java ?

Below java code example shows how to count duplicate elements in a list by using Collection.frequency and Map collection.

Code:

---------------------------------CountDuplicate.java------------------------------------------------------------

import java.util.ArrayList;

import java.util.Collections;

import java.util.HashMap;

import java.util.HashSet;

import java.util.List;

import java.util.Map;

public class CountDuplicate {

       public static void main(String[] args) {

              List<String> items = new ArrayList<>();

              items.add("Zero");

              items.add("One");

              items.add("Two");

              items.add("Two");

              items.add("Two");

              items.add("Two");

              items.add("Three");

              items.add("four");

              items.add("five");

              items.add("five");

              items.add("five");

              System.out.println("count elements by usin frequency");

              // counting an element in a list by using frequency

              int countedItem = Collections.frequency(items, "five");

              System.out.println(countedItem);

             

              System.out.println("");

              System.out.println("count by using set");

              System.out.println("");

              // count all elements which are in list

              HashSet<String> itemsSet = new HashSet<String>(items);

              for (String element : itemsSet) {

                     System.out.println(element + "'s in list : " + Collections.frequency(items, element));

              }

              System.out.println("");

              System.out.println("count elements by using map");

              System.out.println("");

             

              // count number of elements occurance in list by using map

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

              for (String element : itemsSet) {

                     Integer eleCount = countingMap.get(element);

                     countingMap.put(element, (eleCount == null) ? 1 : eleCount + 1);

              }

              for (Map.Entry<String, Integer> elementEntry : countingMap.entrySet()) {

                     System.out.println(elementEntry.getKey() + "'s in list " + elementEntry.getValue());

              }

       }

}

Output:

----------------------------------------------------------

count elements by usin frequency

3

count by using set

Zero's in list : 1

four's in list : 1

One's in list : 1

Two's in list : 4

five's in list : 3

Three's in list : 1

count elements by using map

Zero's in list 1

four's in list 1

One's in list 1

Two's in list 1

five's in list 1

Three's in list 1

Ref: Collections.frequency