How to count duplicated items in Java List

Posted: July 17, 2013 in Java Posts, Java Posts, Java Tutorials

A Java example to show you how to count the total number of duplicated entries in a List, using Collections.frequency and Map.

CountDuplicatedList.java
package com.mkyong;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

public class CountDuplicatedList {

  public static void main(String[] args) {

	List list = new ArrayList();
	list.add("a");
	list.add("b");
	list.add("c");
	list.add("d");
	list.add("b");
	list.add("c");
	list.add("a");
	list.add("a");
	list.add("a");

	System.out.println("\nExample 1 - Count 'a' with frequency");
	System.out.println("a : " + Collections.frequency(list, "a"));

	System.out.println("\nExample 2 - Count all with frequency");
	Set uniqueSet = new HashSet(list);
	for (String temp : uniqueSet) {
		System.out.println(temp + ": " + Collections.frequency(list, temp));
	}

	System.out.println("\nExample 3 - Count all with Map");
	Map<string, integer=""> map = new HashMap<string, integer="">();

	for (String temp : list) {
		Integer count = map.get(temp);
		map.put(temp, (count == null) ? 1 : count + 1);
	}
	printMap(map);

	System.out.println("\nSorted Map");
	Map<string, integer=""> treeMap = new TreeMap<string, integer="">(map);
	printMap(treeMap);

  }

  public static void printMap(Map<string, integer=""> map){

	for (Map.Entry<string, integer=""> entry : map.entrySet()) {
		System.out.println("Key : " + entry.getKey() + " Value : "
			+ entry.getValue());
	}

  }

}

Output

Example 1 - Count 'a' with frequency
a : 4

Example 2 - Count all with frequency
d: 1
b: 2
c: 2
a: 4

Example 3 - Count all with Map
Key : d Value : 1
Key : b Value : 2
Key : c Value : 2
Key : a Value : 4

Sorted Map
Key : a Value : 4
Key : b Value : 2
Key : c Value : 2
Key : d Value : 1
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