## How to find count of duplicates in a List

There are many different ways to find out the duplicates in a List or count of duplicates in a List object. Three best ways have been implemented in the below sample class. I suggest to go with 2nd sample, when there is a requirement in your project.

package in.javatutorials;

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;

public class CountOfDuplicatesInList {

public static void main(String[] args) {

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

// Find out the count of duplicates by passing a String – static way
System.out.println(“\n Output 1 – Count ‘a’ with frequency”);
System.out.println(“a : ” + Collections.frequency(list, “a”));

// Find out the count of duplicates using Unique values – dynamic way
System.out.println(“\n Output 2 – Count all with frequency”);
Set<String> uniqueSet = new HashSet<String>(list);
for (String temp : uniqueSet) {
System.out.println(temp + “: ” + Collections.frequency(list, temp));
}

// Find out the count of duplicates using Map – Lengthy process
System.out.println(“\n Output 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);
}
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(“Key : ” + entry.getKey() + ” Value : ”
+ entry.getValue());
}
}
}

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### 7 thoughts on “How to find count of duplicates in a List”

1. Milind May 28, 2014 / 12:34 pm

Classic examples .. wonderful… keep it up.. Can you please put some examples for the Webservice for SOAP and REST as I am new for it..

• Mallik May 28, 2014 / 10:14 pm

Thank you for your comments. Find below posts for SOAP based web services implementation.

2. Milind May 28, 2014 / 12:35 pm

Wonderful examples.. Highly appreciate… Keep it up..

• Mallik May 28, 2014 / 10:10 pm

Thank you Milind, Keep visit my site.

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