How to count duplicate elements of ArrayList

In this article, we will count of number of duplicate elements present in List using Collections class’s utility frequency() method

 

Count duplicate elements from List

Method signature:

public static int frequency(Collection c, Object o);

CountOfDuplicateElementsOfArrayList.java

package in.bench.resources.java.collection;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;

public class CountOfDuplicateElementsOfArrayList {

	public static void main(String[] args) {

		// creating ArrayList object of type String
		ArrayList<String> al = new ArrayList<String>();

		// adding elements to ArrayList object
		// ArrayList contains duplicates and null
		al.add("Facebook");  
		al.add("LinkedIn"); 
		al.add("YouTube");
		al.add(null);
		al.add("LinkedIn");  
		al.add("Facebook");  
		al.add("LinkedIn");  
		al.add("YouTube");
		al.add("Facebook");  
		al.add("Google");
		al.add(null);


		// create HashSet to find duplicate element 
		HashSet<String> hsUnique = new HashSet<String>(al);

		System.out.println("Duplicates\tElement Name");
		System.out.println("==========\t=================");

		// Iterate using enhanced for-loop
		for (String strElement : hsUnique) {
			System.out.println(
					Collections.frequency(al, strElement) 
					+ "\t\t" 
					+ strElement);
		}
	}
}

Output:

Duplicates	Element Name
==========	=================
2			null
3			Facebook
3			LinkedIn
2			YouTube
1			Google

 

References:

 

Happy Coding !!
Happy Learning !!

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