Tutorials Java How to Remove Duplicate in ArrayList using Comparator

How to Remove Duplicate in ArrayList using Comparator

This example shows how to remove duplicate from ArrayList using Comparator. The easiest way to remove duplicate is by passing the List to an Set. We will use Comparator to remove duplicate elements. Once you have the Set you can again pass it back to ArrayList.

import java.util.ArrayList;
import java.util.Comparator;
import java.util.Set;
import java.util.TreeSet;

public class RemoveDuplicate {

	public static void main(String[] args) {

		final ArrayList students = new ArrayList();
		students.add(new Student("Student1", "1000"));
		students.add(new Student("Student2", "1001"));
		students.add(new Student("Student3", "1002"));
		students.add(new Student("Student4", "1003"));
		students.add(new Student("Student5", "1001"));
		students.add(new Student("Student6", "1004"));

		/** Printing original list **/
		System.out.println(students);

		Set set = new TreeSet(new Comparator() {
			@Override
			public int compare(Student o1, Student o2) {
				if(o1.getId().equalsIgnoreCase(o2.getId())){
	        		return 0;
	        	}
	        	return 1;
			}
		});
		set.addAll(students);

		System.out.println("\n***** After removing duplicates *******\n");

	    final ArrayList newList = new ArrayList(set);

	    /** Printing original list **/
		System.out.println(newList);
	}

}

class Student {
	private String name;
	private String id;

	public Student(String name, String id) {
		this.name = name;
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getId() {
		return id;
	}

	public void setId(String id) {
		this.id = id;
	}

	@Override
	public String toString() {
		return "\n" +"Name=" + name + "   Id=" + id ;
	}
}

Output

[
Name=Student1   Id=1000, 
Name=Student2   Id=1001, 
Name=Student3   Id=1002, 
Name=Student4   Id=1003, 
Name=Student5   Id=1001, 
Name=Student6   Id=1004]

***** After removing duplicates *******

[
Name=Student1   Id=1000, 
Name=Student2   Id=1001, 
Name=Student3   Id=1002, 
Name=Student4   Id=1003, 
Name=Student6   Id=1004]
Total
0
Shares
6 comments
  1. The above code is same as

    class StudentsComparator implements Comparator {
    @Override
    public int compare(Student o1, Student o2) {
    if(o1.getId().equalsIgnoreCase(o2.getId())){
    return 0;
    }
    return 1;
    }
    }

    Set set = new TreeSet(new StudentsComparator());

  2. How to remove duplicates strings from arraylist pojo class .
    like i want to remove names from the ArrayList then how to eliminate duplicates in this.

  3. There is one Problem if we add any duplicate item same as index 0 of list then that will not be removed. For ex :
    students.add(new Student(“Student1”, “1000”));
    students.add(new Student(“Student2”, “1001”));
    students.add(new Student(“Student3”, “1002”));
    students.add(new Student(“Student4”, “1000”));

    Not able to figure out why so.

Leave a Reply

Your email address will not be published.

Previous Post
Java Tutorials

Basic Java Language Constructs

Next Post

TestFlight to terminate Android support as of March 21

Related Posts
By clicking “Allow All”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. Cookie Notice
Allow All