How to find unique elements in multiple ArrayList in Java

Question

I have multiple Integer ArrayList, which contains some duplicate elements. I want to get the unique elements from them. But how?
java.util.ArrayList.removeAll() is not serving my purpose completely. See the below test code-

ArrayList<Integer> d = new ArrayList<Integer>();
d.add(2);
d.add(4);
d.add(5);
d.add(7);
d.add(8);
d.add(9);

ArrayList<Integer> e = new ArrayList<Integer>();
e.add(3);
e.add(7);

d.removeAll(e);

for (int t : d) {
    System.out.print(t+", ");
}

In output, i am getting 2, 4, 5, 8, 9, . Clearly 3 is missing. Also just to keep it simple, I am using only two ArrayList here but in my code, i have more than two ArrayList.

How I can find unique elements in multiple ArrayList in Java

based on your code the output that you are getting is obvious. What I understand is as 7 is common, only 7 should be removed and 3 should be added... right? then use Set. It will not add duplicates... that way you will have all unique list... — Fahim Parkar
– Fahim Parkar, Commented Dec 8, 2012 at 13:20

Community · Accepted Answer · 2017-05-23 10:28:03Z

3

Create a Set<Integer> from your List<Integer>. The set will contain no duplicate objects:

List<Integer> lstNumbers = new ArrayList<Integer>();
//fill the list of integers...
Set<Integer> setNumbers = new HashSet<Integer>(lstNumbers);
//the set will contain no duplicate values...
for (int t : setNumbers) {
    System.out.print(t+", ");
}

Note that you can add more List<Integer> in the Set by using the Set#addAll method (as shown in rahulroc answer) to add more integers in your not duplicated elements collection:

//assuming setNumbers has been initialized before
setNumbers.addAll(anotherListOfNumbers);
setNumbers.addAll(andAnotherListOfNumbers);

Also, as best practice, try to program to interfaces (List, Set, etc), not to class implementations (ArrayList, HashSet), as shown here: What does it mean to “program to an interface”?

edited May 23, 2017 at 10:28

CommunityBot

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answered Dec 8, 2012 at 13:18

Luiggi Mendoza

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6 Comments

ravi Over a year ago

First, i need to add all the ArrayList, then i have to create HashSet. Am i right?

Luiggi Mendoza Over a year ago

@RaviJoshi yes, first fill the ArrayList with duplicate elements, then create the Set.

ravi Over a year ago

+1 for the link What does it mean to program to an interface?

Rahul Over a year ago

@LuiggiMendoza .. great that you updated your answer based on my comment. I guess you missed this point from the question

Luiggi Mendoza Over a year ago

@Muklas yes, this will work fine with Strings, basically because HashSet uses hashCode and equals methods, and String implements it. You can use your own class to check unique results as well, as long as you use HashSet and you class implements these methods as well.

|

Rahul · Accepted Answer · 2012-12-08 13:21:24Z

2

use java.util.HashSet

Set<Integer> uniqueEntries = new HashSet<Integer>();
for(all lists)
     uniqueEntries.addAll(list);

Now the set uniqueEntries will contain all the unique integer values.

answered Dec 8, 2012 at 13:21

Rahul

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2 Comments

Luiggi Mendoza Over a year ago

You can use a Collection<E> as parameter in the HashSet constructor.

Rahul Over a year ago

Yes, we can. but he doesn't have just one arraylist. He needs to add multiple array lists. Hence, the constructor wont fulfil the entire purpose as he has to iterate over the whole list of all the arraylists.

Fahim Parkar · Accepted Answer · 2012-12-08 13:26:53Z

2

Use Set

ArrayList<Integer> d = new ArrayList<Integer>();
d.add(2);
d.add(4);
d.add(5);
d.add(7);
d.add(8);
d.add(9);

d.add(3);
d.add(7);

// this will remove all duplicates
Set setNew = new HashSet(d);

System.out.print(setNew);

Demo

answered Dec 8, 2012 at 13:26

Fahim Parkar

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1 Comment

ravi Over a year ago

I like the DEMO most. You rock buddy. Thank you. I got it now.

Varun Verma · Accepted Answer · 2013-08-10 06:18:58Z

0

 ArrayList<Integer> d = new ArrayList<Integer>();
        d.add(2);
        d.add(4);
        d.add(5);
        d.add(7);
        d.add(8);
        d.add(9);

        ArrayList<Integer> e = new ArrayList<Integer>();
        e.add(3);
        e.add(7);

        List<Integer> temp = new ArrayList<Integer>(d);

        System.out.println("d --> " + d);
        System.out.println("e --> " + e);

        d.removeAll(e);
        e.removeAll(temp);
        d.addAll(e);

        System.out.println("d --> " + d);

answered Aug 10, 2013 at 6:18

Varun Verma

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Comments

Prakash Gujarati · Accepted Answer · 2014-12-29 23:57:02Z

/* ArrayList Unique & Duplicate Records Print */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
ArrayList<Integer> d = new ArrayList<Integer>();
        d.add(2);
        d.add(4);
        d.add(5);
        d.add(7);
        d.add(8);
        d.add(9);

        ArrayList<Integer> e = new ArrayList<Integer>();
        e.add(3);
        e.add(7);

        List<Integer> temp = new ArrayList<Integer>(e);

        System.out.println("d --> " + d);
        System.out.println("e --> " + e);

        e.removeAll(d);
        temp.removeAll(e);

        System.out.println("e (unique)--> " + e);
        System.out.println("temp (duplicate) --> " + temp);
    }
}

Raja Jawahar · Accepted Answer · 2015-10-30 07:46:10Z

0

You can do this much more simpler.

ArrayList first = new Arraylist<>; ArrayList second =new ArrayList<>;

   first.add("one"); 
   first.add("two");  
   first.add("three"); 
   second.add("two");  
   second.add("three");

For find the unique elements.. first.retainall(second);

answered Oct 30, 2015 at 7:46

Raja Jawahar

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