To Remove Duplicates in ArrayList with Example and Use HashSet

Duplicate Elements/Strings can be removed in ArrayList many ways . One way will be using HashSet . Because HashSet does not allow duplicates. Another way can be using another ArrayList . Adding elements one by one from the first ArrayList to Second ArrayList by checking the element exists in the second array list .

//Example 1 : Using the Second Array List to remove duplicates

    


import java.util.*;
public class ArrayListDupRemoval
{
public static void main (String[] args)
{

ArrayList arrayList1 = new ArrayList();

ArrayList arrayList2 = new ArrayList();
  
  arrayList1.add("1");
  arrayList1.add("2");
  arrayList1.add("3");
  arrayList1.add("3");
  arrayList1.add("1");
  arrayList1.add("2");
  arrayList1.add("3");
  arrayList1.add("4");
  

System.out.println("Before Removing Duplicate");  

  for (Object item : arrayList1)
    System.out.println(item);


 for (Object item : arrayList1)

{

if (!arrayList2.contains(item))    // checking the element exists in the second array list , if not then add to second array list.
 {

 arrayList2.add(item);
 }
}

arrayList1.clear();   // Clears the first arraylist with duplicates
arrayList1=arrayList2;   // 
 
System.out.println("After Removing Duplicate");  

  for (Object item : arrayList1)
    System.out.println(item);
}
}

Output :

Before Removing Duplicate
1
2
3
3
1
2
3
4
After Removing Duplicate
1
2
3
4



//Example 2: Using the HashSet 

    

import java.util.*;
public class ArrayListDupRemovalHashSet
{
public static void main (String[] args)
{

ArrayList arrayList1 = new ArrayList();
  
  arrayList1.add("Sunday");
  arrayList1.add("Monday");
  arrayList1.add("Tuesday");
  arrayList1.add("Sunday");
  arrayList1.add("Wednesday");
  arrayList1.add("Tuesday");
  arrayList1.add("Thursday");
  arrayList1.add("Friday");
  arrayList1.add("Monday");
  arrayList1.add("Saturday");

  System.out.println("Before Removing Duplicate");  

  for (Object item : arrayList1)
    System.out.println(item);


System.out.println("------------");

  
  HashSet hashSet = new HashSet(arrayList1); //Creating HashSet which does not allows duplicates

   arrayList1.clear();                    // Clearing arraylist1
   arrayList1 = new ArrayList(hashSet) ; // Assigning Hashset back with no duplicates  to arraylist1   
  
  //Note : Hashset does not any maintain order
    

System.out.println("After Removing Duplicate");  

  for (Object item : arrayList1)
    System.out.println(item);
}
}

Output :

Before Removing Duplicate
Sunday
Monday
Tuesday
Sunday
Wednesday
Tuesday
Thursday
Friday
Monday
Saturday
------------
After Removing Duplicate
Friday
Monday
Saturday
Sunday
Thursday
Tuesday
Wednesday