Get Unique Values from ArrayList in Java

ArrayList is a class of Java Collection Framework that implements List Interface. It is a linear structure that stores and accesses each element sequentially. It allows the storage of duplicate elements however, there are a few approaches that may help to get unique values from an ArrayList. In this article, we are going to see the practical implementation of those approaches through Java example programs.

Java Program to get Unique Values from ArrayList

Before jumping to the solution program for the given problem, let’s discuss the following concepts of Collection Interface −

HashSet

It is a class of Java Collection Interface that implements the set interface. It is quite similar to the mathematical set. In it, all the elements are stored in a sorted manner that reduces the retrieval time and also, it doesn’t allow duplicate values. This is the reason for using HashSet to filter duplicates and to get unique values from a list.

Syntax

HashSet< Type >nameOfSet = new HashSet<>();

Here, Type is the wrapper class object, not a primitive datatype.

Syntax for ArrayList

ArrayList<Type> nameOflist = new ArrayList< Type >();

Approach 1

• Create an ArrayList object of type Integer and store some elements to it by using a built-in method named ‘add()’. This method takes an argument and adds it to the specified list.

• Now, create a HashSet of the same type and store all the elements of previous ArrayList to it using the in-built method ‘addAll()’. This method accepts an ArrayList as an argument and adds all its elements to the new specified list.

• In the end, print the values by calling the list.

Example 

The following example demonstrates the use of HashSet class to get unique values from an ArrayList.

import java.util.*;
public class Unique {
   public static void main(String[] args) {
      // Creating arraylist
      ArrayList<Integer> araylist = new ArrayList<Integer>();
      
      // Adding elements in arraylist
      araylist.add(8);
      araylist.add(5);
      araylist.add(2);
      araylist.add(9);
      araylist.add(2);
      araylist.add(4);
      araylist.add(9);
      araylist.add(7);
      System.out.println("List with duplicate elements: ");
      
      // loop to iterate through elements
      for(int i = 0; i < araylist.size(); i++ ) {
         // to print the elements in the list
         System.out.println(araylist.get(i));
      }
      HashSet<Integer> set = new HashSet<>(); // creating hashset
      set.addAll(araylist); // adding all elements to set
      System.out.println("List with unique elements: " + set);
   }
}     

Output

List with duplicate elements:
8
5
2
9
2
4
9
7
List with unique elements: [2, 4, 5, 7, 8, 9]

Approach 2

• Create an ArrayList object of type Integer and store some elements to it by using a built-in method named ‘add()’. This method takes an argument and adds it to the specified list.

• Now, create another ArrayList object of the same type.

• Define a for loop that will iterate till the ArrayList size. Inside this loop, take an if block that will check whether a value at index ‘i’ is unique or not. If it is unique then add that value to the new ArrayList otherwise discard it.

• the end, print the values by calling the list.

Example 

This is another example in which we will try to get unique values from an ArrayList.

import java.util.*;
public class Unique {
   public static void main(String[] args) {
      // Creating arraylist
      ArrayList<Integer> araylist1 = new ArrayList<Integer>();
      // Adding elements in arraylist
      araylist1.add(8);
      araylist1.add(5);
      araylist1.add(2);
      araylist1.add(9);
      araylist1.add(2);
      araylist1.add(4);
      araylist1.add(9);
      araylist1.add(7);
      System.out.println("List with duplicate elements: " + araylist1);
      ArrayList<Integer>araylist2 = new ArrayList<Integer>();
      for(int i = 0; i < araylist1.size(); i++ ) {
         int elem;
         // to filter the duplicate elements
         if(!araylist2.contains(araylist1.get(i))) {
            // storing unique elements to elem and adding to new list
            elem = araylist1.get(i);
            araylist2.add(elem);
         }
      }
      System.out.println("List with unique elements: " + araylist2);
   }
}

Output

List with duplicate elements: [8, 5, 2, 9, 2, 4, 9, 7]
List with unique elements: [8, 5, 2, 9, 4, 7]

Conclusion

We started this article by defining ArrayList and in the next section we understand the use of HashSet in finding unique values from a list. We discussed two approaches to get unique values from an ArrayList, first using the HashSet class and the second without using this class