Remove repeated elements from ArrayList in Java

Prerequisite: ArrayList in Java

Given an ArrayList, the task is to remove repeated elements of the ArrayList in Java.

Examples:

Input: ArrayList = [1, 2, 2, 3, 4, 4, 4] 
Output: [1, 2, 3, 4] 

Input: ArrayList = [12, 23, 23, 34, 45, 45, 45, 45, 57, 67, 89] 
Output: [12, 23, 34, 45, 57, 67, 89]

Below are the various methods to remove repeated elements an ArrayList in Java:

• Using a Set: Since Set is a collection which do not includes any duplicate elements. Hence the solution can be achieved with the help of a Set.

  Approach:

  1. Get the ArrayList with repeated elements.
  2. Convert the ArrayList to Set.
  3. Now convert the Set back to ArrayList. This will remove all the repeated elements.

  Below is the implementation of the above approach:

  // Java code to illustrate remove duolicate
  // of ArrayList using hashSet<> method
  
  import java.util.*;
  
  public class GFG {
      public static void main(String args[])
      {
  
          // create a ArrayList String type
          ArrayList<String>
              gfg = new ArrayList<String>();
  
          // Initialize an ArrayList
          gfg.add("Geeks");
          gfg.add("for");
          gfg.add("Geeks");
  
          // print ArrayList
          System.out.println("Original ArrayList : "
                             + gfg);
  
          // -----Using LinkedHashSet-----
          System.out.println("\nUsing LinkedHashSet:\n");
  
          // create a set and copy all value of list
          Set<String> set = new LinkedHashSet<>(gfg);
  
          // create a list and copy all value of set
          List<String> gfg1 = new ArrayList<>(set);
  
          // print ArrayList
          System.out.println("Modified ArrayList : "
                             + gfg1);
  
          // -----Using HashSet-----
          System.out.println("\nUsing HashSet:\n");
  
          // create a set and copy all value of list
          Set<String> set1 = new HashSet<>(gfg);
  
          // create a list and copy all value of set
          List<String> gfg2 = new ArrayList<>(set);
  
          // print ArrayList
          System.out.println("Modified ArrayList : "
                             + gfg2);
      }
  }
  Output:

  Original ArrayList : [Geeks, for, Geeks]
  
  Using LinkedHashSet:
  
  Modified ArrayList : [Geeks, for]
  
  Using HashSet:
  
  Modified ArrayList : [Geeks, for]
  

• Using Java 8 Lambdas:

  Approach:

  1. Get the ArrayList with repeated elements.
  2. Convert the ArrayList to Stream using stream() method.
  3. Set the filter condition to be distinct using distinct() method.
  4. Collect the filtered values as List using collect() method. This list will be with repeated elements removed

  Below is the implementation of the above approach:

  Example:

  // Java code to illustrate remove duolicate
  // of ArrayList using hashSet<> method
  
  import java.util.*;
  import java.util.stream.Collectors;
  
  public class GFG {
      public static void main(String args[])
      {
          // create a ArrayList String type
          ArrayList<String>
              gfg = new ArrayList<String>();
  
          // Initialize an ArrayList
          gfg.add("Geeks");
          gfg.add("for");
          gfg.add("Geeks");
  
          // print ArrayList
          System.out.println("Original ArrayList : "
                             + gfg);
  
          // create a list and copy all distinct value of list
          List<String> gfg1 = gfg.stream()
                                  .distinct()
                                  .collect(Collectors.toList());
  
          // print modified list
          System.out.println("Modified List : " + gfg1);
      }
  }
  Output:

  Original ArrayList : [Geeks, for, Geeks]
  Modified List : [Geeks, for]