ITV-JAVA-COLLECTION-Programming-How to Remove Duplicates from ArrayList in Java

How to Remove Duplicates from ArrayList in Java

Given an ArrayList with duplicate values, the task is to remove the duplicate values from this ArrayList in Java.

Examples:

Input: List = [1, 10, 2, 2, 10, 3, 3, 3, 4, 5, 5]
Output: List = [1, 10, 2, 3, 4, 5]

Input: List = ["G", "e", "e", "k", "s"]
Output: List = ["G", "e", "k", "s"]

1. Using Iterator

   Approach:
   1. Get the ArrayList with duplicate values.
   2. Create another ArrayList.
   3. Traverse through the first arraylist and store the first appearance of each element into the second arraylist using contains() method.
   4. The second ArrayList contains the elements with duplicates removed.
   Below is the implementation of the above approach:
   
   

   // Java program to remove duplicates from ArrayList    import java.util.*;   public class GFG {       // Function to remove duplicates from an ArrayList     public static <T> ArrayList<T> removeDuplicates(ArrayList<T> list)     {           // Create a new ArrayList         ArrayList<T> newList = new ArrayList<T>();           // Traverse through the first list         for (T element : list) {               // If this element is not present in newList             // then add it             if (!newList.contains(element)) {                   newList.add(element);             }         }           // return the new list         return newList;     }       // Driver code     public static void main(String args[])     {           // Get the ArrayList with duplicate values         ArrayList<Integer>             list = new ArrayList<>(                 Arrays                     .asList(1, 10, 1, 2, 2, 3, 3, 10, 3, 4, 5, 5));           // Print the Arraylist         System.out.println("ArrayList with duplicates: "                            + list);           // Remove duplicates         ArrayList<Integer>             newList = removeDuplicates(list);            // Print the ArrayList with duplicates removed         System.out.println("ArrayList with duplicates removed: "                            + newList);     } }

   Output:
   
   

   ArrayList with duplicates: [1, 10, 1, 2, 2, 3, 3, 10, 3, 4, 5, 5]
   ArrayList with duplicates removed: [1, 10, 2, 3, 4, 5]
   

2. Using LinkedHashSet

   A better way (both time complexity and ease of implementation wise) is to remove duplicates from an ArrayList is to convert it into a Set that does not allow duplicates. Hence LinkedHashSet is the best option available as this do not allows duplicates as well it preserves the insertion order.
   Approach:
   1. Get the ArrayList with duplicate values.
   2. Create a LinkedHashSet from this ArrayList. This will remove the duplicates
   3. Convert this LinkedHashSet back to Arraylist.
   4. The second ArrayList contains the elements with duplicates removed.
   Below is the implementation of the above approach:
   
   

   
   

   
   

   // Java program to remove duplicates from ArrayList    import java.util.*;    public class GFG {        // Function to remove duplicates from an ArrayList     public static <T> ArrayList<T> removeDuplicates(ArrayList<T> list)     {            // Create a new LinkedHashSet         Set<T> set = new LinkedHashSet<>();            // Add the elements to set         set.addAll(list);            // Clear the list         list.clear();            // add the elements of set         // with no duplicates to the list         list.addAll(set);            // return the list         return list;     }        // Driver code     public static void main(String args[])     {            // Get the ArrayList with duplicate values         ArrayList<Integer>             list = new ArrayList<>(                 Arrays                     .asList(1, 10, 1, 2, 2, 3, 10, 3, 3, 4, 5, 5));            // Print the Arraylist         System.out.println("ArrayList with duplicates: "                            + list);            // Remove duplicates         ArrayList<Integer>             newList = removeDuplicates(list);            // Print the ArrayList with duplicates removed         System.out.println("ArrayList with duplicates removed: "                            + newList);     } }

   Output:
   

   ArrayList with duplicates: [1, 10, 1, 2, 2, 3, 10, 3, 3, 4, 5, 5]
   ArrayList with duplicates removed: [1, 10, 2, 3, 4, 5]
   

3. Using Java 8 Stream.distinct()

   You can use the distinct() method from the Stream API. The distinct() method return a new Stream without duplicates elements based on the result returned by equals() method, which can be used for further processing. The actual processing of Stream pipeline starts only after calling terminal methods like forEach() or collect().
   Approach:
   1. Get the ArrayList with duplicate values.
   2. Create a new List from this ArrayList.
   3. Using Stream().distinct() method which return distinct object stream.
   4. convert this object stream into List
   Below is the implementation of the above approach:
   
   

   // Java program to remove duplicates from ArrayList    import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors;    // Program to remove duplicates from a List in Java 8 class GFG {     public static void main(String[] args)     {         // input list with duplicates         List<Integer> list = new ArrayList<>(             Arrays.asList(1, 10, 1, 2, 2, 3, 10, 3, 3, 4, 5, 5));             // Print the Arraylist         System.out.println("ArrayList with duplicates: "                            + list);            // Construct a new list from the set constucted from elements         // of the original list         List<Integer> newList = list.stream()                                       .distinct()                                       .collect(Collectors.toList());            // Print the ArrayList with duplicates removed         System.out.println("ArrayList with duplicates removed: "                            + newList);     } }

   Output:
   

   ArrayList with duplicates: [1, 10, 1, 2, 2, 3, 10, 3, 3, 4, 5, 5]
   ArrayList with duplicates removed: [1, 10, 2, 3, 4, 5]
   

Recommended Posts:

• Remove all elements from the ArrayList in Java
• How to remove an element from ArrayList in Java?
• Remove repeated elements from ArrayList in Java
• ArrayList and LinkedList remove() methods in Java with Examples
• ArrayList of ArrayList in Java
• Java.util.ArrayList.addall() method in Java
• ArrayList in Java
• Arraylist.contains() in Java
• Java.util.ArrayList.add() Method in Java
• Java.util.Arraylist.indexOf() in Java
• ArrayList isEmpty() in Java with example
• ArrayList trimToSize() in Java with example
• Initialize an ArrayList in Java
• Array of ArrayList in Java
• ArrayList vs LinkedList in Java