How to remove duplicate elements from an ArrayList
In this shot, we will learn two common ways to remove duplicate elements from an ArrayList. The methods involve the use of:
- Set collection
- The
distinctmethod of the Stream API
1. Using a set to remove duplicate elements
We can use a set
In the code below, we will use LinkedHashSet
import java.util.ArrayList;
import java.util.LinkedHashSet;
class RemoveDuplicate {
public static void main( String args[] ) {
ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(1);
numbers.add(2);
numbers.add(1);
numbers.add(3);
numbers.add(1);
numbers.add(4);
numbers.add(1);
numbers.add(5);
System.out.println("Array list with duplicates");
System.out.println(numbers);
LinkedHashSet<Integer> uniqueNumbers = new LinkedHashSet<>(numbers);
ArrayList<Integer> uniqueList = new ArrayList<>(uniqueNumbers);
System.out.println("Array list without duplicates");
System.out.println(uniqueList);
}
}In the code above, we:
-
Created an ArrayList
numberswith duplicate values. -
Created a new
LinkedHashSetfrom the ArrayList. This will:- Remove the duplicate elements.
- Maintain the List order.
-
Created an ArrayList from the
LinkedHashSetthat only contains the unique values.
2. Using the distinct() method of the Stream API
The distinct() method of the Stream API returns a stream with distinct elements. The elements are compared through the use of the object’s equals method.
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
class RemoveDuplicate {
public static void main( String args[] ) {
ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(1);
numbers.add(2);
numbers.add(1);
numbers.add(3);
numbers.add(1);
numbers.add(4);
numbers.add(1);
numbers.add(5);
System.out.println("Array list with duplicates");
System.out.println(numbers);
List<Integer> uniqueNumbers;
uniqueNumbers = numbers
.stream()
.distinct()
.collect(Collectors.toList());
System.out.println("Array list without duplicates");
System.out.println(uniqueNumbers);
}
}In the code above, we removed the duplicated elements of the numbers ArrayList through the use of the Stream().distinct() method that will return a distinct object stream.
The stream of a distinct object is collected into a List through the collect method. We passed Collectors.toList() as an argument to tell the collect method to create a List from the stream.
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