Remove duplicates from list of integers using Lambda expression in Java

Last update on May 20 2023 13:07:42 (UTC/GMT +8 hours)

Java Lambda Program: Exercise-7 with Solution

Write a Java program to implement a lambda expression to remove duplicates from a list of integers.

Sample Solution:

Java Code:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        // Create a list of integers with duplicates
        List<Integer> nums = Arrays.asList(1, 2, 3, 3, 4, 3, 2, 5, 6, 1, 7, 7, 8, 10);
        // Print the list
        System.out.println("List elements " + nums);     
        // Remove duplicates using lambda expression
        List<Integer> unique_nums = new ArrayList<>();
        nums.stream()
                .distinct()
                .forEach(unique_nums::add);

        // Print the list without duplicates
        System.out.println("\nList elements without duplicates: " + unique_nums);
    }
}

Sample Output:

List elements [1, 2, 3, 3, 4, 3, 2, 5, 6, 1, 7, 7, 8, 10]

List elements without duplicates: [1, 2, 3, 4, 5, 6, 7, 8, 10]

Explanation:

In the above exercise we use the stream() method on the nums list to convert it into a stream. We call the distinct() method to filter out duplicate elements. The distinct() method ensures that only distinct elements are retained in the stream.

Finally, we use the forEach() method and a lambda expression unique_nums::add to add each unique element to the unique_nums list.

Flowchart:

Live Demo:

Java Code Editor:

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Java Lambda Exercises Previous: Calculate average of doubles using Lambda expression in Java.
Java Lambda Exercises Next: Calculate factorial using Lambda expression in Java.

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Java: Tips of the Day

Hashset vs Treeset:

HashSet is much faster than TreeSet (constant-time versus log-time for most operations like add, remove and contains) but offers no ordering guarantees like TreeSet.

HashSet

the class offers constant time performance for the basic operations (add, remove, contains and size).
it does not guarantee that the order of elements will remain constant over time
iteration performance depends on the initial capacity and the load factor of the HashSet.

It's quite safe to accept default load factor but you may want to specify an initial capacity that's about twice the size to which you expect the set to grow.

TreeSet

guarantees log(n) time cost for the basic operations (add, remove and contains)
guarantees that elements of set will be sorted (ascending, natural, or the one specified by you via its constructor) (implements SortedSet)
doesn't offer any tuning parameters for iteration performance
offers a few handy methods to deal with the ordered set like first(), last(), headSet(), and tailSet() etc

Important points:

Both guarantee duplicate-free collection of elements
It is generally faster to add elements to the HashSet and then convert the collection to a TreeSet for a duplicate-free sorted traversal.
None of these implementations are synchronized. That is if multiple threads access a set concurrently, and at least one of the threads modifies the set, it must be synchronized externally.
LinkedHashSet is in some sense intermediate between HashSet and TreeSet. Implemented as a hash table with a linked list running through it, however,it provides insertion-ordered iteration which is not same as sorted traversal guaranteed by TreeSet.

So a choice of usage depends entirely on your needs but I feel that even if you need an ordered collection then you should still prefer HashSet to create the Set and then convert it into TreeSet.

e.g. SortedSet<String> s = new TreeSet<String>(hashSet);

Ref: https://bit.ly/3d3waGh

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