How to Create an ArrayList of Unique Values in Java?

Question

How can I eliminate duplicate values from an ArrayList in Java?

import java.util.ArrayList;
import java.util.HashSet;

public class UniqueValues {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        // adding values to the list
        list.add("A");
        list.add("B");
        list.add("A");
        list.add("C");
        
        // Method to remove duplicates
        ArrayList<String> uniqueList = removeDuplicates(list);
        System.out.println(uniqueList);
    }

    // Method to remove duplicates
    public static ArrayList<String> removeDuplicates(ArrayList<String> list) {
        return new ArrayList<>(new HashSet<>(list));
    }
}

Answer

To create an ArrayList of unique values from another ArrayList, Java provides several methods. The most efficient way is to leverage the HashSet, which does not allow duplicate values. This overview will explain a method of filtering unique values from your data.

import java.util.ArrayList;
import java.util.HashSet;

public class UniqueValues {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("20/03/2013 23:31:46 6870");
        list.add("20/03/2013 23:31:46 6800");
        // Add more data as needed
        
        ArrayList<String> uniqueList = removeDuplicates(list);
        System.out.println(uniqueList);
    }

    public static ArrayList<String> removeDuplicates(ArrayList<String> list) {
        return new ArrayList<>(new HashSet<>(list));
    }
}

Causes

Duplicate values in the list that need removal.
Multiple occurrences of the same identifiers causing redundancy.

Solutions

Utilize a HashSet to filter out duplicates efficiently.
Implement a loop to check and add only unique values to a new ArrayList.

Common Mistakes

Mistake: Attempting to use nested loops to compare elements, which can lead to inefficiency and complexity.

Solution: Use a HashSet to automatically filter duplicates.

Mistake: Not handling nulls or edge cases that can lead to exceptions.

Solution: Ensure to validate inputs before processing.

Helpers

Java ArrayList
remove duplicates in ArrayList
unique values in Java
Java collections framework
Java HashSet