An ArrayList in Java allows duplicate elements. Sometimes, we need to remove these duplicate values while keeping only one occurrence of each element. Java provides several ways to remove duplicates from an ArrayList, such as using the contains() method, LinkedHashSet, and the Stream API.
- Using contains() is simple but can be less efficient for large lists.
- LinkedHashSet removes duplicates and maintains insertion order.
Illustration:
Input: ArrayList = [1, 10, 1, 2, 2, 3, 10, 3, 3, 4, 5, 5]
Output: ArrayList after removing duplicates = [1, 10, 2, 3, 4, 5]Input: ArrayList = [Apple, Banana, Apple, Mango, Banana, Orange]
Output: ArrayList after removing duplicates = [Apple, Banana, Mango, Orange]
Different Approaches to Remove Duplicates
1. Using contains() Method
This approach traverses the list and adds only the first occurrence of each element to a new list.
import java.util.*;
public class GFG {
public static <T> ArrayList<T> removeDuplicates(ArrayList<T> list) {
ArrayList<T> newList = new ArrayList<>();
for (T element : list) {
if (!newList.contains(element)) {
newList.add(element);
}
}
return newList;
}
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<>(
Arrays.asList(1, 10, 1, 2, 2, 3, 3, 10, 3, 4, 5, 5)
);
System.out.println("ArrayList with duplicates: " + list);
ArrayList<Integer> newList = removeDuplicates(list);
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]
Explanation:
- if (!newList.contains(element)) ensures only the first occurrence is added.
- The original list remains unchanged, and newList contains all elements without duplicates
2. Using LinkedHashSet
This code removes duplicates from an ArrayList by adding only the first occurrence of each element to a new list.
import java.util.*;
public class GFG {
public static <T> ArrayList<T> removeDuplicates(ArrayList<T> list) {
Set<T> set = new LinkedHashSet<>(list);
list.clear();
list.addAll(set);
return list;
}
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<>(
Arrays.asList(1, 10, 1, 2, 2, 3, 10, 3, 3, 4, 5, 5)
);
System.out.println("ArrayList with duplicates: " + list);
ArrayList<Integer> newList = removeDuplicates(list);
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]
Explanation:
- if (!newList.contains(element)) ensures only the first occurrence is added.
- The original list remains unchanged, and newList contains all elements without duplicates.
3. Using Java 8 Stream API
This code shows how to remove duplicates from an ArrayList using Java 8 Stream API’s distinct() method.
import java.util.*;
import java.util.stream.Collectors;
public class GFG {
public static void main(String[] args) {
List<Integer> list = new ArrayList<>(
Arrays.asList(1, 10, 1, 2, 2, 3, 10, 3, 3, 4, 5, 5)
);
System.out.println("ArrayList with duplicates: " + list);
List<Integer> newList = list.stream()
.distinct()
.collect(Collectors.toList());
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]
Explanation:
- distinct() ensures only unique elements are retained.
- collect(Collectors.toList()) converts the distinct elements back into a List.
