Efficient Methods to Remove Duplicates from a List in Java

General Discussions
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1

What is the best way to remove duplicates from a list in Java? I have an ArrayList<String> and want to remove any repeated strings from it. How can I do this efficiently in Java?

2

One of the quickest ways to remove duplicates from a list in Java is by converting the ArrayList into a Set. Since sets don’t allow duplicates, this automatically handles the cleanup. The only thing to note is that the order may not be maintained when using a HashSet.

import java.util.*;

List<String> list = new ArrayList<>(Arrays.asList("apple", "banana", "apple", "orange", "banana"));
Set<String> uniqueSet = new HashSet<>(list);
list = new ArrayList<>(uniqueSet);

System.out.println(list); // Output: [banana, orange, apple] (Order may change)

:white_check_mark: Why this works? The HashSet removes duplicates automatically, but it doesn’t preserve the original order. If maintaining the order is important, check out the next method.

3

Great point, Tim! Now, if you’re looking to remove duplicates from a list in Java but want to keep the original order intact, a LinkedHashSet is your friend.

import java.util.*;

List<String> list = new ArrayList<>(Arrays.asList("apple", "banana", "apple", "orange", "banana"));
list = new ArrayList<>(new LinkedHashSet<>(list));

System.out.println(list); // Output: [apple, banana, orange]

:white_check_mark: Why this works? Unlike HashSet, a LinkedHashSet maintains the insertion order, so the duplicates are removed, but the order remains exactly as in the original list.

4

Exactly, Alveera! If you prefer a more manual approach and want to remove duplicates from a list in Java without relying on collections like Set, you can always do it using a for loop. You can iterate over the list, checking if the item already exists in a new list before adding it.

import java.util.*;

List<String> list = new ArrayList<>(Arrays.asList("apple", "banana", "apple", "orange", "banana"));
List<String> uniqueList = new ArrayList<>();

for (String item : list) {
    if (!uniqueList.contains(item)) {
        uniqueList.add(item);
    }
}

System.out.println(uniqueList); // Output: [apple, banana, orange]

:white_check_mark: Why this works? It checks each item before adding it to the uniqueList, ensuring that no duplicates are included. However, be mindful that this approach can be slower for large lists because the contains() method checks each element one by one.