Remove duplicates in an array without changing order of elements

Question

I have an array, say List<Integer> 139, 127, 127, 139, 130

How to remove duplicates of it and keep its order unchanged? i.e. 139, 127, 130

Solution 1

Use an instance of java.util.LinkedHashSet.

Set<Integer> set = new LinkedHashSet<>(list);

Solution 2

With this one-liner:

yourList = new ArrayList<Integer>(new LinkedHashSet<Integer>(yourList))

Solution 3

Without LinkedHashSet overhead (uses HashSet for seen elements instead which is slightly faster):

List<Integer> noDuplicates = list
        .stream()
        .distinct()
        .collect(Collectors.toList());

Note that the order is guaranteed by the Stream.distinct() contract:

For ordered streams, the selection of distinct elements is stable (for duplicated elements, the element appearing first in the encounter order is preserved.)

Solution 4

Construct Set from your list - "A collection that contains no duplicate elements":

Set<Integer> yourSet = new HashSet<Integer>(yourList);

And convert it back to whatever you want.

Note: If you want to preserve order, use LinkedHashSet instead.

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Comments

[Marco Forberg]

does HashSet preserve insertion order?

[Axel]

You have to use LinkedHashSet to preserve insertion order.

[Andreas]

You need a LinkedHashSet

[Paul Vargas]

Humm... That was a downgrade, I was using the diamond notation of Java 7 .

[Maroun]

Sorry about that, after my coffee I'll be a totally different person.

[user1156544]

This doesn't preserve order afak