LinkedHashSet combines HashSet and linked list features in Java, which not only ensures the uniqueness of elements but also maintains the insertion order. It records the order of elements added by the linked list, so that the traversal results are consistent with the order of insertion. It is suitable for scenarios where deduplication and order preservation is required, such as processing user operation logs or reading file deduplication. The checking is repeated based on the equals() and hashCode() methods, and custom objects need to rewrite these two methods correctly. In terms of performance, add/remove/contains operation is O(1), slightly inferior to HashSet but not much difference. Suitable for scenarios without index access and thread safety, not for memory sensitive or sequential maintenance.
A LinkedHashSet is a special type of collection in Java that combines the features of a HashSet and a linked list. It lets you store unique elements like a HashSet, but unlike HashSet, it maintains the insertion order. So when you loop through a LinkedHashSet, the elements will appear in the same order you added them.
Maintains Insertion Order
One of the main differences between a LinkedHashSet and a regular HashSet is that LinkedHashSet remembers the order in which you add elements. This can be super useful if you need to keep track of the order but still want to avoid duplicates.
For example:
- You have a list of user actions and you want to process them in the order they happened.
- Or you're reading lines from a file and want to ignore duplicates but preserve the original order.
Under the hood, it uses a linked list to maintain this order, which adds a bit more overhead compared to a normal HashSet, but not enough to worry about unless you're working with huge data sets.
No Duplicates (Like HashSet)
Just like a regular HashSet, a LinkedHashSet doesn't allow duplicate elements. When you try to add an element that already exists in the set, it simply ignores the new addition.
How does it check for duplicates?
It uses the equals()
method and the hashCode()
method of the objects you're adding. That means if you're putting custom objects into a LinkedHashSet, you should override these two methods correctly — otherwise, two logically equal objects might end up both being stored, breaking the no-duplicate rule.
So if you're using something like:
Set<String> set = new LinkedHashSet<>(); set.add("apple"); set.add("apple"); // This won't be added again
The second "apple" just gets skipped silently.
Performance Considerations
LinkedHashSet has pretty good performance for most operations like add, remove, and contains — they're usually O(1) time complexity, just like HashSet. The only difference is that maintaining the linked list behind the scenes makes each operation slightly slower than a pure HashSet, but again, the difference is negligible for most applications.
When to use it:
- You want to eliminate duplicates.
- You care about the order in which items are inserted.
- You don't need access by index (like with a List).
- You don't need thread safety (if you do, consider wrapping it or using another structure).
When not to use it:
- If memory usage matters a lot — because it stores extra links for the linked list.
- If order doesn't matter — then just use a regular HashSet for slightly better efficiency.
Basically that's it. It's a handy tool when you want uniqueness and order without having to manage that logic yourself.
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