LinkedHashSet在Java中結(jié)合了HashSet和鏈表特性，既保證元素唯一性又維護(hù)插入順序。它通過鏈表記錄元素添加順序，使遍歷結(jié)果與插入順序一致，適用于需去重且保序的場景，如處理用戶操作日志或讀取文件去重。其檢查重復(fù)基于equals()和hashCode()方法，自定義對象需正確重寫這兩個(gè)方法。性能上add/remove/contains操作為O(1)，略遜于HashSet但差異不大。適合無需索引訪問和線程安全的場景，不適合內(nèi)存敏感或無需順序維護(hù)的情況。

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.

基本上就這些。It's a handy tool when you want uniqueness and order without having to manage that logic yourself.

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