Hash table and linked list implementation of theSet interface, with predictable iteration order. This implementation differs fromHashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order). Note that insertion order isnot affected if an element isre-inserted into the set. (An elemente is reinserted into a sets ifs.add(e) is invoked whens.contains(e) would returntrue immediately prior to the invocation.)

This implementation spares its clients from the unspecified, generally chaotic ordering provided byHashSet, without incurring the increased cost associated withTreeSet. It can be used to produce a copy of a set that has the same order as the original, regardless of the original set's implementation:

     void foo(Set<String> s) {         Set<String> copy = new LinkedHashSet<>(s);         ...     }

This technique is particularly useful if a module takes a set on input, copies it, and later returns results whose order is determined by that of the copy. (Clients generally appreciate having things returned in the same order they were presented.)

This class provides all of the optionalSet operations, and permits null elements. LikeHashSet, it provides constant-time performance for the basic operations (add,contains andremove), assuming the hash function disperses elements properly among the buckets. Performance is likely to be just slightly below that ofHashSet, due to the added expense of maintaining the linked list, with one exception: Iteration over aLinkedHashSet requires time proportional to thesize of the set, regardless of its capacity. Iteration over aHashSet is likely to be more expensive, requiring time proportional to itscapacity.

A linked hash set has two parameters that affect its performance:initial capacity andload factor. They are defined precisely as forHashSet. Note, however, that the penalty for choosing an excessively high value for initial capacity is less severe for this class than forHashSet, as iteration times for this class are unaffected by capacity.

Note that this implementation is not synchronized. If multiple threads access a linked hash set concurrently, and at least one of the threads modifies the set, itmust be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be "wrapped" using theCollections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:

   Set s = Collections.synchronizedSet(new LinkedHashSet(...));

The iterators returned by this class'siterator method arefail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's ownremove method, the iterator will throw aConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throwConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness:the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.