Module java.base
Package java.util

Class LinkedHashSet<E>

  • Type Parameters:
    E - the type of elements maintained by this set
    All Implemented Interfaces:
    Serializable, Cloneable, Iterable<E>, Collection<E>, Set<E>

    public class LinkedHashSet<E>
    extends HashSet<E>
    implements Set<E>, Cloneable, Serializable

    Hash table and linked list implementation of the Set interface, with predictable iteration order. This implementation differs from HashSet 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 is not affected if an element is re-inserted into the set. (An element e is reinserted into a set s if s.add(e) is invoked when s.contains(e) would return true immediately prior to the invocation.)

    This implementation spares its clients from the unspecified, generally chaotic ordering provided by HashSet, without incurring the increased cost associated with TreeSet. 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 s) {
             Set 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 optional Set operations, and permits null elements. Like HashSet, it provides constant-time performance for the basic operations (add, contains and remove), assuming the hash function disperses elements properly among the buckets. Performance is likely to be just slightly below that of HashSet, due to the added expense of maintaining the linked list, with one exception: Iteration over a LinkedHashSet requires time proportional to the size of the set, regardless of its capacity. Iteration over a HashSet is likely to be more expensive, requiring time proportional to its capacity.

    A linked hash set has two parameters that affect its performance: initial capacity and load factor. They are defined precisely as for HashSet. Note, however, that the penalty for choosing an excessively high value for initial capacity is less severe for this class than for HashSet, 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, it must 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 the Collections.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's iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. 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 throw ConcurrentModificationException 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.

    See Also:
    Object.hashCode(), Collection, Set, HashSet, TreeSet, Hashtable, Serialized Form
    • Constructor Detail

      • LinkedHashSet

        public LinkedHashSet​(int initialCapacity,
                             float loadFactor)
        Constructs a new, empty linked hash set with the specified initial capacity and load factor.
        initialCapacity - the initial capacity of the linked hash set
        loadFactor - the load factor of the linked hash set
        IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive
      • LinkedHashSet

        public LinkedHashSet​(int initialCapacity)
        Constructs a new, empty linked hash set with the specified initial capacity and the default load factor (0.75).
        initialCapacity - the initial capacity of the LinkedHashSet
        IllegalArgumentException - if the initial capacity is less than zero
      • LinkedHashSet

        public LinkedHashSet()
        Constructs a new, empty linked hash set with the default initial capacity (16) and load factor (0.75).
      • LinkedHashSet

        public LinkedHashSet​(Collection<? extends E> c)
        Constructs a new linked hash set with the same elements as the specified collection. The linked hash set is created with an initial capacity sufficient to hold the elements in the specified collection and the default load factor (0.75).
        c - the collection whose elements are to be placed into this set
        NullPointerException - if the specified collection is null