Module java.base
Package java.util

Class LinkedHashSet<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractSet<E>
java.util.HashSet<E>
java.util.LinkedHashSet<E>
Type Parameters:
E - the type of elements maintained by this set
All Implemented Interfaces:
Serializable, Cloneable, Iterable<E>, Collection<E>, SequencedCollection<E>, SequencedSet<E>, Set<E>
public class LinkedHashSet<E> extends HashSet<E> implements SequencedSet<E>, Cloneable, Serializable

Hash table and linked list implementation of the Set interface, with well-defined encounter 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 encounter order (iteration order), which is the order in which elements were inserted into the set (insertion-order). The least recently inserted element (the eldest) is first, and the youngest element is last. Note that encounter order is not affected if an element is re-inserted into the set with the add method. (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.) The reverse-ordered view of this set is in the opposite order, with the youngest element appearing first and the eldest element appearing last. The encounter order of elements already in the set can be changed by using the addFirst and addLast methods.

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<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 optional Set and SequencedSet operations, and it 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.

Since:
1.4
See Also:

  • Constructor Details

    • LinkedHashSet

      public LinkedHashSet(int initialCapacity, float loadFactor)
      Constructs a new, empty linked hash set with the specified initial capacity and load factor.
      API Note:
      To create a LinkedHashSet with an initial capacity that accommodates an expected number of elements, use newLinkedHashSet.
      Parameters:
      initialCapacity - the initial capacity of the linked hash set
      loadFactor - the load factor of the linked hash set
      Throws:
      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).
      API Note:
      To create a LinkedHashSet with an initial capacity that accommodates an expected number of elements, use newLinkedHashSet.
      Parameters:
      initialCapacity - the initial capacity of the LinkedHashSet
      Throws:
      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).
      Parameters:
      c - the collection whose elements are to be placed into this set
      Throws:
      NullPointerException - if the specified collection is null

  • Method Details

    • spliterator

      public Spliterator<E> spliterator()
      Creates a late-binding and fail-fast Spliterator over the elements in this set.

      The Spliterator reports Spliterator.SIZED, Spliterator.DISTINCT, and ORDERED. Implementations should document the reporting of additional characteristic values.

      Specified by:
      spliterator in interface Collection<E>
      Specified by:
      spliterator in interface Iterable<E>
      Specified by:
      spliterator in interface Set<E>
      Overrides:
      spliterator in class HashSet<E>
      Implementation Note:
      The implementation creates a late-binding spliterator from the set's Iterator. The spliterator inherits the fail-fast properties of the set's iterator. The created Spliterator additionally reports Spliterator.SUBSIZED.
      Returns:
      a Spliterator over the elements in this set
      Since:
      1.8

    • newLinkedHashSet

      public static <T> LinkedHashSet<T> newLinkedHashSet(int numElements)
      Creates a new, empty LinkedHashSet suitable for the expected number of elements. The returned set uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of elements can be added without resizing the set.
      Type Parameters:
      T - the type of elements maintained by the new set
      Parameters:
      numElements - the expected number of elements
      Returns:
      the newly created set
      Throws:
      IllegalArgumentException - if numElements is negative
      Since:
      19

    • addFirst

      public void addFirst(E e)
      Adds an element as the first element of this collection (optional operation). After this operation completes normally, the given element will be a member of this collection, and it will be the first element in encounter order.

      If this set already contains the element, it is relocated if necessary so that it is first in encounter order.

      Specified by:
      addFirst in interface SequencedCollection<E>
      Parameters:
      e - the element to be added
      Since:
      21

    • addLast

      public void addLast(E e)
      Adds an element as the last element of this collection (optional operation). After this operation completes normally, the given element will be a member of this collection, and it will be the last element in encounter order.

      If this set already contains the element, it is relocated if necessary so that it is last in encounter order.

      Specified by:
      addLast in interface SequencedCollection<E>
      Parameters:
      e - the element to be added.
      Since:
      21

    • getFirst

      public E getFirst()
      Gets the first element of this collection.
      Specified by:
      getFirst in interface SequencedCollection<E>
      Returns:
      the retrieved element
      Throws:
      NoSuchElementException - if this collection is empty
      Since:
      21

    • getLast

      public E getLast()
      Gets the last element of this collection.
      Specified by:
      getLast in interface SequencedCollection<E>
      Returns:
      the retrieved element
      Throws:
      NoSuchElementException - if this collection is empty
      Since:
      21

    • removeFirst

      public E removeFirst()
      Removes and returns the first element of this collection (optional operation).
      Specified by:
      removeFirst in interface SequencedCollection<E>
      Returns:
      the removed element
      Throws:
      NoSuchElementException - if this collection is empty
      Since:
      21

    • removeLast

      public E removeLast()
      Removes and returns the last element of this collection (optional operation).
      Specified by:
      removeLast in interface SequencedCollection<E>
      Returns:
      the removed element
      Throws:
      NoSuchElementException - if this collection is empty
      Since:
      21

    • reversed

      public SequencedSet<E> reversed()
      Returns a reverse-ordered view of this collection. The encounter order of elements in the returned view is the inverse of the encounter order of elements in this collection. The reverse ordering affects all order-sensitive operations, including those on the view collections of the returned view. If the collection implementation permits modifications to this view, the modifications "write through" to the underlying collection. Changes to the underlying collection might or might not be visible in this reversed view, depending upon the implementation.

      Modifications to the reversed view are permitted and will be propagated to this set. In addition, modifications to this set will be visible in the reversed view.

      Specified by:
      reversed in interface SequencedCollection<E>
      Specified by:
      reversed in interface SequencedSet<E>
      Returns:
      a reverse-ordered view of this collection, as a SequencedSet
      Since:
      21