E
- the type of elements maintained by this setSerializable
, 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.
Constructor | Description |
---|---|
LinkedHashSet() |
Constructs a new, empty linked hash set with the default initial capacity (16) and load factor (0.75). |
LinkedHashSet |
Constructs a new, empty linked hash set with the specified initial capacity and the default load factor (0.75). |
LinkedHashSet |
Constructs a new, empty linked hash set with the specified initial capacity and load factor. |
LinkedHashSet |
Constructs a new linked hash set with the same elements as the specified collection. |
Modifier and Type | Method | Description |
---|---|---|
void |
addFirst |
Adds an element as the first element of this collection (optional operation). |
void |
addLast |
Adds an element as the last element of this collection (optional operation). |
E |
getFirst() |
Gets the first element of this collection. |
E |
getLast() |
Gets the last element of this collection. |
static <T> LinkedHashSet |
newLinkedHashSet |
Creates a new, empty LinkedHashSet suitable for the expected number of elements. |
E |
removeFirst() |
Removes and returns the first element of this collection (optional operation). |
E |
removeLast() |
Removes and returns the last element of this collection (optional operation). |
SequencedSet |
reversed() |
Returns a reverse-ordered view of this collection. |
Spliterator |
spliterator() |
Creates a late-binding and fail-fast Spliterator over the elements in this set. |
add, clear, clone, contains, isEmpty, iterator, newHashSet, remove, size, toArray, toArray
equals, hashCode, removeAll
addAll, containsAll, retainAll, toArray, toArray, toString
parallelStream, removeIf, stream, toArray
public LinkedHashSet(int initialCapacity, float loadFactor)
LinkedHashSet
with an initial capacity that accommodates an expected number of elements, use newLinkedHashSet
.initialCapacity
- the initial capacity of the linked hash setloadFactor
- the load factor of the linked hash setIllegalArgumentException
- if the initial capacity is less than zero, or if the load factor is nonpositivepublic LinkedHashSet(int initialCapacity)
LinkedHashSet
with an initial capacity that accommodates an expected number of elements, use newLinkedHashSet
.initialCapacity
- the initial capacity of the LinkedHashSetIllegalArgumentException
- if the initial capacity is less than zeropublic LinkedHashSet()
public LinkedHashSet(Collection<? extends E> c)
c
- the collection whose elements are to be placed into this setNullPointerException
- if the specified collection is nullpublic Spliterator<E> spliterator()
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.
spliterator
in interface Collection<E>
spliterator
in interface Iterable<E>
spliterator
in interface Set<E>
spliterator
in class HashSet<E>
Iterator
. The spliterator inherits the fail-fast properties of the set's iterator. The created Spliterator
additionally reports Spliterator.SUBSIZED
.Spliterator
over the elements in this setpublic static <T> LinkedHashSet<T> newLinkedHashSet(int numElements)
T
- the type of elements maintained by the new setnumElements
- the expected number of elementsIllegalArgumentException
- if numElements is negativepublic void addFirst(E e)
If this set already contains the element, it is relocated if necessary so that it is first in encounter order.
addFirst
in interface SequencedCollection<E>
e
- the element to be addedpublic void addLast(E e)
If this set already contains the element, it is relocated if necessary so that it is last in encounter order.
addLast
in interface SequencedCollection<E>
e
- the element to be added.public E getFirst()
getFirst
in interface SequencedCollection<E>
NoSuchElementException
- if this collection is emptypublic E getLast()
getLast
in interface SequencedCollection<E>
NoSuchElementException
- if this collection is emptypublic E removeFirst()
removeFirst
in interface SequencedCollection<E>
NoSuchElementException
- if this collection is emptypublic E removeLast()
removeLast
in interface SequencedCollection<E>
NoSuchElementException
- if this collection is emptypublic SequencedSet<E> reversed()
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.
reversed
in interface SequencedCollection<E>
reversed
in interface SequencedSet<E>
SequencedSet
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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/LinkedHashSet.html