Added in API level 1

Summary: Ctors | Methods | Inherited Methods

LinkedHashSet

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

java.lang.Object
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractSet<E>
		↳	java.util.HashSet<E>
			↳	java.util.LinkedHashSet<E>

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.

See also:

Summary

Public constructors
`LinkedHashSet()` Constructs a new, empty linked hash set with the default initial capacity (16) and load factor (0.75).
`LinkedHashSet(int initialCapacity)` Constructs a new, empty linked hash set with the specified initial capacity and the default load factor (0.75).
`LinkedHashSet(int initialCapacity, float loadFactor)` Constructs a new, empty linked hash set with the specified initial capacity and load factor.
`LinkedHashSet(Collection<? extends E> c)` Constructs a new linked hash set with the same elements as the specified collection.

Public methods
`void`	`addFirst(E e)` Adds an element as the first element of this collection (optional operation). If this set already contains the element, it is relocated if necessary so that it is first in encounter order.
`void`	`addLast(E e)` Adds an element as the last element of this collection (optional operation). If this set already contains the element, it is relocated if necessary so that it is last in encounter order.
`E`	`getFirst()` Gets the first element of this collection.
`E`	`getLast()` Gets the last element of this collection.
`static <T> LinkedHashSet<T>`	`newLinkedHashSet(int numElements)` 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<E>`	`reversed()` Returns a reverse-ordered view of this collection. Modifications to the reversed view are permitted and will be propagated to this set.
`Spliterator<E>`	`spliterator()` Creates a late-binding and fail-fast `Spliterator` over the elements in this set.

Inherited methods

From class


        
          java.util.HashSet

`boolean`	`add(E e)` Adds the specified element to this set if it is not already present.
`void`	`clear()` Removes all of the elements from this set.
`Object`	`clone()` Returns a shallow copy of this `HashSet` instance: the elements themselves are not cloned.
`boolean`	`contains(Object o)` Returns `true` if this set contains the specified element.
`boolean`	`isEmpty()` Returns `true` if this set contains no elements.
`Iterator<E>`	`iterator()` Returns an iterator over the elements in this set.
`static <T> HashSet<T>`	`newHashSet(int numElements)` Creates a new, empty HashSet suitable for the expected number of elements.
`boolean`	`remove(Object o)` Removes the specified element from this set if it is present.
`int`	`size()` Returns the number of elements in this set (its cardinality).
`Spliterator<E>`	`spliterator()` Creates a late-binding and fail-fast `Spliterator` over the elements in this set.
`<T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`Object[]`	`toArray()` Returns an array containing all of the elements in this collection.

From class


        
          java.util.AbstractSet

`boolean`	`equals(Object o)` Compares the specified object with this set for equality.
`int`	`hashCode()` Returns the hash code value for this set.
`boolean`	`removeAll(Collection<?> c)` Removes from this set all of its elements that are contained in the specified collection (optional operation).

From class


        
          java.util.AbstractCollection

`boolean`	`add(E e)` Ensures that this collection contains the specified element (optional operation).
`boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`void`	`clear()` Removes all of the elements from this collection (optional operation).
`boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>`	`iterator()` Returns an iterator over the elements contained in this collection.
`boolean`	`remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int`	`size()` Returns the number of elements in this collection.
`<T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`String`	`toString()` Returns a string representation of this collection.

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

From interface


        
          java.util.Set

`abstract boolean`	`add(E e)` Adds the specified element to this set if it is not already present (optional operation).
`abstract boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this set if they're not already present (optional operation).
`abstract void`	`clear()` Removes all of the elements from this set (optional operation).
`abstract boolean`	`contains(Object o)` Returns `true` if this set contains the specified element.
`abstract boolean`	`containsAll(Collection<?> c)` Returns `true` if this set contains all of the elements of the specified collection.
`static <E> Set<E>`	`copyOf(Collection<? extends E> coll)` Returns an unmodifiable Set containing the elements of the given Collection.
`abstract boolean`	`equals(Object o)` Compares the specified object with this set for equality.
`abstract int`	`hashCode()` Returns the hash code value for this set.
`abstract boolean`	`isEmpty()` Returns `true` if this set contains no elements.
`abstract Iterator<E>`	`iterator()` Returns an iterator over the elements in this set.
`static <E> Set<E>`	`of(E e1, E e2, E e3)` Returns an unmodifiable set containing three elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)` Returns an unmodifiable set containing nine elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5)` Returns an unmodifiable set containing five elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5, E e6)` Returns an unmodifiable set containing six elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)` Returns an unmodifiable set containing eight elements.
`static <E> Set<E>`	`of(E... elements)` Returns an unmodifiable set containing an arbitrary number of elements.
`static <E> Set<E>`	`of()` Returns an unmodifiable set containing zero elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)` Returns an unmodifiable set containing ten elements.
`static <E> Set<E>`	`of(E e1, E e2)` Returns an unmodifiable set containing two elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)` Returns an unmodifiable set containing seven elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4)` Returns an unmodifiable set containing four elements.
`static <E> Set<E>`	`of(E e1)` Returns an unmodifiable set containing one element.
`abstract boolean`	`remove(Object o)` Removes the specified element from this set if it is present (optional operation).
`abstract boolean`	`removeAll(Collection<?> c)` Removes from this set all of its elements that are contained in the specified collection (optional operation).
`abstract boolean`	`retainAll(Collection<?> c)` Retains only the elements in this set that are contained in the specified collection (optional operation).
`abstract int`	`size()` Returns the number of elements in this set (its cardinality).
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements in this set.
`abstract Object[]`	`toArray()` Returns an array containing all of the elements in this set.
`abstract <T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.

From interface


        
          java.util.Collection

`abstract boolean`	`add(E e)` Ensures that this collection contains the specified element (optional operation).
`abstract boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`abstract void`	`clear()` Removes all of the elements from this collection (optional operation).
`abstract boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`abstract boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`abstract boolean`	`equals(Object o)` Compares the specified object with this collection for equality.
`abstract int`	`hashCode()` Returns the hash code value for this collection.
`abstract boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>`	`iterator()` Returns an iterator over the elements in this collection.
`default Stream<E>`	`parallelStream()` Returns a possibly parallel `Stream` with this collection as its source.
`abstract boolean`	`remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`abstract boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`default boolean`	`removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`abstract boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int`	`size()` Returns the number of elements in this collection.
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements in this collection.
`default Stream<E>`	`stream()` Returns a sequential `Stream` with this collection as its source.
`abstract <T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`abstract Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`default <T> T[]`	`toArray(IntFunction<T[]> generator)` Returns an array containing all of the elements in this collection, using the provided `generator` function to allocate the returned array.

From interface


        
          java.util.SequencedSet


        abstract
        
        
        
        
        SequencedSet<E>


      reversed()

Returns a reverse-ordered view of this collection.

From interface


        
          java.lang.Iterable

`default void`	`forEach(Consumer<? super T> action)` Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
`abstract Iterator<E>`	`iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.

From interface


        
          java.util.SequencedCollection

`default void`	`addFirst(E e)` Adds an element as the first element of this collection (optional operation).
`default void`	`addLast(E e)` Adds an element as the last element of this collection (optional operation).
`default E`	`getFirst()` Gets the first element of this collection.
`default E`	`getLast()` Gets the last element of this collection.
`default E`	`removeFirst()` Removes and returns the first element of this collection (optional operation).
`default E`	`removeLast()` Removes and returns the last element of this collection (optional operation).
`abstract SequencedCollection<E>`	`reversed()` Returns a reverse-ordered view of this collection.

Public constructors

LinkedHashSet

Added in API level 1

public LinkedHashSet ()

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

LinkedHashSet

Added in API level 1

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`	`int`: the initial capacity of the LinkedHashSet

Throws
`IllegalArgumentException`	if the initial capacity is less than zero

LinkedHashSet

Added in API level 1

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`	`int`: the initial capacity of the linked hash set
`loadFactor`	`float`: 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

Added in API level 1

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`	`Collection`: the collection whose elements are to be placed into this set

Throws
`NullPointerException`	if the specified collection is null

Public methods

addFirst

Added in API level 35

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.

Parameters
`e`	`E`: the element to be added

addLast

Added in API level 35

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.

Parameters
`e`	`E`: the element to be added.

getFirst

Added in API level 35

public E getFirst ()

Gets the first element of this collection.

Returns
`E`	the retrieved element

Throws
`NoSuchElementException`

getLast

Added in API level 35

public E getLast ()

Gets the last element of this collection.

Returns
`E`	the retrieved element

Throws
`NoSuchElementException`

newLinkedHashSet

Added in API level 35

public static 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.

Parameters
`numElements`	`int`: the expected number of elements

Returns
`LinkedHashSet<T>`	the newly created set

Throws
`IllegalArgumentException`	if numElements is negative

removeFirst

Added in API level 35

public E removeFirst ()

Removes and returns the first element of this collection (optional operation).

Returns
`E`	the removed element

Throws
`NoSuchElementException`

removeLast

Added in API level 35

public E removeLast ()

Removes and returns the last element of this collection (optional operation).

Returns
`E`	the removed element

Throws
`NoSuchElementException`

reversed

Added in API level 35

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.

Returns
`SequencedSet<E>`	a reverse-ordered view of this collection, as a `SequencedSet`

spliterator

Added in API level 24

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.

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
`Spliterator<E>`	a `Spliterator` over the elements in this set