Added in API level 1

LinkedHashSet

open class LinkedHashSet<E : Any!> : HashSet<E>, Cloneable, SequencedSet<E>, Serializable, MutableSet<E>

kotlin.Any
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractSet<E>
		↳	java.util.HashSet<E>
			↳	java.util.LinkedHashSet

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:

<code>void foo(Set&lt;String&gt; s) {
          Set&lt;String&gt; copy = new LinkedHashSet&lt;&gt;(s);
          ...
      }
  </code>

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.

Summary

Public constructors
LinkedHashSet() Constructs a new, empty linked hash set with the default initial capacity (16) and load factor (0.75).
LinkedHashSet(initialCapacity: Int) Constructs a new, empty linked hash set with the specified initial capacity and the default load factor (0.75).
LinkedHashSet(initialCapacity: Int, loadFactor: Float) Constructs a new, empty linked hash set with the specified initial capacity and load factor.
LinkedHashSet(c: MutableCollection<out E>!) Constructs a new linked hash set with the same elements as the specified collection.

Public methods
open Unit	addFirst(e: E) Adds an element as the first element of this collection (optional operation).
open Unit	addLast(e: E) Adds an element as the last element of this collection (optional operation).
open E	getFirst() Gets the first element of this collection.
open E	getLast() Gets the last element of this collection.
open static LinkedHashSet<T>!	newLinkedHashSet(numElements: Int) Creates a new, empty LinkedHashSet suitable for the expected number of elements.
open E	removeFirst() Removes and returns the first element of this collection (optional operation).
open E	removeLast() Removes and returns the last element of this collection (optional operation).
open SequencedSet<E>!	reversed() Returns a reverse-ordered view of this collection.
open Spliterator<E>	spliterator() Creates a late-binding and fail-fast Spliterator over the elements in this set.

Inherited functions
From class HashSet Boolean add(element: E) Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that Objects.equals(e, e2). If this set already contains the element, the call leaves the set unchanged and returns false. Unit clear() Removes all of the elements from this set. The set will be empty after this call returns. Any clone() Returns a shallow copy of this HashSet instance: the elements themselves are not cloned. Boolean contains(element: E) Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that Objects.equals(o, e). Boolean isEmpty() Returns true if this set contains no elements. MutableIterator<E> iterator() Returns an iterator over the elements in this set. The elements are returned in no particular order. HashSet<T> newHashSet(numElements: Int) Creates a new, empty HashSet 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. Boolean remove(element: E) Removes the specified element from this set if it is present. More formally, removes an element e such that Objects.equals(o, e), if this set contains such an element. Returns true if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.) Array<Any!> toArray() Array<T> toArray(a: Array<T>)
From class AbstractCollection Boolean add(element: E) Ensures that this collection contains the specified element (optional operation). Returns true if this collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.) Collections that support this operation may place limitations on what elements may be added to this collection. In particular, some collections will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added. If a collection refuses to add a particular element for any reason other than that it already contains the element, it must throw an exception (rather than returning false). This preserves the invariant that a collection always contains the specified element after this call returns. Boolean addAll(elements: Collection<E>) Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.) If the specified collection has a defined encounter order, processing of its elements generally occurs in that order. Unit clear() Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns. Boolean contains(element: E?) Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that Objects.equals(o, e). Boolean containsAll(elements: Collection<E>) Returns true if this collection contains all of the elements in the specified collection. Boolean isEmpty() Returns true if this collection contains no elements. MutableIterator<E> iterator() Returns an iterator over the elements contained in this collection. Boolean remove(element: E?) Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that Objects.equals(o, e), if this collection contains one or more such elements. Returns true if this collection contained the specified element (or equivalently, if this collection changed as a result of the call). Boolean retainAll(elements: Collection<E>) Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection. Array<Any!> toArray() Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array's runtime component type is Object. The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array. Array<T> toArray(a: Array<T>) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection. If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to null. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any null elements.) If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. String toString() Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).
From class AbstractSet Boolean equals(other: Any?) Compares the specified object with this set for equality. Returns true if the given object is also a set, the two sets have the same size, and every member of the given set is contained in this set. This ensures that the equals method works properly across different implementations of the Set interface. This implementation first checks if the specified object is this set; if so it returns true. Then, it checks if the specified object is a set whose size is identical to the size of this set; if not, it returns false. If so, it returns containsAll((Collection) o). Int hashCode() Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a null element is defined to be zero. This ensures that s1.equals(s2) implies that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of Object.hashCode. This implementation iterates over the set, calling the hashCode method on each element in the set, and adding up the results. Boolean removeAll(elements: Collection<E>) Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets. This implementation determines which is the smaller of this set and the specified collection, by invoking the size method on each. If this set has fewer elements, then the implementation iterates over this set, checking each element returned by the iterator in turn to see if it is contained in the specified collection. If it is so contained, it is removed from this set with the iterator's remove method. If the specified collection has fewer elements, then the implementation iterates over the specified collection, removing from this set each element returned by the iterator, using this set's remove method. Note that this implementation will throw an UnsupportedOperationException if the iterator returned by the iterator method does not implement the remove method.

Inherited properties
From class HashSet Int size Returns the number of elements in this set (its cardinality).
From class AbstractCollection Int size

Public constructors

LinkedHashSet

Added in API level 1

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

LinkedHashSet(initialCapacity: Int)

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

Parameters
initialCapacity	Int: the initial capacity of the LinkedHashSet

Exceptions
java.lang.IllegalArgumentException	if the initial capacity is less than zero

LinkedHashSet

Added in API level 1

LinkedHashSet(
    initialCapacity: Int, 
    loadFactor: Float)

Constructs a new, empty linked hash set with the specified initial capacity and load factor.

Parameters
initialCapacity	Int: the initial capacity of the linked hash set
loadFactor	Float: the load factor of the linked hash set

Exceptions
java.lang.IllegalArgumentException	if the initial capacity is less than zero, or if the load factor is nonpositive

LinkedHashSet

Added in API level 1

LinkedHashSet(c: MutableCollection<out E>!)

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	MutableCollection<out E>!: the collection whose elements are to be placed into this set

Exceptions
java.lang.NullPointerException	if the specified collection is null

Public methods

addFirst

Added in API level 35

open fun addFirst(e: E): Unit

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

Exceptions
java.lang.NullPointerException	if the specified element is null and this collection does not permit null elements
java.lang.UnsupportedOperationException	if this collection implementation does not support this operation

addLast

Added in API level 35

open fun addLast(e: E): Unit

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.

Exceptions
java.lang.NullPointerException	if the specified element is null and this collection does not permit null elements
java.lang.UnsupportedOperationException	if this collection implementation does not support this operation

getFirst

Added in API level 35

open fun getFirst(): E

Gets the first element of this collection.

Return
E	the retrieved element

Exceptions
java.util.NoSuchElementException	if this collection is empty

getLast

Added in API level 35

open fun getLast(): E

Gets the last element of this collection.

Return
E	the retrieved element

Exceptions
java.util.NoSuchElementException	if this collection is empty

newLinkedHashSet

Added in API level 35

open static fun <T : Any!> newLinkedHashSet(numElements: Int): LinkedHashSet<T>!

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
<T>	the type of elements maintained by the new set

Return
LinkedHashSet<T>!	the newly created set

Exceptions
java.lang.IllegalArgumentException	if numElements is negative

removeFirst

Added in API level 35

open fun removeFirst(): E

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

Return
E	the removed element

Exceptions
java.util.NoSuchElementException	if this collection is empty
java.lang.UnsupportedOperationException	if this collection implementation does not support this operation

removeLast

Added in API level 35

open fun removeLast(): E

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

Return
E	the removed element

Exceptions
java.util.NoSuchElementException	if this collection is empty
java.lang.UnsupportedOperationException	if this collection implementation does not support this operation

reversed

Added in API level 35

open fun reversed(): SequencedSet<E>!

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.

Return
SequencedSet<E>!	a reverse-ordered view of this collection, as a SequencedSet

spliterator

Added in API level 24

open fun spliterator(): Spliterator<E>

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.

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