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# LinkedHashSet

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

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 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`

## Summary

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

### Public methods
`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.
`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.
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). This implementation always throws an `UnsupportedOperationException`.
`boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
`void` | `clear()` Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
`boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
`boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
`boolean` | `isEmpty()` Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
`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). This implementation iterates over the collection looking for the specified element.
`boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`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. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index `0`.
`Object[]` | `toArray()` Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index `0`.
`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 timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
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.
`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 immutable 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 immutable set containing nine elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5)` Returns an immutable set containing five elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6)` Returns an immutable 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 immutable set containing eight elements.
`static <E> Set<E>` | `of()` Returns an immutable 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 immutable set containing ten elements.
`static <E> Set<E>` | `of(E e1, E e2)` Returns an immutable set containing two elements.
`static <E> Set<E>` | `of(E... elements)` Returns an immutable set containing an arbitrary number of elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)` Returns an immutable set containing seven elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4)` Returns an immutable set containing four elements.
`static <E> Set<E>` | `of(E e1)` Returns an immutable 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.
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`.

## Public constructors

### LinkedHashSet

    public LinkedHashSet (int initialCapacity,
                    float loadFactor)

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

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).

Parameters
`initialCapacity` | `int`: 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` | `Collection`: the collection whose elements are to be placed into this set

Throws
`NullPointerException` | if the specified collection is null

## Public methods

### 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.

##### 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

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