java.util

## Class HashSet<E>

- java.lang.Object
- - java.util.AbstractCollection<E>
  - - java.util.AbstractSet<E>
    - - java.util.HashSet<E>

- Type Parameters:
    `E` - the type of elements maintained by this set
  All Implemented Interfaces:
    Serializable, Cloneable, Iterable<E>, Collection<E>, Set<E>
  Direct Known Subclasses:
    LinkedHashSet

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

  This class implements the `Set` interface, backed by a hash table (actually a `HashMap` instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the `null` element.

  This class offers constant time performance for the basic operations (`add`, `remove`, `contains` and `size`), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the `HashSet` instance's size (the number of elements) plus the "capacity" of the backing `HashMap` instance (the number of buckets). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

  Note that this implementation is not synchronized. If multiple threads access a 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 HashSet(...));

  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 throws 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:
    1.2
  Author:
    Josh Bloch, Neal Gafter
  See Also:
    `Collection`, `Set`, `TreeSet`, `HashMap`, Serialized Form

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `HashSet()` Constructs a new, empty set; the backing `HashMap` instance has default initial capacity (16) and load factor (0.75).
    `HashSet(Collection<? extends E> c)` Constructs a new set containing the elements in the specified collection.
    `HashSet(int initialCapacity)` Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and default load factor (0.75).
    `HashSet(int initialCapacity, float loadFactor)` Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and the specified load factor.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `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).

    - ### Methods inherited from class java.util.AbstractSet

      `equals, hashCode, removeAll`

    - ### Methods inherited from class java.util.AbstractCollection

      `addAll, containsAll, retainAll, toArray, toArray, toString`

    - ### Methods inherited from class java.lang.Object

      `finalize, getClass, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.Set

      `addAll, containsAll, equals, hashCode, removeAll, retainAll, toArray, toArray`

    - ### Methods inherited from interface java.util.Collection

      `parallelStream, removeIf, spliterator, stream`

    - ### Methods inherited from interface java.lang.Iterable

      `forEach`

- - ### Constructor Detail

    - #### HashSet

          public HashSet()

      Constructs a new, empty set; the backing `HashMap` instance has default initial capacity (16) and load factor (0.75).

    - #### HashSet

          public HashSet(Collection<? extends E> c)

      Constructs a new set containing the elements in the specified collection. The `HashMap` is created with default load factor (0.75) and an initial capacity sufficient to contain the elements in the specified collection.
      Parameters:
        `c` - the collection whose elements are to be placed into this set
      Throws:
        `NullPointerException` - if the specified collection is null

    - #### HashSet

          public HashSet(int initialCapacity,
                         float loadFactor)

      Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and the specified load factor.
      Parameters:
        `initialCapacity` - the initial capacity of the hash map
        `loadFactor` - the load factor of the hash map
      Throws:
        `IllegalArgumentException` - if the initial capacity is less than zero, or if the load factor is nonpositive

    - #### HashSet

          public HashSet(int initialCapacity)

      Constructs a new, empty set; the backing `HashMap` instance has the specified initial capacity and default load factor (0.75).
      Parameters:
        `initialCapacity` - the initial capacity of the hash table
      Throws:
        `IllegalArgumentException` - if the initial capacity is less than zero

  - ### Method Detail

    - #### iterator

          public Iterator<E> iterator()

      Returns an iterator over the elements in this set. The elements are returned in no particular order.
      Specified by:
        `iterator` in interface `Iterable<E>`
      Specified by:
        `iterator` in interface `Collection<E>`
      Specified by:
        `iterator` in interface `Set<E>`
      Specified by:
        `iterator` in class `AbstractCollection<E>`
      Returns:
        an Iterator over the elements in this set
      See Also:
        `ConcurrentModificationException`

    - #### size

          public int size()

      Returns the number of elements in this set (its cardinality).
      Specified by:
        `size` in interface `Collection<E>`
      Specified by:
        `size` in interface `Set<E>`
      Specified by:
        `size` in class `AbstractCollection<E>`
      Returns:
        the number of elements in this set (its cardinality)

    - #### isEmpty

          public boolean isEmpty()

      Returns `true` if this set contains no elements.
      Specified by:
        `isEmpty` in interface `Collection<E>`
      Specified by:
        `isEmpty` in interface `Set<E>`
      Overrides:
        `isEmpty` in class `AbstractCollection<E>`
      Returns:
        `true` if this set contains no elements

    - #### contains

          public boolean contains(Object o)

      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 `(o==null ? e==null : o.equals(e))`.
      Specified by:
        `contains` in interface `Collection<E>`
      Specified by:
        `contains` in interface `Set<E>`
      Overrides:
        `contains` in class `AbstractCollection<E>`
      Parameters:
        `o` - element whose presence in this set is to be tested
      Returns:
        `true` if this set contains the specified element

    - #### add

          public boolean add(E 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 `(e==null ? e2==null : e.equals(e2))`. If this set already contains the element, the call leaves the set unchanged and returns `false`.
      Specified by:
        `add` in interface `Collection<E>`
      Specified by:
        `add` in interface `Set<E>`
      Overrides:
        `add` in class `AbstractCollection<E>`
      Parameters:
        `e` - element to be added to this set
      Returns:
        `true` if this set did not already contain the specified element

    - #### remove

          public boolean remove(Object o)

      Removes the specified element from this set if it is present. More formally, removes an element `e` such that `(o==null ? e==null : o.equals(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.)
      Specified by:
        `remove` in interface `Collection<E>`
      Specified by:
        `remove` in interface `Set<E>`
      Overrides:
        `remove` in class `AbstractCollection<E>`
      Parameters:
        `o` - object to be removed from this set, if present
      Returns:
        `true` if the set contained the specified element

    - #### clear

          public void clear()

      Removes all of the elements from this set. The set will be empty after this call returns.
      Specified by:
        `clear` in interface `Collection<E>`
      Specified by:
        `clear` in interface `Set<E>`
      Overrides:
        `clear` in class `AbstractCollection<E>`

    - #### clone

          public Object clone()

      Returns a shallow copy of this `HashSet` instance: the elements themselves are not cloned.
      Overrides:
        `clone` in class `Object`
      Returns:
        a shallow copy of this set
      See Also:
        `Cloneable`

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