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## java.util
Class HashSet

    java.lang.Object
      |
      +--java.util.AbstractCollection
            |
            +--java.util.AbstractSet
                  |
                  +--java.util.HashSet

All Implemented Interfaces:
  Cloneable, Collection, Serializable, Set

public class HashSet
extends AbstractSet
implements Set, 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 intial 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 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 `HashSet` instance:

         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.

Since:
  1.2
See Also:
  `Collection`, `Set`, `TreeSet`, `Collections.synchronizedSet(Set)`, `HashMap`, Serialized Form

Constructor Summary
`HashSet()` Constructs a new, empty set; the backing `HashMap` instance has default capacity and load factor, which is `0.75`.
`HashSet(Collection 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, which is `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
`boolean` | `add(Object o)` 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` | `iterator()` Returns an iterator over the elements in this set.
`boolean` | `remove(Object o)` Removes the given 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`

Constructor Detail

### HashSet

    public HashSet()

  Constructs a new, empty set; the backing `HashMap` instance has default capacity and load factor, which is `0.75`.

### HashSet

    public HashSet(Collection c)

  Constructs a new set containing the elements in the specified collection. The capacity of the backing `HashMap` instance is twice the size of the specified collection or eleven (whichever is greater), and the default load factor (which is `0.75`) is used.
  Parameters:
    `c` - the collection whose elements are to be placed into this set.

### 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, which is `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 iterator()

  Returns an iterator over the elements in this set. The elements are returned in no particular order.
  Specified by:
    `iterator` in interface `Set`
  Overrides:
    `iterator` in class `AbstractCollection`
  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 `Set`
  Overrides:
    `size` in class `AbstractCollection`
  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 `Set`
  Overrides:
    `isEmpty` in class `AbstractCollection`
  Returns:
    `true` if this set contains no elements.

### contains

    public boolean contains(Object o)

  Returns `true` if this set contains the specified element.
  Specified by:
    `contains` in interface `Set`
  Overrides:
    `contains` in class `AbstractCollection`
  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(Object o)

  Adds the specified element to this set if it is not already present.
  Specified by:
    `add` in interface `Set`
  Overrides:
    `add` in class `AbstractCollection`
  Parameters:
    `o` - element to be added to this set.
  Returns:
    `true` if the set did not already contain the specified element.

### remove

    public boolean remove(Object o)

  Removes the given element from this set if it is present.
  Specified by:
    `remove` in interface `Set`
  Overrides:
    `remove` in class `AbstractCollection`
  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.
  Specified by:
    `clear` in interface `Set`
  Overrides:
    `clear` in class `AbstractCollection`
  Following copied from interface: `java.util.Set`
  Throws:
    `UnsupportedOperationException` - if the `clear` method is not supported by this set.

### 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.

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