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## java.util
Class WeakHashMap

    java.lang.Object
      |
      +--java.util.AbstractMap
            |
            +--java.util.WeakHashMap

All Implemented Interfaces:
  Map

public class WeakHashMap
extends AbstractMap
implements Map

A hashtable-based `Map` implementation with weak keys. An entry in a `WeakHashMap` will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently than other `Map` implementations.

Both null values and the null key are supported. This class has performance characteristics similar to those of the `HashMap` class, and has the same efficiency parameters of initial capacity and load factor.

Like most collection classes, this class is not synchronized. A synchronized `WeakHashMap` may be constructed using the `Collections.synchronizedMap` method.

This class is intended primarily for use with key objects whose `equals` methods test for object identity using the `==` operator. Once such a key is discarded it can never be recreated, so it is impossible to do a lookup of that key in a `WeakHashMap` at some later time and be surprised that its entry has been removed. This class will work perfectly well with key objects whose `equals` methods are not based upon object identity, such as `String` instances. With such recreatable key objects, however, the automatic removal of `WeakHashMap` entries whose keys have been discarded may prove to be confusing.

The behavior of the `WeakHashMap` class depends in part upon the actions of the garbage collector, so several familiar (though not required) `Map` invariants do not hold for this class. Because the garbage collector may discard keys at any time, a `WeakHashMap` may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a `WeakHashMap` instance and invoke none of its mutator methods, it is possible for the `size` method to return smaller values over time, for the `isEmpty` method to return `false` and then `true`, for the `containsKey` method to return `true` and later `false` for a given key, for the `get` method to return a value for a given key but later return `null`, for the `put` method to return `null` and the `remove` method to return `false` for a key that previously appeared to be in the map, and for successive examinations of the key set, the value set, and the entry set to yield successively smaller numbers of elements.

Each key object in a `WeakHashMap` is stored indirectly as the referent of a weak reference. Therefore a key will automatically be removed only after the weak references to it, both inside and outside of the map, have been cleared by the garbage collector.

Implementation note: The value objects in a `WeakHashMap` are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded. Note that a value object may refer indirectly to its key via the `WeakHashMap` itself; that is, a value object may strongly refer to some other key object whose associated value object, in turn, strongly refers to the key of the first value object. This problem may be fixed in a future release.

Since:
  1.2
See Also:
  `HashMap`, `WeakReference`

Inner classes inherited from class java.util.Map
`Map.Entry`

Constructor Summary
`WeakHashMap()` Constructs a new, empty `WeakHashMap` with the default initial capacity and the default load factor, which is `0.75`.
`WeakHashMap(int initialCapacity)` Constructs a new, empty `WeakHashMap` with the given initial capacity and the default load factor, which is `0.75`.
`WeakHashMap(int initialCapacity, float loadFactor)` Constructs a new, empty `WeakHashMap` with the given initial capacity and the given load factor.
`WeakHashMap(Map t)` Constructs a new `WeakHashMap` with the same mappings as the specified `Map`.

Method Summary
`void` | `clear()` Removes all mappings from this map.
`boolean` | `containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`Set` | `entrySet()` Returns a `Set` view of the mappings in this map.
`Object` | `get(Object key)` Returns the value to which this map maps the specified `key`.
`boolean` | `isEmpty()` Returns `true` if this map contains no key-value mappings.
`Object` | `put(Object key, Object value)` Updates this map so that the given `key` maps to the given `value`.
`Object` | `remove(Object key)` Removes the mapping for the given `key` from this map, if present.
`int` | `size()` Returns the number of key-value mappings in this map.

Methods inherited from class java.util.AbstractMap
`containsValue, equals, hashCode, keySet, putAll, toString, values`

Methods inherited from class java.lang.Object
`clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

Methods inherited from interface java.util.Map
`containsValue, equals, hashCode, keySet, putAll, values`

Constructor Detail

### WeakHashMap

    public WeakHashMap(int initialCapacity,
                       float loadFactor)

  Constructs a new, empty `WeakHashMap` with the given initial capacity and the given load factor.
  Parameters:
    `initialCapacity` - The initial capacity of the `WeakHashMap`
    `loadFactor` - The load factor of the `WeakHashMap`
  Throws:
    `IllegalArgumentException` - If the initial capacity is less than zero, or if the load factor is nonpositive

### WeakHashMap

    public WeakHashMap(int initialCapacity)

  Constructs a new, empty `WeakHashMap` with the given initial capacity and the default load factor, which is `0.75`.
  Parameters:
    `initialCapacity` - The initial capacity of the `WeakHashMap`
  Throws:
    `IllegalArgumentException` - If the initial capacity is less than zero

### WeakHashMap

    public WeakHashMap()

  Constructs a new, empty `WeakHashMap` with the default initial capacity and the default load factor, which is `0.75`.

### WeakHashMap

    public WeakHashMap(Map t)

  Constructs a new `WeakHashMap` with the same mappings as the specified `Map`. The `WeakHashMap` is created with an initial capacity of twice the number of mappings in the specified map or 11 (whichever is greater), and a default load factor, which is `0.75`.
  Parameters:
    `t` - the map whose mappings are to be placed in this map.
  Since:
    1.3

Method Detail

### size

    public int size()

  Returns the number of key-value mappings in this map. Note: In contrast with most implementations of the `Map` interface, the time required by this operation is linear in the size of the map.
  Specified by:
    `size` in interface `Map`
  Overrides:
    `size` in class `AbstractMap`
  Following copied from interface: `java.util.Map`
  Returns:
    the number of key-value mappings in this map.

### isEmpty

    public boolean isEmpty()

  Returns `true` if this map contains no key-value mappings.
  Specified by:
    `isEmpty` in interface `Map`
  Overrides:
    `isEmpty` in class `AbstractMap`
  Following copied from interface: `java.util.Map`
  Returns:
    `true` if this map contains no key-value mappings.

### containsKey

    public boolean containsKey(Object key)

  Returns `true` if this map contains a mapping for the specified key.
  Specified by:
    `containsKey` in interface `Map`
  Overrides:
    `containsKey` in class `AbstractMap`
  Parameters:
    `key` - The key whose presence in this map is to be tested

### get

    public Object get(Object key)

  Returns the value to which this map maps the specified `key`. If this map does not contain a value for this key, then return `null`.
  Specified by:
    `get` in interface `Map`
  Overrides:
    `get` in class `AbstractMap`
  Parameters:
    `key` - The key whose associated value, if any, is to be returned

### put

    public Object put(Object key,
                      Object value)

  Updates this map so that the given `key` maps to the given `value`. If the map previously contained a mapping for `key` then that mapping is replaced and the previous value is returned.
  Specified by:
    `put` in interface `Map`
  Overrides:
    `put` in class `AbstractMap`
  Parameters:
    `key` - The key that is to be mapped to the given `value`
    `value` - The value to which the given `key` is to be mapped
  Returns:
    The previous value to which this key was mapped, or `null` if if there was no mapping for the key

### remove

    public Object remove(Object key)

  Removes the mapping for the given `key` from this map, if present.
  Specified by:
    `remove` in interface `Map`
  Overrides:
    `remove` in class `AbstractMap`
  Parameters:
    `key` - The key whose mapping is to be removed
  Returns:
    The value to which this key was mapped, or `null` if there was no mapping for the key

### clear

    public void clear()

  Removes all mappings from this map.
  Specified by:
    `clear` in interface `Map`
  Overrides:
    `clear` in class `AbstractMap`
  Following copied from interface: `java.util.Map`
  Throws:
    `UnsupportedOperationException` - clear is not supported by this map.

### entrySet

    public Set entrySet()

  Returns a `Set` view of the mappings in this map.
  Specified by:
    `entrySet` in interface `Map`
  Overrides:
    `entrySet` in class `AbstractMap`
  Following copied from interface: `java.util.Map`
  Returns:
    a set view of the mappings contained in this map.

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