Module java.base
Package java.util

# Class WeakHashMap

≠
public class WeakHashMap<K, V>
extends java.util.AbstractMap<K, V>
implements java.util.Map<K, V>
Comparing jdk-master and valhalla-csr-open

Type Parameters:
  `K` - the type of keys maintained by this map
  `V` - the type of mapped values

All Implemented Interfaces:
  `Map<K, V>`

public class WeakHashMap<K, V> extends AbstractMap<K, V> implements Map<K, V>
Hash table based implementation of the `Map` interface, 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 from 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 collection, 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. If the values in the map do not rely on the map holding strong references to them, one way to deal with this is to wrap values themselves within `WeakReferences` before inserting, as in: `m.put(key, new WeakReference(value))` , and then unwrapping upon each `get` .

The iterators returned by the `iterator` method of the collections returned by all of this class's "collection view methods" are fail-fast: if the map is structurally 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.

API Note:
  Objects that are value objectsPREVIEW do not have identity and can not be used as keys in a `WeakHashMap` . References such as WeakReference used by `WeakhashMap` to hold the key cannot refer to a value object. Methods such as get or containsKey will always return `null` or `false` respectively. The methods such as put, putAll, compute, and computeIfAbsent or any method putting a value object, as a key, throw `IdentityException` PREVIEW .
Since:
  1.2
See Also:
  - `HashMap`
  - `WeakReference`

## Constructors

- =
  public WeakHashMap()
- =
  public WeakHashMap(int arg0)
- =
  public WeakHashMap(int arg0, float arg1)
- =
  public WeakHashMap(java.util.Map<? extends K, ? extends V> arg0)

## Methods

- =
  public void clear()
- =
  public boolean containsKey(java.lang.Object arg0)
- =
  public boolean containsValue(java.lang.Object arg0)
- =
  public java.util.Set<java.util.Map.Entry<K, V>> entrySet()
- =
  public void forEach(java.util.function.BiConsumer<? super K, ? super V> arg0)
- =
  public V get(java.lang.Object arg0)
- =
  public boolean isEmpty()
- =
  public java.util.Set<K> keySet()
- =
  public static <K, V> java.util.WeakHashMap<K, V> newWeakHashMap(int arg0)
- ≠
  public V put(K arg0, V arg1)
  Comparing jdk-master and valhalla-csr-open

  ### put

  public V put (K key, V value)
  Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.
  Specified by:
    `put` in interface `Map<K, V>`
  Overrides:
    `put` in class `AbstractMap<K, V>`
  Parameters:
    `key` - key with which the specified value is to be associated.
    `value` - value to be associated with the specified key.
  Returns:
    the previous value associated with `key` , or `null` if there was no mapping for `key` . (A `null` return can also indicate that the map previously associated `null` with `key` .)
  Throws:
    `IdentityExceptionPREVIEW` - if `key` is a value object
- ≠
  public void putAll(java.util.Map<? extends K, ? extends V> arg0)
  Comparing jdk-master and valhalla-csr-open

  ### putAll

  public void putAll (Map<? extends K, ? extends V> m)
  Copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.
  Specified by:
    `putAll` in interface `Map<K, V>`
  Overrides:
    `putAll` in class `AbstractMap<K, V>`
  API Note:
    If the specified map contains keys that are value objectsPREVIEW an IdentityException is thrown when the first value object key is encountered. Zero or more mappings may have already been copied to this map.
  Parameters:
    `m` - mappings to be stored in this map.
  Throws:
    `NullPointerException` - if the specified map is null.
    `IdentityExceptionPREVIEW` - if any of the `keys` is a value object
- =
  public V remove(java.lang.Object arg0)
- =
  public void replaceAll(java.util.function.BiFunction<? super K, ? super V, ? extends V> arg0)
- =
  public int size()
- =
  public java.util.Collection<V> values()

## Summary

Differences
| Elements | Comments | Descriptions | Total
Added | Changed | Removed | Added | Changed | Removed | Added | Changed | Removed
--- | --- | --- | --- | --- | --- | --- | --- | ---
WeakHashMap |  |  |  |  |  |  | 1 |  |  | 1
put(K,V) |  |  |  |  |  |  | 4 |  |  | 4
putAll(Map) |  |  |  |  |  |  | 9 |  |  | 9
Total |  |  |  |  |  |  | 14 |  |  | 14