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## org.apache.lucene.util
Class WeakIdentityMap<K,V>

    java.lang.Object
      org.apache.lucene.util.WeakIdentityMap<K,V>

    public final class WeakIdentityMap<K,V>

extends Object

Implements a combination of `WeakHashMap` and `IdentityHashMap`. Useful for caches that need to key off of a `==` comparison instead of a `.equals`.

This class is not a general-purpose `Map` implementation! It intentionally violates Map's general contract, which mandates the use of the equals method when comparing objects. This class is designed for use only in the rare cases wherein reference-equality semantics are required.

This implementation was forked from Apache CXF but modified to not implement the `Map` interface and without any set views on it, as those are error-prone and inefficient, if not implemented carefully. The map only contains `Iterator` implementations on the values and not-GCed keys. Lucene's implementation also supports `null` keys, but those are never weak!

The map supports two modes of operation:

- `reapOnRead = true`: This behaves identical to a `WeakHashMap` where it also cleans up the reference queue on every read operation (`get(Object)`, `containsKey(Object)`, `size()`, `valueIterator()`), freeing map entries of already GCed keys.
- `reapOnRead = false`: This mode does not call `reap()` on every read operation. In this case, the reference queue is only cleaned up on write operations (like `put(Object, Object)`). This is ideal for maps with few entries where the keys are unlikely be garbage collected, but there are lots of `get(Object)` operations. The code can still call `reap()` to manually clean up the queue without doing a write operation.

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

Method Summary
`void` | `clear()` Removes all of the mappings from this map.
`boolean` | `containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`V` | `get(Object key)` Returns the value to which the specified key is mapped.
`boolean` | `isEmpty()` Returns `true` if this map contains no key-value mappings.
`Iterator<K>` | `keyIterator()` Returns an iterator over all weak keys of this map.
`static
`<K,V> WeakIdentityMap<K,V>`
` | `newConcurrentHashMap()` Creates a new `WeakIdentityMap` based on a `ConcurrentHashMap`.
`static
`<K,V> WeakIdentityMap<K,V>`
` | `newConcurrentHashMap(boolean reapOnRead)` Creates a new `WeakIdentityMap` based on a `ConcurrentHashMap`.
`static
`<K,V> WeakIdentityMap<K,V>`
` | `newHashMap()` Creates a new `WeakIdentityMap` based on a non-synchronized `HashMap`.
`static
`<K,V> WeakIdentityMap<K,V>`
` | `newHashMap(boolean reapOnRead)` Creates a new `WeakIdentityMap` based on a non-synchronized `HashMap`.
`V` | `put(K key, V value)` Associates the specified value with the specified key in this map.
`void` | `reap()` This method manually cleans up the reference queue to remove all garbage collected key/value pairs from the map.
`V` | `remove(Object key)` Removes the mapping for a key from this weak hash map if it is present.
`int` | `size()` Returns the number of key-value mappings in this map.
`Iterator<V>` | `valueIterator()` Returns an iterator over all values of this map.

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

Method Detail

### newHashMap

    public static <K,V> WeakIdentityMap<K,V> newHashMap()

  Creates a new `WeakIdentityMap` based on a non-synchronized `HashMap`. The map cleans up the reference queue on every read operation.

### newHashMap

    public static <K,V> WeakIdentityMap<K,V> newHashMap(boolean reapOnRead)

  Creates a new `WeakIdentityMap` based on a non-synchronized `HashMap`.

  Parameters:
    `reapOnRead` - controls if the map cleans up the reference queue on every read operation.

### newConcurrentHashMap

    public static <K,V> WeakIdentityMap<K,V> newConcurrentHashMap()

  Creates a new `WeakIdentityMap` based on a `ConcurrentHashMap`. The map cleans up the reference queue on every read operation.

### newConcurrentHashMap

    public static <K,V> WeakIdentityMap<K,V> newConcurrentHashMap(boolean reapOnRead)

  Creates a new `WeakIdentityMap` based on a `ConcurrentHashMap`.

  Parameters:
    `reapOnRead` - controls if the map cleans up the reference queue on every read operation.

### clear

    public void clear()

  Removes all of the mappings from this map.

### containsKey

    public boolean containsKey(Object key)

  Returns `true` if this map contains a mapping for the specified key.

### get

    public V get(Object key)

  Returns the value to which the specified key is mapped.

### 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.

### isEmpty

    public boolean isEmpty()

  Returns `true` if this map contains no key-value mappings.

### remove

    public V remove(Object key)

  Removes the mapping for a key from this weak hash map if it is present. Returns the value to which this map previously associated the key, or `null` if the map contained no mapping for the key. A return value of `null` does not necessarily indicate that the map contained.

### size

    public int size()

  Returns the number of key-value mappings in this map. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced.

### keyIterator

    public Iterator<K> keyIterator()

  Returns an iterator over all weak keys of this map. Keys already garbage collected will not be returned. This Iterator does not support removals.

### valueIterator

    public Iterator<V> valueIterator()

  Returns an iterator over all values of this map. This iterator may return values whose key is already garbage collected while iterator is consumed, especially if `reapOnRead` is `false`.

### reap

    public void reap()

  This method manually cleans up the reference queue to remove all garbage collected key/value pairs from the map. Calling this method is not needed if `reapOnRead = true`. Otherwise it might be a good idea to call this method when there is spare time (e.g. from a background thread).

  See Also:
    Information about the `reapOnRead` setting

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