# Class ConcurrentReferenceHashMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
org.apache.commons.collections4.map.ConcurrentReferenceHashMap<K,V>

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

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

public class ConcurrentReferenceHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>
An advanced hash map supporting configurable garbage collection semantics of keys and values, optional referential-equality, full concurrency of retrievals, and adjustable expected concurrency for updates.

This map is designed around specific advanced use-cases. If there is any doubt whether this map is for you, you most likely should be using `ConcurrentHashMap` instead.

This map supports strong, weak, and soft keys and values. By default, keys are weak, and values are strong. Such a configuration offers similar behavior to `WeakHashMap`, entries of this map are periodically removed once their corresponding keys are no longer referenced outside of this map. In other words, this map will not prevent a key from being discarded by the garbage collector. Once a key has been discarded by the collector, the corresponding entry is no longer visible to this map; however, the entry may occupy space until a future map operation decides to reclaim it. For this reason, summary functions such as `size` and `isEmpty` might return a value greater than the observed number of entries. In order to support a high level of concurrency, stale entries are only reclaimed during blocking (usually mutating) operations.

Enabling soft keys allows entries in this map to remain until their space is absolutely needed by the garbage collector. This is unlike weak keys which can be reclaimed as soon as they are no longer referenced by a normal strong reference. The primary use case for soft keys is a cache, which ideally occupies memory that is not in use for as long as possible.

By default, values are held using a normal strong reference. This provides the commonly desired guarantee that a value will always have at least the same life-span as its key. For this reason, care should be taken to ensure that a value never refers, either directly or indirectly, to its key, thereby preventing reclamation. If this is unavoidable, then it is recommended to use the same reference type in use for the key. However, it should be noted that non-strong values may disappear before their corresponding key.

While this map does allow the use of both strong keys and values, it is recommended you use `ConcurrentHashMap` for such a configuration, since it is optimized for that case.

Just like `ConcurrentHashMap`, this class obeys the same functional specification as `Hashtable`, and includes versions of methods corresponding to each method of `Hashtable`. However, even though all operations are thread-safe, retrieval operations do not entail locking, and there is not any support for locking the entire map in a way that prevents all access. This class is fully interoperable with `Hashtable` in programs that rely on its thread safety but not on its synchronization details.

Retrieval operations (including `get`) generally do not block, so they may overlap with update operations (including `put` and `remove`). Retrievals reflect the results of the most recently completed update operations holding upon their onset. For aggregate operations such as `putAll` and `clear`, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash map at some point at or since the creation of the iterator/enumeration. They do not throw `ConcurrentModificationException`. However, iterators are designed to be used by only one thread at a time.

The allowed concurrency among update operations is guided by the optional `concurrencyLevel` constructor argument (default 16), which is used as a hint for internal sizing. The map is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the map. Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention. But overestimates and underestimates within an order of magnitude do not usually have much noticeable impact. A value of one is appropriate when it is known that only one thread will modify and all others will only read. Also, resizing this or any other kind of hash map is a relatively slow operation, so, when possible, it is a good idea that you provide estimates of expected map sizes in constructors.

This class and its views and iterators implement all of the optional methods of the `Map` and `Iterator` interfaces.

Like `Hashtable` but unlike `HashMap`, this class does not allow `null` to be used as a key or value.

Provenance: Copied and edited from Apache Groovy git master at commit 77dc80a7512ceb2168b1bc866c3d0c69b002fe11; via Doug Lea, Jason T. Greene, with assistance from members of JCP JSR-166, and Hazelcast.

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `static class`
  `ConcurrentReferenceHashMap.Builder<K,V>`
  Builds new ConcurrentReferenceHashMap instances.
  `static enum`
  `ConcurrentReferenceHashMap.Option`
  Behavior-changing configuration options for the map
  `static enum`
  `ConcurrentReferenceHashMap.ReferenceType`
  An option specifying which Java reference type should be used to refer to a key and/or value.

  ### Nested classes/interfaces inherited from class java.util.AbstractMap

  `AbstractMap.SimpleImmutableEntry<K,V>`

  ### Nested classes/interfaces inherited from interface java.util.Map

  `Map.Entry<K,V>`

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static <K, V> ConcurrentReferenceHashMap.Builder<K,V>`
  `builder()`
  Creates a new Builder.
  `void`
  `clear()`
  Removes all of the mappings from this map.
  `V`
  `compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)`
  `V`
  `computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)`
  The default implementation is equivalent to the following steps for this `map`, then returning the current value or `null` if now absent:
  `V`
  `computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)`
  `boolean`
  `containsKey(Object key)`
  Tests if the specified object is a key in this table.
  `boolean`
  `containsValue(Object value)`
  Returns `true` if this map maps one or more keys to the specified value.
  `Set<Map.Entry<K,V>>`
  `entrySet()`
  Returns a `Set` view of the mappings contained in this map.
  `V`
  `get(Object key)`
  Gets the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.
  `boolean`
  `isEmpty()`
  Returns `true` if this map contains no key-value mappings.
  `Set<K>`
  `keySet()`
  Returns a `Set` view of the keys contained in this map.
  `void`
  `purgeStaleEntries()`
  Removes any stale entries whose keys have been finalized.
  `V`
  `put(K key, V value)`
  Maps the specified key to the specified value in this table.
  `void`
  `putAll(Map<? extends K,? extends V> m)`
  Copies all of the mappings from the specified map to this one.
  `V`
  `putIfAbsent(K key, V value)`
  `V`
  `remove(Object key)`
  Removes the key (and its corresponding value) from this map.
  `boolean`
  `remove(Object key, Object value)`
  `V`
  `replace(K key, V value)`
  `boolean`
  `replace(K key, V oldValue, V newValue)`
  `int`
  `size()`
  Returns the number of key-value mappings in this map.
  `Collection<V>`
  `values()`
  Returns a `Collection` view of the values contained in this map.

  ### Methods inherited from class java.util.AbstractMap

  `clone, equals, hashCode, toString`

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

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

  ### Methods inherited from interface java.util.concurrent.ConcurrentMap

  `forEach, getOrDefault, merge, replaceAll`

  ### Methods inherited from interface java.util.Map

  `equals, hashCode`

- ## Method Details

  - ### builder

    public static <K, V> ConcurrentReferenceHashMap.Builder<K,V> builder()
    Creates a new Builder.

    By default, keys are weak, and values are strong.

    The default values are:

    - concurrency level: 16
    - initial capacity: 16
    - key reference type: `ConcurrentReferenceHashMap.ReferenceType.WEAK`
    - load factor: 0.75f
    - options: `null`
    - source map: `null`
    - value reference type: `ConcurrentReferenceHashMap.ReferenceType.STRONG`
    Type Parameters:
      `K` - the type of keys.
      `V` - the type of values.
    Returns:
      a new Builder.

  - ### clear

    public void clear()
    Removes all of the mappings from this map.
    Specified by:
      `clear` in interface `Map<K,V>`
    Overrides:
      `clear` in class `AbstractMap<K,V>`

  - ### compute

    public V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
    Specified by:
      `compute` in interface `ConcurrentMap<K,V>`
    Specified by:
      `compute` in interface `Map<K,V>`

  - ### computeIfAbsent

    public V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
    The default implementation is equivalent to the following steps for this `map`, then returning the current value or `null` if now absent:

         if (map.get(key) == null) {
             V newValue = mappingFunction.apply(key);
             if (newValue != null)
                 return map.putIfAbsent(key, newValue);
         }

    The default implementation may retry these steps when multiple threads attempt updates including potentially calling the mapping function multiple times.

    This implementation assumes that the ConcurrentMap cannot contain null values and `get()` returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

    Specified by:
      `computeIfAbsent` in interface `ConcurrentMap<K,V>`
    Specified by:
      `computeIfAbsent` in interface `Map<K,V>`

  - ### computeIfPresent

    public V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
    Specified by:
      `computeIfPresent` in interface `ConcurrentMap<K,V>`
    Specified by:
      `computeIfPresent` in interface `Map<K,V>`

  - ### containsKey

    public boolean containsKey(Object key)
    Tests if the specified object is a key in this table.
    Specified by:
      `containsKey` in interface `Map<K,V>`
    Overrides:
      `containsKey` in class `AbstractMap<K,V>`
    Parameters:
      `key` - possible key
    Returns:
      `true` if and only if the specified object is a key in this table, as determined by the `equals` method; `false` otherwise.
    Throws:
      `NullPointerException` - if the specified key is null

  - ### containsValue

    public boolean containsValue(Object value)
    Returns `true` if this map maps one or more keys to the specified value. Note: This method requires a full internal traversal of the hash table, therefore it is much slower than the method `containsKey`.
    Specified by:
      `containsValue` in interface `Map<K,V>`
    Overrides:
      `containsValue` in class `AbstractMap<K,V>`
    Parameters:
      `value` - value whose presence in this map is to be tested
    Returns:
      `true` if this map maps one or more keys to the specified value
    Throws:
      `NullPointerException` - if the specified value is null

  - ### entrySet

    public Set<Map.Entry<K,V>> entrySet()
    Returns a `Set` view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from the map, via the `Iterator.remove`, `Set.remove`, `removeAll`, `retainAll`, and `clear` operations. It does not support the `add` or `addAll` operations.

    The view's `iterator` is a "weakly consistent" iterator that will never throw `ConcurrentModificationException`, and is guaranteed to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

    Specified by:
      `entrySet` in interface `Map<K,V>`
    Specified by:
      `entrySet` in class `AbstractMap<K,V>`

  - ### get

    public V get(Object key)
    Gets the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.

    If this map contains a mapping from a key `k` to a value `v` such that `key.equals(k)`, then this method returns `v`; otherwise it returns `null`. (There can be at most one such mapping.)

    Specified by:
      `get` in interface `Map<K,V>`
    Overrides:
      `get` in class `AbstractMap<K,V>`
    Throws:
      `NullPointerException` - if the specified key is null

  - ### isEmpty

    public boolean isEmpty()
    Returns `true` if this map contains no key-value mappings.
    Specified by:
      `isEmpty` in interface `Map<K,V>`
    Overrides:
      `isEmpty` in class `AbstractMap<K,V>`
    Returns:
      `true` if this map contains no key-value mappings

  - ### keySet

    public Set<K> keySet()
    Returns a `Set` view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from this map, via the `Iterator.remove`, `Set.remove`, `removeAll`, `retainAll`, and `clear` operations. It does not support the `add` or `addAll` operations.

    The view's `iterator` is a "weakly consistent" iterator that will never throw `ConcurrentModificationException`, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

    Specified by:
      `keySet` in interface `Map<K,V>`
    Overrides:
      `keySet` in class `AbstractMap<K,V>`

  - ### purgeStaleEntries

    public void purgeStaleEntries()
    Removes any stale entries whose keys have been finalized. Use of this method is normally not necessary since stale entries are automatically removed lazily, when blocking operations are required. However, there are some cases where this operation should be performed eagerly, such as cleaning up old references to a ClassLoader in a multi-classloader environment.

    Note: this method will acquire locks one at a time across all segments of this table, so this method should be used sparingly.

  - ### put

    public V put(K key, V value)
    Maps the specified key to the specified value in this table. Neither the key nor the value can be null.

    The value can be retrieved by calling the `get` method with a key that is equal to the original key.

    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`
    Throws:
      `NullPointerException` - if the specified key or value is null

  - ### putAll

    public void putAll(Map<? extends K,? extends V> m)
    Copies all of the mappings from the specified map to this one. These mappings 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>`
    Parameters:
      `m` - mappings to be stored in this map

  - ### putIfAbsent

    public V putIfAbsent(K key, V value)
    Specified by:
      `putIfAbsent` in interface `ConcurrentMap<K,V>`
    Specified by:
      `putIfAbsent` in interface `Map<K,V>`
    Returns:
      the previous value associated with the specified key, or `null` if there was no mapping for the key
    Throws:
      `NullPointerException` - if the specified key or value is null

  - ### remove

    public V remove(Object key)
    Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.
    Specified by:
      `remove` in interface `Map<K,V>`
    Overrides:
      `remove` in class `AbstractMap<K,V>`
    Parameters:
      `key` - the key that needs to be removed
    Returns:
      the previous value associated with `key`, or `null` if there was no mapping for `key`
    Throws:
      `NullPointerException` - if the specified key is null

  - ### remove

    public boolean remove(Object key, Object value)
    Specified by:
      `remove` in interface `ConcurrentMap<K,V>`
    Specified by:
      `remove` in interface `Map<K,V>`
    Throws:
      `NullPointerException` - if the specified key is null

  - ### replace

    public V replace(K key, V value)
    Specified by:
      `replace` in interface `ConcurrentMap<K,V>`
    Specified by:
      `replace` in interface `Map<K,V>`
    Returns:
      the previous value associated with the specified key, or `null` if there was no mapping for the key
    Throws:
      `NullPointerException` - if the specified key or value is null

  - ### replace

    public boolean replace(K key, V oldValue, V newValue)
    Specified by:
      `replace` in interface `ConcurrentMap<K,V>`
    Specified by:
      `replace` in interface `Map<K,V>`
    Throws:
      `NullPointerException` - if any of the arguments are null

  - ### size

    public int size()
    Returns the number of key-value mappings in this map. If the map contains more than `Integer.MAX_VALUE` elements, returns `Integer.MAX_VALUE`.
    Specified by:
      `size` in interface `Map<K,V>`
    Overrides:
      `size` in class `AbstractMap<K,V>`
    Returns:
      the number of key-value mappings in this map

  - ### values

    public Collection<V> values()
    Returns a `Collection` view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. The collection supports element removal, which removes the corresponding mapping from this map, via the `Iterator.remove`, `Collection.remove`, `removeAll`, `retainAll`, and `clear` operations. It does not support the `add` or `addAll` operations.

    The view's `iterator` is a "weakly consistent" iterator that will never throw `ConcurrentModificationException`, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

    Specified by:
      `values` in interface `Map<K,V>`
    Overrides:
      `values` in class `AbstractMap<K,V>`