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## org.apache.openjpa.lib.util.concurrent
Class ConcurrentHashMap

    java.lang.Object
      java.util.AbstractMap
          org.apache.openjpa.lib.util.concurrent.ConcurrentHashMap

All Implemented Interfaces:
  Serializable, Cloneable, Map, ConcurrentMap, SizedMap

    public class ConcurrentHashMap

extends AbstractMap
implements ConcurrentMap, SizedMap, Cloneable, Serializable

This class implements a HashMap which has limited synchronization. In particular mutators are generally synchronized while accessors are generally not. Additionally the Iterators returned by this class are not "fail-fast", but instead try to continue to iterate over the data structure after changes have been made. The synchronization semantics are built right in to the implementation rather than using a delegating wrapper like the other collection classes do because it wasn't clear to me that the how the two should be seperated or that it would be useful to do so. This can probably be a topic for further debate in the future. This class is based heavily on the HashMap class in the Java collections package.

See Also:
  Serialized Form

Nested Class Summary
`protected static class` | `ConcurrentHashMap.Entry`

Nested classes/interfaces inherited from class java.util.AbstractMap
`AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>`

Field Summary
`(package private) static double[]` | `RANDOMS` Cache of random numbers used in "random" methods, since generating them is expensive.

Constructor Summary
`ConcurrentHashMap()` Constructs an empty `ConcurrentHashMap` with the default initial capacity(16) and the default load factor(0.75).
`ConcurrentHashMap(int initialCapacity)` Constructs an empty `ConcurrentHashMap` with the specified initial capacity and the default load factor(0.75).
`ConcurrentHashMap(int initialCapacity, float loadFactor)` Constructs an empty `ConcurrentHashMap` with the specified initial capacity and load factor.
`ConcurrentHashMap(Map m)` Constructs a new `ConcurrentHashMap` with the same mappings as the specified `Map`.

Method Summary
`int` | `capacity()` Returns the current capacity of backing table in this map.
`void` | `clear()` Removes all mappings from this map.
`Object` | `clone()` Returns a shallow copy of this `ConcurrentHashMap` instance: the keys and values themselves are not cloned.
`boolean` | `containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`boolean` | `containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
`protected ConcurrentHashMap.Entry` | `createEntry(int h, Object k, Object v, ConcurrentHashMap.Entry n)`
`Set` | `entrySet()` Returns a collection view of the mappings contained in this map.
`Object` | `get(Object key)` Returns the value to which the specified key is mapped in this identity hash map, or `null` if the map contains no mapping for this key.
`protected ConcurrentHashMap.Entry` | `getEntry(Object key)` Returns the entry associated with the specified key in the ConcurrentHashMap.
`int` | `getMaxSize()` The maximum number of entries, or Integer.MAX_VALUE for no limit.
`boolean` | `isEmpty()` Returns `true` if this map contains no key-value mappings.
`boolean` | `isFull()` Whether the map is full.
`Set` | `keySet()` Returns a set view of the keys contained in this map.
`float` | `loadFactor()` Returns the load factor for this map.
`void` | `overflowRemoved(Object key, Object value)` Overridable callback for when an overflow entry is automatically removed.
`Object` | `put(Object key, Object value)` Associates the specified value with the specified key in this map.
`void` | `putAll(Map t)` 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.
`Object` | `putIfAbsent(Object key, Object value)`
`Iterator` | `randomEntryIterator()` Iterate over map entries, beginning at an arbitrary (not strictly random) entry.
`Object` | `remove(Object key)` Removes the mapping for this key from this map if present.
`Map.Entry` | `removeRandom()` Remove an arbitrary(not strictly random) entry from the map.
`void` | `setMaxSize(int maxSize)` The maximum number of entries, or Integer.MAX_VALUE for no limit.
`int` | `size()` Returns the number of key-value mappings in this map.
`Collection` | `values()` Returns a collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap
`equals, hashCode, toString`

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

Methods inherited from interface java.util.Map
`equals, hashCode`

Field Detail

### RANDOMS

    static final double[] RANDOMS

  Cache of random numbers used in "random" methods, since generating them is expensive. We hope each map changes enough between cycling through this list that the overall effect is random enough.

Constructor Detail

### ConcurrentHashMap

    public ConcurrentHashMap(int initialCapacity,
                             float loadFactor)

  Constructs an empty `ConcurrentHashMap` with the specified initial capacity and load factor.

  Parameters:
    `initialCapacity` - The initial capacity.
    `loadFactor` - The load factor.
  Throws:
    `IllegalArgumentException` - if the initial capacity is negative or the load factor is nonpositive.

### ConcurrentHashMap

    public ConcurrentHashMap(int initialCapacity)

  Constructs an empty `ConcurrentHashMap` with the specified initial capacity and the default load factor(0.75).

  Parameters:
    `initialCapacity` - the initial capacity.
  Throws:
    `IllegalArgumentException` - if the initial capacity is negative.

### ConcurrentHashMap

    public ConcurrentHashMap()

  Constructs an empty `ConcurrentHashMap` with the default initial capacity(16) and the default load factor(0.75).

### ConcurrentHashMap

    public ConcurrentHashMap(Map m)

  Constructs a new `ConcurrentHashMap` with the same mappings as the specified `Map`. The `ConcurrentHashMap` is created with default load factor(0.75) and an initial capacity sufficient to hold the mappings in the specified `Map`.

  Parameters:
    `m` - the map whose mappings are to be placed in this map.
  Throws:
    `NullPointerException` - if the specified map is null.

Method Detail

### capacity

    public final int capacity()

  Returns the current capacity of backing table in this map.

  Returns:
    the current capacity of backing table in this map.

### loadFactor

    public final float loadFactor()

  Returns the load factor for this map.

  Returns:
    the load factor for this map.

### getMaxSize

    public int getMaxSize()

  Description copied from interface: `SizedMap`
  The maximum number of entries, or Integer.MAX_VALUE for no limit.

  Specified by:
    `getMaxSize` in interface `SizedMap`

### setMaxSize

    public void setMaxSize(int maxSize)

  Description copied from interface: `SizedMap`
  The maximum number of entries, or Integer.MAX_VALUE for no limit.

  Specified by:
    `setMaxSize` in interface `SizedMap`

### isFull

    public boolean isFull()

  Description copied from interface: `SizedMap`
  Whether the map is full.

  Specified by:
    `isFull` in interface `SizedMap`

### overflowRemoved

    public void overflowRemoved(Object key,
                                Object value)

  Description copied from interface: `SizedMap`
  Overridable callback for when an overflow entry is automatically removed.

  Specified by:
    `overflowRemoved` in interface `SizedMap`

### size

    public final int size()

  Returns the number of key-value mappings in this map.

  Specified by:
    `size` in interface `Map`
  Overrides:
    `size` in class `AbstractMap`
  Returns:
    the number of key-value mappings in this map.

### isEmpty

    public final boolean isEmpty()

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

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

### get

    public Object get(Object key)

  Returns the value to which the specified key is mapped in this identity hash map, or `null` if the map contains no mapping for this key. A return value of `null` does not necessarily indicate that the map contains no mapping for the key; it is also possible that the map explicitly maps the key to `null`. The `containsKey` method may be used to distinguish these two cases.

  Specified by:
    `get` in interface `Map`
  Overrides:
    `get` in class `AbstractMap`
  Parameters:
    `key` - the key whose associated value is to be returned.
  Returns:
    the value to which this map maps the specified key, or `null` if the map contains no mapping for this key.
  See Also:
    `put(Object, Object)`

### containsKey

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

### getEntry

    protected ConcurrentHashMap.Entry getEntry(Object key)

  Returns the entry associated with the specified key in the ConcurrentHashMap. Returns null if the ConcurrentHashMap contains no mapping for this key.

### put

    public Object put(Object key,
                      Object 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`
  Overrides:
    `put` in class `AbstractMap`
  Parameters:
    `key` - key with which the specified value is to be associated.
    `value` - value to be associated with the specified key.
  Returns:
    previous value associated with specified key, or `null` if there was no mapping for key. A `null` return can also indicate that the ConcurrentHashMap previously associated `null` with the specified key.

### putIfAbsent

    public Object putIfAbsent(Object key,
                              Object value)

### putAll

    public final void putAll(Map t)

  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`
  Overrides:
    `putAll` in class `AbstractMap`
  Parameters:
    `t` - mappings to be stored in this map.
  Throws:
    `NullPointerException` - if the specified map is null.

### remove

    public Object remove(Object key)

  Removes the mapping for this key from this map if present.

  Specified by:
    `remove` in interface `Map`
  Overrides:
    `remove` in class `AbstractMap`
  Parameters:
    `key` - key whose mapping is to be removed from the map.
  Returns:
    previous value associated with specified key, or `null` if there was no mapping for key. A `null` return can also indicate that the map previously associated `null` with the specified key.

### clear

    public void clear()

  Removes all mappings from this map.

  Specified by:
    `clear` in interface `Map`
  Overrides:
    `clear` in class `AbstractMap`

### removeRandom

    public Map.Entry removeRandom()

  Description copied from interface: `ConcurrentMap`
  Remove an arbitrary(not strictly random) entry from the map. This allows implementation of concurrent caches with size ceilings.

  Specified by:
    `removeRandom` in interface `ConcurrentMap`
  Returns:
    the removed entry, or null if map is empty

### randomEntryIterator

    public Iterator randomEntryIterator()

  Description copied from interface: `ConcurrentMap`
  Iterate over map entries, beginning at an arbitrary (not strictly random) entry.

  Specified by:
    `randomEntryIterator` in interface `ConcurrentMap`

### containsValue

    public final boolean containsValue(Object value)

  Returns `true` if this map maps one or more keys to the specified value.

  Specified by:
    `containsValue` in interface `Map`
  Overrides:
    `containsValue` in class `AbstractMap`
  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.

### clone

    public final Object clone()

  Returns a shallow copy of this `ConcurrentHashMap` instance: the keys and values themselves are not cloned.

  Overrides:
    `clone` in class `AbstractMap`
  Returns:
    a shallow copy of this map.

### createEntry

    protected ConcurrentHashMap.Entry createEntry(int h,
                                                  Object k,
                                                  Object v,
                                                  ConcurrentHashMap.Entry n)

### keySet

    public final Set 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.

  Specified by:
    `keySet` in interface `Map`
  Overrides:
    `keySet` in class `AbstractMap`
  Returns:
    a set view of the keys contained in this map.

### values

    public final Collection 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.

  Specified by:
    `values` in interface `Map`
  Overrides:
    `values` in class `AbstractMap`
  Returns:
    a collection view of the values contained in this map.

### entrySet

    public final Set entrySet()

  Returns a collection view of the mappings contained in this map. Each element in the returned collection is a `Map.Entry`. 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 the map, via the `Iterator.remove`, `Collection.remove`, `removeAll`, `retainAll`, and `clear` operations. It does not support the `add` or `addAll` operations.

  Specified by:
    `entrySet` in interface `Map`
  Specified by:
    `entrySet` in class `AbstractMap`
  Returns:
    a collection view of the mappings contained in this map.
  See Also:
    `ConcurrentHashMap.Entry`

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