## org.apache.commons.collections.map
Class AbstractReferenceMap

    java.lang.Object
      java.util.AbstractMap
          org.apache.commons.collections.map.AbstractHashedMap
              org.apache.commons.collections.map.AbstractReferenceMap

All Implemented Interfaces:
  IterableMap, Map

Direct Known Subclasses:
  ReferenceIdentityMap, ReferenceMap

public abstract class AbstractReferenceMap
extends AbstractHashedMap

An abstract implementation of a hash-based map that allows the entries to be removed by the garbage collector.

This class implements all the features necessary for a subclass reference hash-based map. Key-value entries are stored in instances of the `ReferenceEntry` class which can be overridden and replaced. The iterators can similarly be replaced, without the need to replace the KeySet, EntrySet and Values view classes.

Overridable methods are provided to change the default hashing behaviour, and to change how entries are added to and removed from the map. Hopefully, all you need for unusual subclasses is here.

When you construct an `AbstractReferenceMap`, you can specify what kind of references are used to store the map's keys and values. If non-hard references are used, then the garbage collector can remove mappings if a key or value becomes unreachable, or if the JVM's memory is running low. For information on how the different reference types behave, see `Reference`.

Different types of references can be specified for keys and values. The keys can be configured to be weak but the values hard, in which case this class will behave like a `WeakHashMap`. However, you can also specify hard keys and weak values, or any other combination. The default constructor uses hard keys and soft values, providing a memory-sensitive cache.

This `Map` implementation does not allow null elements. Attempting to add a null key or value to the map will raise a `NullPointerException`.

All the available iterators can be reset back to the start by casting to `ResettableIterator` and calling `reset()`.

This implementation is not synchronized. You can use `Collections.synchronizedMap(java.util.Map)` to provide synchronized access to a `ReferenceMap`.

Since:
  Commons Collections 3.1 (extracted from ReferenceMap in 3.0)
Version:
  $Revision: 1.3 $ $Date: 2004/06/07 22:14:42 $
Author:
  Paul Jack, Stephen Colebourne
See Also:
  `Reference`

Nested Class Summary
`protected static class` `AbstractReferenceMap.ReferenceEntry` A MapEntry implementation for the map.

Nested classes inherited from class org.apache.commons.collections.map.AbstractHashedMap
`AbstractHashedMap.EntrySet, AbstractHashedMap.EntrySetIterator, AbstractHashedMap.HashEntry, AbstractHashedMap.HashIterator, AbstractHashedMap.HashMapIterator, AbstractHashedMap.KeySet, AbstractHashedMap.KeySetIterator, AbstractHashedMap.Values, AbstractHashedMap.ValuesIterator`

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

Field Summary
`static int` `HARD` Constant indicating that hard references should be used
`protected int` `keyType` The reference type for keys.
`protected boolean` `purgeValues` Should the value be automatically purged when the associated key has been collected?
`static int` `SOFT` Constant indicating that soft references should be used
`protected int` `valueType` The reference type for values.
`static int` `WEAK` Constant indicating that weak references should be used

Fields inherited from class org.apache.commons.collections.map.AbstractHashedMap
`data, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_THRESHOLD, entrySet, GETKEY_INVALID, GETVALUE_INVALID, keySet, loadFactor, MAXIMUM_CAPACITY, modCount, NO_NEXT_ENTRY, NO_PREVIOUS_ENTRY, NULL, REMOVE_INVALID, SETVALUE_INVALID, size, threshold, values`

Constructor Summary
`protected` `AbstractReferenceMap()` Constructor used during deserialization.
`protected` `AbstractReferenceMap(int keyType, int valueType, int capacity, float loadFactor, boolean purgeValues)` Constructs a new empty map with the specified reference types, load factor and initial capacity.

Method Summary
`void` `clear()` Clears this map.
`boolean` `containsKey(Object key)` Checks whether the map contains the specified key.
`boolean` `containsValue(Object value)` Checks whether the map contains the specified value.
`protected AbstractHashedMap.HashEntry` `createEntry(AbstractHashedMap.HashEntry next, int hashCode, Object key, Object value)` Creates a ReferenceEntry instead of a HashEntry.
`protected Iterator` `createEntrySetIterator()` Creates an entry set iterator.
`protected Iterator` `createKeySetIterator()` Creates an key set iterator.
`protected Iterator` `createValuesIterator()` Creates an values iterator.
`protected void` `doReadObject(ObjectInputStream in)` Replaces the superclassm method to read the state of this class.
`protected void` `doWriteObject(ObjectOutputStream out)` Replaces the superclass method to store the state of this class.
`Set` `entrySet()` Returns a set view of this map's entries.
`Object` `get(Object key)` Gets the value mapped to the key specified.
`protected AbstractHashedMap.HashEntry` `getEntry(Object key)` Gets the entry mapped to the key specified.
`protected int` `hashEntry(Object key, Object value)` Gets the hash code for a MapEntry.
`protected void` `init()` Initialise this subclass during construction, cloning or deserialization.
`boolean` `isEmpty()` Checks whether the map is currently empty.
`protected boolean` `isEqualKey(Object key1, Object key2)` Compares two keys, in internal converted form, to see if they are equal.
`Set` `keySet()` Returns a set view of this map's keys.
`MapIterator` `mapIterator()` Gets a MapIterator over the reference map.
`protected void` `purge()` Purges stale mappings from this map.
`protected void` `purge(Reference ref)` Purges the specified reference.
`protected void` `purgeBeforeRead()` Purges stale mappings from this map before read operations.
`protected void` `purgeBeforeWrite()` Purges stale mappings from this map before write operations.
`Object` `put(Object key, Object value)` Puts a key-value mapping into this map.
`Object` `remove(Object key)` Removes the specified mapping from this map.
`int` `size()` Gets the size of the map.
`Collection` `values()` Returns a collection view of this map's values.

Methods inherited from class org.apache.commons.collections.map.AbstractHashedMap
`addEntry, addMapping, calculateNewCapacity, calculateThreshold, checkCapacity, clone, convertKey, destroyEntry, ensureCapacity, entryHashCode, entryKey, entryNext, entryValue, equals, hash, hashCode, hashIndex, isEqualValue, putAll, removeEntry, removeMapping, reuseEntry, toString, updateEntry`

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

Field Detail

### HARD

    public static final int HARD

  Constant indicating that hard references should be used

  See Also:
    Constant Field Values

### SOFT

    public static final int SOFT

  Constant indicating that soft references should be used

  See Also:
    Constant Field Values

### WEAK

    public static final int WEAK

  Constant indicating that weak references should be used

  See Also:
    Constant Field Values

### keyType

    protected int keyType

  The reference type for keys. Must be HARD, SOFT, WEAK.

### valueType

    protected int valueType

  The reference type for values. Must be HARD, SOFT, WEAK.

### purgeValues

    protected boolean purgeValues

  Should the value be automatically purged when the associated key has been collected?

Constructor Detail

### AbstractReferenceMap

    protected AbstractReferenceMap()

  Constructor used during deserialization.

### AbstractReferenceMap

    protected AbstractReferenceMap(int keyType,
                                   int valueType,
                                   int capacity,
                                   float loadFactor,
                                   boolean purgeValues)

  Constructs a new empty map with the specified reference types, load factor and initial capacity.

Parameters:
  `keyType` - the type of reference to use for keys; must be `HARD`, `SOFT`, `WEAK`
  `valueType` - the type of reference to use for values; must be `HARD`, `SOFT`, `WEAK`
  `capacity` - the initial capacity for the map
  `loadFactor` - the load factor for the map
  `purgeValues` - should the value be automatically purged when the key is garbage collected

Method Detail

### init

    protected void init()

  Initialise this subclass during construction, cloning or deserialization.

  Overrides:
    `init` in class `AbstractHashedMap`

### size

    public int size()

  Gets the size of the map.

  Specified by:
    `size` in interface `Map`
  Overrides:
    `size` in class `AbstractHashedMap`
  Returns:
    the size

### isEmpty

    public boolean isEmpty()

  Checks whether the map is currently empty.

  Specified by:
    `isEmpty` in interface `Map`
  Overrides:
    `isEmpty` in class `AbstractHashedMap`
  Returns:
    true if the map is currently size zero

### containsKey

    public boolean containsKey(Object key)

  Checks whether the map contains the specified key.

  Specified by:
    `containsKey` in interface `Map`
  Overrides:
    `containsKey` in class `AbstractHashedMap`
  Parameters:
    `key` - the key to search for
  Returns:
    true if the map contains the key

### containsValue

    public boolean containsValue(Object value)

  Checks whether the map contains the specified value.

  Specified by:
    `containsValue` in interface `Map`
  Overrides:
    `containsValue` in class `AbstractHashedMap`
  Parameters:
    `value` - the value to search for
  Returns:
    true if the map contains the value

### get

    public Object get(Object key)

  Gets the value mapped to the key specified.

  Specified by:
    `get` in interface `Map`
  Overrides:
    `get` in class `AbstractHashedMap`
  Parameters:
    `key` - the key
  Returns:
    the mapped value, null if no match

### put

    public Object put(Object key,
                      Object value)

  Puts a key-value mapping into this map. Neither the key nor the value may be null.

  Specified by:
    `put` in interface `Map`
  Overrides:
    `put` in class `AbstractHashedMap`
  Parameters:
    `key` - the key to add, must not be null
    `value` - the value to add, must not be null
  Returns:
    the value previously mapped to this key, null if none
  Throws:
    `NullPointerException` - if either the key or value is null

### remove

    public Object remove(Object key)

  Removes the specified mapping from this map.

  Specified by:
    `remove` in interface `Map`
  Overrides:
    `remove` in class `AbstractHashedMap`
  Parameters:
    `key` - the mapping to remove
  Returns:
    the value mapped to the removed key, null if key not in map

### clear

    public void clear()

  Clears this map.

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

### mapIterator

    public MapIterator mapIterator()

  Gets a MapIterator over the reference map. The iterator only returns valid key/value pairs.

  Specified by:
    `mapIterator` in interface `IterableMap`
  Overrides:
    `mapIterator` in class `AbstractHashedMap`
  Returns:
    a map iterator

### entrySet

    public Set entrySet()

  Returns a set view of this map's entries. An iterator returned entry is valid until `next()` is called again. The `setValue()` method on the `toArray` entries has no effect.

  Specified by:
    `entrySet` in interface `Map`
  Overrides:
    `entrySet` in class `AbstractHashedMap`
  Returns:
    a set view of this map's entries

### keySet

    public Set keySet()

  Returns a set view of this map's keys.

  Specified by:
    `keySet` in interface `Map`
  Overrides:
    `keySet` in class `AbstractHashedMap`
  Returns:
    a set view of this map's keys

### values

    public Collection values()

  Returns a collection view of this map's values.

  Specified by:
    `values` in interface `Map`
  Overrides:
    `values` in class `AbstractHashedMap`
  Returns:
    a set view of this map's values

### purgeBeforeRead

    protected void purgeBeforeRead()

  Purges stale mappings from this map before read operations.

  This implementation calls `purge()` to maintain a consistent state.

### purgeBeforeWrite

    protected void purgeBeforeWrite()

  Purges stale mappings from this map before write operations.

  This implementation calls `purge()` to maintain a consistent state.

### purge

    protected void purge()

  Purges stale mappings from this map.

  Note that this method is not synchronized! Special care must be taken if, for instance, you want stale mappings to be removed on a periodic basis by some background thread.

### purge

    protected void purge(Reference ref)

  Purges the specified reference.

  Parameters:
    `ref` - the reference to purge

### getEntry

    protected AbstractHashedMap.HashEntry getEntry(Object key)

  Gets the entry mapped to the key specified.

  Overrides:
    `getEntry` in class `AbstractHashedMap`
  Parameters:
    `key` - the key
  Returns:
    the entry, null if no match

### hashEntry

    protected int hashEntry(Object key,
                            Object value)

  Gets the hash code for a MapEntry. Subclasses can override this, for example to use the identityHashCode.

  Parameters:
    `key` - the key to get a hash code for, may be null
    `value` - the value to get a hash code for, may be null
  Returns:
    the hash code, as per the MapEntry specification

### isEqualKey

    protected boolean isEqualKey(Object key1,
                                 Object key2)

  Compares two keys, in internal converted form, to see if they are equal.

  This implementation converts the key from the entry to a real reference before comparison.

  Overrides:
    `isEqualKey` in class `AbstractHashedMap`
  Parameters:
    `key1` - the first key to compare passed in from outside
    `key2` - the second key extracted from the entry via `entry.key`
  Returns:
    true if equal

### createEntry

    protected AbstractHashedMap.HashEntry createEntry(AbstractHashedMap.HashEntry next,
                                                      int hashCode,
                                                      Object key,
                                                      Object value)

  Creates a ReferenceEntry instead of a HashEntry.

  Overrides:
    `createEntry` in class `AbstractHashedMap`
  Parameters:
    `next` - the next entry in sequence
    `hashCode` - the hash code to use
    `key` - the key to store
    `value` - the value to store
  Returns:
    the newly created entry

### createEntrySetIterator

    protected Iterator createEntrySetIterator()

  Creates an entry set iterator.

  Overrides:
    `createEntrySetIterator` in class `AbstractHashedMap`
  Returns:
    the entrySet iterator

### createKeySetIterator

    protected Iterator createKeySetIterator()

  Creates an key set iterator.

  Overrides:
    `createKeySetIterator` in class `AbstractHashedMap`
  Returns:
    the keySet iterator

### createValuesIterator

    protected Iterator createValuesIterator()

  Creates an values iterator.

  Overrides:
    `createValuesIterator` in class `AbstractHashedMap`
  Returns:
    the values iterator

### doWriteObject

    protected void doWriteObject(ObjectOutputStream out)
                          throws IOException

  Replaces the superclass method to store the state of this class.

  Serialization is not one of the JDK's nicest topics. Normal serialization will initialise the superclass before the subclass. Sometimes however, this isn't what you want, as in this case the `put()` method on read can be affected by subclass state.

  The solution adopted here is to serialize the state data of this class in this protected method. This method must be called by the `writeObject()` of the first serializable subclass.

  Subclasses may override if they have a specific field that must be present on read before this implementation will work. Generally, the read determines what must be serialized here, if anything.

  Overrides:
    `doWriteObject` in class `AbstractHashedMap`
  Parameters:
    `out` - the output stream
  Throws:
    `IOException`

### doReadObject

    protected void doReadObject(ObjectInputStream in)
                         throws IOException,
                                ClassNotFoundException

  Replaces the superclassm method to read the state of this class.

  Serialization is not one of the JDK's nicest topics. Normal serialization will initialise the superclass before the subclass. Sometimes however, this isn't what you want, as in this case the `put()` method on read can be affected by subclass state.

  The solution adopted here is to deserialize the state data of this class in this protected method. This method must be called by the `readObject()` of the first serializable subclass.

  Subclasses may override if the subclass has a specific field that must be present before `put()` or `calculateThreshold()` will work correctly.

  Overrides:
    `doReadObject` in class `AbstractHashedMap`
  Parameters:
    `in` - the input stream
  Throws:
    `IOException`
    `ClassNotFoundException`

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