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

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

All Implemented Interfaces:
  java.io.Serializable, java.lang.Cloneable, java.util.Map, BoundedMap, IterableMap, OrderedMap

    public class LRUMap

extends AbstractLinkedMap
implements BoundedMap, java.io.Serializable, java.lang.Cloneable

A `Map` implementation with a fixed maximum size which removes the least recently used entry if an entry is added when full.

The least recently used algorithm works on the get and put operations only. Iteration of any kind, including setting the value by iteration, does not change the order. Queries such as containsKey and containsValue or access via views also do not change the order.

The map implements `OrderedMap` and entries may be queried using the bidirectional `OrderedMapIterator`. The order returned is least recently used to most recently used. Iterators from map views can also be cast to `OrderedIterator` if required.

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

Note that LRUMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. The simplest approach is to wrap this map using `Collections.synchronizedMap(Map)`. This class may throw `NullPointerException`'s when accessed by concurrent threads.

Since:
  Commons Collections 3.0 (previously in main package v1.0)
Version:
  $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
Author:
  James Strachan, Morgan Delagrange, Stephen Colebourne, Mike Pettypiece, Mario Ivankovits
See Also:
  Serialized Form

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.commons.collections.map.AbstractLinkedMap
`AbstractLinkedMap.EntrySetIterator, AbstractLinkedMap.KeySetIterator, AbstractLinkedMap.LinkEntry, AbstractLinkedMap.LinkIterator, AbstractLinkedMap.LinkMapIterator, AbstractLinkedMap.ValuesIterator`

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

Nested classes/interfaces inherited from interface java.util.Map
`java.util.Map.Entry`

Field Summary
`protected static int` | `DEFAULT_MAX_SIZE` Default maximum size

Fields inherited from class org.apache.commons.collections.map.AbstractLinkedMap
`header`

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
`LRUMap()` Constructs a new empty map with a maximum size of 100.
`LRUMap(int maxSize)` Constructs a new, empty map with the specified maximum size.
`LRUMap(int maxSize, boolean scanUntilRemovable)` Constructs a new, empty map with the specified maximum size.
`LRUMap(int maxSize, float loadFactor)` Constructs a new, empty map with the specified initial capacity and load factor.
`LRUMap(int maxSize, float loadFactor, boolean scanUntilRemovable)` Constructs a new, empty map with the specified initial capacity and load factor.
`LRUMap(java.util.Map map)` Constructor copying elements from another map.
`LRUMap(java.util.Map map, boolean scanUntilRemovable)` Constructor copying elements from another map.

Method Summary
`protected void` | `addMapping(int hashIndex, int hashCode, java.lang.Object key, java.lang.Object value)` Adds a new key-value mapping into this map.
`java.lang.Object` | `clone()` Clones the map without cloning the keys or values.
`protected void` | `doReadObject(java.io.ObjectInputStream in)` Reads the data necessary for `put()` to work in the superclass.
`protected void` | `doWriteObject(java.io.ObjectOutputStream out)` Writes the data necessary for `put()` to work in deserialization.
`java.lang.Object` | `get(java.lang.Object key)` Gets the value mapped to the key specified.
`boolean` | `isFull()` Returns true if this map is full and no new mappings can be added.
`boolean` | `isScanUntilRemovable()` Whether this LRUMap will scan until a removable entry is found when the map is full.
`int` | `maxSize()` Gets the maximum size of the map (the bound).
`protected void` | `moveToMRU(AbstractLinkedMap.LinkEntry entry)` Moves an entry to the MRU position at the end of the list.
`protected boolean` | `removeLRU(AbstractLinkedMap.LinkEntry entry)` Subclass method to control removal of the least recently used entry from the map.
`protected void` | `reuseMapping(AbstractLinkedMap.LinkEntry entry, int hashIndex, int hashCode, java.lang.Object key, java.lang.Object value)` Reuses an entry by removing it and moving it to a new place in the map.
`protected void` | `updateEntry(AbstractHashedMap.HashEntry entry, java.lang.Object newValue)` Updates an existing key-value mapping.

Methods inherited from class org.apache.commons.collections.map.AbstractLinkedMap
`addEntry, clear, containsValue, createEntry, createEntrySetIterator, createKeySetIterator, createValuesIterator, entryAfter, entryBefore, firstKey, getEntry, init, lastKey, mapIterator, nextKey, orderedMapIterator, previousKey, removeEntry`

Methods inherited from class org.apache.commons.collections.map.AbstractHashedMap
`calculateNewCapacity, calculateThreshold, checkCapacity, containsKey, convertKey, destroyEntry, ensureCapacity, entryHashCode, entryKey, entryNext, entrySet, entryValue, equals, getEntry, hash, hashCode, hashIndex, isEmpty, isEqualKey, isEqualValue, keySet, put, putAll, remove, removeMapping, reuseEntry, size, toString, values`

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

Methods inherited from interface java.util.Map
`clear, containsKey, containsValue, entrySet, equals, hashCode, isEmpty, keySet, put, putAll, remove, size, values`

Field Detail

### DEFAULT_MAX_SIZE

    protected static final int DEFAULT_MAX_SIZE

  Default maximum size

  See Also:
    Constant Field Values

Constructor Detail

### LRUMap

    public LRUMap()

  Constructs a new empty map with a maximum size of 100.

### LRUMap

    public LRUMap(int maxSize)

  Constructs a new, empty map with the specified maximum size.

  Parameters:
    `maxSize` - the maximum size of the map
  Throws:
    `java.lang.IllegalArgumentException` - if the maximum size is less than one

### LRUMap

    public LRUMap(int maxSize,
                  boolean scanUntilRemovable)

  Constructs a new, empty map with the specified maximum size.

  Parameters:
    `maxSize` - the maximum size of the map
    `scanUntilRemovable` - scan until a removeable entry is found, default false
  Throws:
    `java.lang.IllegalArgumentException` - if the maximum size is less than one
  Since:
    Commons Collections 3.1

### LRUMap

    public LRUMap(int maxSize,
                  float loadFactor)

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

  Parameters:
    `maxSize` - the maximum size of the map, -1 for no limit,
    `loadFactor` - the load factor
  Throws:
    `java.lang.IllegalArgumentException` - if the maximum size is less than one
    `java.lang.IllegalArgumentException` - if the load factor is less than zero

### LRUMap

    public LRUMap(int maxSize,
                  float loadFactor,
                  boolean scanUntilRemovable)

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

  Parameters:
    `maxSize` - the maximum size of the map, -1 for no limit,
    `loadFactor` - the load factor
    `scanUntilRemovable` - scan until a removeable entry is found, default false
  Throws:
    `java.lang.IllegalArgumentException` - if the maximum size is less than one
    `java.lang.IllegalArgumentException` - if the load factor is less than zero
  Since:
    Commons Collections 3.1

### LRUMap

    public LRUMap(java.util.Map map)

  Constructor copying elements from another map.

  The maximum size is set from the map's size.

  Parameters:
    `map` - the map to copy
  Throws:
    `java.lang.NullPointerException` - if the map is null
    `java.lang.IllegalArgumentException` - if the map is empty

### LRUMap

    public LRUMap(java.util.Map map,
                  boolean scanUntilRemovable)

  Constructor copying elements from another map.

  The maximum size is set from the map's size.

  Parameters:
    `map` - the map to copy
    `scanUntilRemovable` - scan until a removeable entry is found, default false
  Throws:
    `java.lang.NullPointerException` - if the map is null
    `java.lang.IllegalArgumentException` - if the map is empty
  Since:
    Commons Collections 3.1

Method Detail

### get

    public java.lang.Object get(java.lang.Object key)

  Gets the value mapped to the key specified.

  This operation changes the position of the key in the map to the most recently used position (first).

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

### moveToMRU

    protected void moveToMRU(AbstractLinkedMap.LinkEntry entry)

  Moves an entry to the MRU position at the end of the list.

  This implementation moves the updated entry to the end of the list.

  Parameters:
    `entry` - the entry to update

### updateEntry

    protected void updateEntry(AbstractHashedMap.HashEntry entry,
                               java.lang.Object newValue)

  Updates an existing key-value mapping.

  This implementation moves the updated entry to the top of the list using `moveToMRU(AbstractLinkedMap.LinkEntry)`.

  Overrides:
    `updateEntry` in class `AbstractHashedMap`
  Parameters:
    `entry` - the entry to update
    `newValue` - the new value to store

### addMapping

    protected void addMapping(int hashIndex,
                              int hashCode,
                              java.lang.Object key,
                              java.lang.Object value)

  Adds a new key-value mapping into this map.

  This implementation checks the LRU size and determines whether to discard an entry or not using `removeLRU(AbstractLinkedMap.LinkEntry)`.

  From Commons Collections 3.1 this method uses `isFull()` rather than accessing `size` and `maxSize` directly. It also handles the scanUntilRemovable functionality.

  Overrides:
    `addMapping` in class `AbstractHashedMap`
  Parameters:
    `hashIndex` - the index into the data array to store at
    `hashCode` - the hash code of the key to add
    `key` - the key to add
    `value` - the value to add

### reuseMapping

    protected void reuseMapping(AbstractLinkedMap.LinkEntry entry,
                                int hashIndex,
                                int hashCode,
                                java.lang.Object key,
                                java.lang.Object value)

  Reuses an entry by removing it and moving it to a new place in the map.

  This method uses `AbstractLinkedMap.removeEntry(org.apache.commons.collections.map.AbstractHashedMap.HashEntry, int, org.apache.commons.collections.map.AbstractHashedMap.HashEntry)`, `AbstractHashedMap.reuseEntry(org.apache.commons.collections.map.AbstractHashedMap.HashEntry, int, int, java.lang.Object, java.lang.Object)` and `AbstractLinkedMap.addEntry(org.apache.commons.collections.map.AbstractHashedMap.HashEntry, int)`.

  Parameters:
    `entry` - the entry to reuse
    `hashIndex` - the index into the data array to store at
    `hashCode` - the hash code of the key to add
    `key` - the key to add
    `value` - the value to add

### removeLRU

    protected boolean removeLRU(AbstractLinkedMap.LinkEntry entry)

  Subclass method to control removal of the least recently used entry from the map.

  This method exists for subclasses to override. A subclass may wish to provide cleanup of resources when an entry is removed. For example:

       protected boolean removeLRU(LinkEntry entry) {
         releaseResources(entry.getValue());  // release resources held by entry
         return true;  // actually delete entry
       }

  Alternatively, a subclass may choose to not remove the entry or selectively keep certain LRU entries. For example:

       protected boolean removeLRU(LinkEntry entry) {
         if (entry.getKey().toString().startsWith("System.")) {
           return false;  // entry not removed from LRUMap
         } else {
           return true;  // actually delete entry
         }
       }

  The effect of returning false is dependent on the scanUntilRemovable flag. If the flag is true, the next LRU entry will be passed to this method and so on until one returns false and is removed, or every entry in the map has been passed. If the scanUntilRemovable flag is false, the map will exceed the maximum size.

  NOTE: Commons Collections 3.0 passed the wrong entry to this method. This is fixed in version 3.1 onwards.

  Parameters:
    `entry` - the entry to be removed

### isFull

    public boolean isFull()

  Returns true if this map is full and no new mappings can be added.

  Specified by:
    `isFull` in interface `BoundedMap`
  Returns:
    `true` if the map is full

### maxSize

    public int maxSize()

  Gets the maximum size of the map (the bound).

  Specified by:
    `maxSize` in interface `BoundedMap`
  Returns:
    the maximum number of elements the map can hold

### isScanUntilRemovable

    public boolean isScanUntilRemovable()

  Whether this LRUMap will scan until a removable entry is found when the map is full.

  Returns:
    true if this map scans
  Since:
    Commons Collections 3.1

### clone

    public java.lang.Object clone()

  Clones the map without cloning the keys or values.

  Overrides:
    `clone` in class `AbstractHashedMap`
  Returns:
    a shallow clone

### doWriteObject

    protected void doWriteObject(java.io.ObjectOutputStream out)
                          throws java.io.IOException

  Writes the data necessary for `put()` to work in deserialization.

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

### doReadObject

    protected void doReadObject(java.io.ObjectInputStream in)
                         throws java.io.IOException,
                                java.lang.ClassNotFoundException

  Reads the data necessary for `put()` to work in the superclass.

  Overrides:
    `doReadObject` in class `AbstractHashedMap`
  Parameters:
    `in` - the input stream
  Throws:
    `java.io.IOException`
    `java.lang.ClassNotFoundException`

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