Package org.apache.commons.collections4.map

Class LRUMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
org.apache.commons.collections4.map.AbstractHashedMap<K,V>
org.apache.commons.collections4.map.AbstractLinkedMap<K,V>
org.apache.commons.collections4.map.LRUMap<K,V>
Type Parameters:
K - the type of the keys in this map
V - the type of the values in this map
All Implemented Interfaces:
Serializable, Cloneable, Map<K,V>, BoundedMap<K,V>, Get<K,V>, IterableGet<K,V>, IterableMap<K,V>, OrderedMap<K,V>, Put<K,V>
public class LRUMap<K,V> extends AbstractLinkedMap<K,V> implements BoundedMap<K,V>, Serializable, 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.

A somewhat subtle ramification of the least recently used algorithm is that calls to get(Object) stand a very good chance of modifying the map's iteration order and thus invalidating any iterators currently in use. It is therefore suggested that iterations over an LRUMap instance access entry values only through a MapIterator or AbstractHashedMap.entrySet() iterator.

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:
3.0 (previously in main package v1.0)
See Also:

  • Constructor Details

    • 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:
      IllegalArgumentException - if the maximum size is less than one

    • LRUMap

      public LRUMap(int maxSize, int initialSize)
      Constructs a new, empty map with the specified maximum size.
      Parameters:
      maxSize - the maximum size of the map
      initialSize - the initial size of the map
      Throws:
      IllegalArgumentException - if the maximum size is less than one
      IllegalArgumentException - if the initial size is negative or larger than the maximum size
      Since:
      4.1

    • 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:
      IllegalArgumentException - if the maximum size is less than one
      Since:
      3.1

    • LRUMap

      public LRUMap(int maxSize, float loadFactor)
      Constructs a new, empty map with the specified max capacity and load factor.
      Parameters:
      maxSize - the maximum size of the map
      loadFactor - the load factor
      Throws:
      IllegalArgumentException - if the maximum size is less than one
      IllegalArgumentException - if the load factor is less than zero

    • LRUMap

      public LRUMap(int maxSize, int initialSize, float loadFactor)
      Constructs a new, empty map with the specified max / initial capacity and load factor.
      Parameters:
      maxSize - the maximum size of the map
      initialSize - the initial size of the map
      loadFactor - the load factor
      Throws:
      IllegalArgumentException - if the maximum size is less than one
      IllegalArgumentException - if the initial size is negative or larger than the maximum size
      IllegalArgumentException - if the load factor is less than zero
      Since:
      4.1

    • LRUMap

      public LRUMap(int maxSize, float loadFactor, boolean scanUntilRemovable)
      Constructs a new, empty map with the specified max capacity and load factor.
      Parameters:
      maxSize - the maximum size of the map
      loadFactor - the load factor
      scanUntilRemovable - scan until a removeable entry is found, default false
      Throws:
      IllegalArgumentException - if the maximum size is less than one
      IllegalArgumentException - if the load factor is less than zero
      Since:
      3.1

    • LRUMap

      public LRUMap(int maxSize, int initialSize, float loadFactor, boolean scanUntilRemovable)
      Constructs a new, empty map with the specified max / initial capacity and load factor.
      Parameters:
      maxSize - the maximum size of the map
      initialSize - the initial size of the map
      loadFactor - the load factor
      scanUntilRemovable - scan until a removeable entry is found, default false
      Throws:
      IllegalArgumentException - if the maximum size is less than one
      IllegalArgumentException - if the initial size is negative or larger than the maximum size
      IllegalArgumentException - if the load factor is less than zero
      Since:
      4.1

    • LRUMap

      public LRUMap(Map<? extends K,? extends V> map)
      Constructor copying elements from another map.

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

      Parameters:
      map - the map to copy
      Throws:
      NullPointerException - if the map is null
      IllegalArgumentException - if the map is empty

    • LRUMap

      public LRUMap(Map<? extends K,? extends V> 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:
      NullPointerException - if the map is null
      IllegalArgumentException - if the map is empty
      Since:
      3.1

  • Method Details

    • get

      public V get(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 (last).

      Specified by:
      get in interface Get<K,V>
      Specified by:
      get in interface Map<K,V>
      Overrides:
      get in class AbstractHashedMap<K,V>
      Parameters:
      key - the key
      Returns:
      the mapped value, null if no match
      See Also:

    • get

      public V get(Object key, boolean updateToMRU)
      Gets the value mapped to the key specified.

      If updateToMRU is true, the position of the key in the map is changed to the most recently used position (last), otherwise the iteration order is not changed by this operation.

      Parameters:
      key - the key
      updateToMRU - whether the key shall be updated to the most recently used position
      Returns:
      the mapped value, null if no match
      Since:
      4.1

    • isFull

      public boolean isFull()
      Returns true if this map is full and no new mappings can be added.
      Specified by:
      isFull in interface BoundedMap<K,V>
      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<K,V>
      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:
      3.1

    • clone

      public LRUMap<K,V> clone()
      Clones the map without cloning the keys or values.
      Returns:
      a shallow clone