Package org.apache.commons.collections.map

Class LRUMap

  • All Implemented Interfaces:
    Serializable, Cloneable, Map, BoundedMap, IterableMap, OrderedMap
    public class LRUMap
extends AbstractLinkedMap
implements BoundedMap, 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.

    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 14:33:15 +0200 (Thu, 10 Apr 2008) $
    Author:
    James Strachan, Morgan Delagrange, Stephen Colebourne, Mike Pettypiece, Mario Ivankovits
    See Also:
    Serialized Form

    • 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:
        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:
        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:
        IllegalArgumentException - if the maximum size is less than one
        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:
        IllegalArgumentException - if the maximum size is less than one
        IllegalArgumentException - if the load factor is less than zero
        Since:
        Commons Collections 3.1

      • LRUMap

        public LRUMap​(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:
        NullPointerException - if the map is null
        IllegalArgumentException - if the map is empty

      • LRUMap

        public LRUMap​(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:
        NullPointerException - if the map is null
        IllegalArgumentException - if the map is empty
        Since:
        Commons Collections 3.1

    • Method Detail

      • get

        public Object 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 (first).

        Specified by:
        get in interface 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

      • addMapping

        protected void addMapping​(int hashIndex,
                          int hashCode,
                          Object key,
                          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

      • 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 Object clone()
        Clones the map without cloning the keys or values.
        Overrides:
        clone in class AbstractHashedMap
        Returns:
        a shallow clone