# 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:
  - Serialized Form

- ## Nested Class Summary

  ### Nested classes/interfaces inherited from class org.apache.commons.collections4.map.AbstractLinkedMap

  `AbstractLinkedMap.EntrySetIterator<K,V>, AbstractLinkedMap.KeySetIterator<K>, AbstractLinkedMap.LinkEntry<K,V>, AbstractLinkedMap.LinkIterator<K,V>, AbstractLinkedMap.LinkMapIterator<K,V>, AbstractLinkedMap.ValuesIterator<V>`

  ### Nested classes/interfaces inherited from class org.apache.commons.collections4.map.AbstractHashedMap

  `AbstractHashedMap.EntrySet<K,V>, AbstractHashedMap.HashEntry<K,V>, AbstractHashedMap.HashIterator<K,V>, AbstractHashedMap.HashMapIterator<K,V>, AbstractHashedMap.KeySet<K>, AbstractHashedMap.Values<V>`

  ### Nested classes/interfaces inherited from class java.util.AbstractMap

  `AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>`

  ### Nested classes/interfaces inherited from interface java.util.Map

  `Map.Entry<K,V>`

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `protected static final int`
  `DEFAULT_MAX_SIZE`
  Default maximum size

  ### Fields inherited from class org.apache.commons.collections4.map.AbstractHashedMap

  `DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_THRESHOLD, GETKEY_INVALID, GETVALUE_INVALID, MAXIMUM_CAPACITY, NO_NEXT_ENTRY, NO_PREVIOUS_ENTRY, NULL, REMOVE_INVALID, SETVALUE_INVALID`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `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 max capacity and load factor.
  `LRUMap(int maxSize, float loadFactor, boolean scanUntilRemovable)`
  Constructs a new, empty map with the specified max capacity and load factor.
  `LRUMap(int maxSize, int initialSize)`
  Constructs a new, empty map with the specified maximum size.
  `LRUMap(int maxSize, int initialSize, float loadFactor)`
  Constructs a new, empty map with the specified max / initial capacity and load factor.
  `LRUMap(int maxSize, int initialSize, float loadFactor, boolean scanUntilRemovable)`
  Constructs a new, empty map with the specified max / initial capacity and load factor.
  `LRUMap(Map<? extends K,? extends V> map)`
  Constructor copying elements from another map.
  `LRUMap(Map<? extends K,? extends V> map, boolean scanUntilRemovable)`
  Constructor copying elements from another map.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected void`
  `addMapping(int hashIndex, int hashCode, K key, V value)`
  Adds a new key-value mapping into this map.
  `LRUMap<K,V>`
  `clone()`
  Clones the map without cloning the keys or values.
  `protected void`
  `doReadObject(ObjectInputStream in)`
  Reads the data necessary for `put()` to work in the superclass.
  `protected void`
  `doWriteObject(ObjectOutputStream out)`
  Writes the data necessary for `put()` to work in deserialization.
  `V`
  `get(Object key)`
  Gets the value mapped to the key specified.
  `V`
  `get(Object key, boolean updateToMRU)`
  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()`
  Tests 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<K,V> entry)`
  Moves an entry to the MRU position at the end of the list.
  `protected boolean`
  `removeLRU(AbstractLinkedMap.LinkEntry<K,V> entry)`
  Subclass method to control removal of the least recently used entry from the map.
  `protected void`
  `reuseMapping(AbstractLinkedMap.LinkEntry<K,V> entry, int hashIndex, int hashCode, K key, V value)`
  Reuses an entry by removing it and moving it to a new place in the map.
  `protected void`
  `updateEntry(AbstractHashedMap.HashEntry<K,V> entry, V newValue)`
  Updates an existing key-value mapping.

  ### Methods inherited from class org.apache.commons.collections4.map.AbstractLinkedMap

  `addEntry, clear, containsValue, createEntry, createEntrySetIterator, createKeySetIterator, createValuesIterator, entryAfter, entryBefore, firstKey, getEntry, getEntry, init, lastKey, mapIterator, nextKey, previousKey, removeEntry`

  ### Methods inherited from class org.apache.commons.collections4.map.AbstractHashedMap

  `calculateNewCapacity, calculateThreshold, checkCapacity, containsKey, convertKey, destroyEntry, ensureCapacity, entryHashCode, entryKey, entryNext, entrySet, entryValue, equals, 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 org.apache.commons.collections4.Get

  `containsKey, containsValue, entrySet, isEmpty, keySet, remove, size, values`

  ### Methods inherited from interface java.util.Map

  `clear, compute, computeIfAbsent, computeIfPresent, containsKey, containsValue, entrySet, equals, forEach, getOrDefault, hashCode, isEmpty, keySet, merge, put, putAll, putIfAbsent, remove, remove, replace, replace, replaceAll, size, values`

  ### Methods inherited from interface org.apache.commons.collections4.Put

  `clear, put, putAll`

- ## Field Details

  - ### DEFAULT_MAX_SIZE

    protected static final int DEFAULT_MAX_SIZE
    Default maximum size
    See Also:
      - Constant Field Values

- ## 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, boolean scanUntilRemovable)
    Constructs a new, empty map with the specified maximum size.
    Parameters:
      `maxSize` - the maximum size of the map
      `scanUntilRemovable` - scan until a removable 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, 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 removable 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)
    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, 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, 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 removable 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 removable entry is found, default false
    Throws:
      `NullPointerException` - if the map is null
      `IllegalArgumentException` - if the map is empty
    Since:
      3.1

- ## Method Details

  - ### addMapping

    protected void addMapping(int hashIndex, int hashCode, K key, V 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<K,V>`
    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

  - ### clone

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

  - ### doReadObject

    protected void doReadObject(ObjectInputStream in) throws IOException, ClassNotFoundException
    Reads the data necessary for `put()` to work in the superclass.
    Overrides:
      `doReadObject` in class `AbstractHashedMap<K,V>`
    Parameters:
      `in` - the input stream
    Throws:
      `IOException` - if an error occurs while reading from the stream
      `ClassNotFoundException` - if an object read from the stream cannot be loaded

  - ### doWriteObject

    protected void doWriteObject(ObjectOutputStream out) throws IOException
    Writes the data necessary for `put()` to work in deserialization.
    Overrides:
      `doWriteObject` in class `AbstractHashedMap<K,V>`
    Parameters:
      `out` - the output stream
    Throws:
      `IOException` - if an error occurs while writing to the stream

  - ### 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:
      - `Map.get(Object)`

  - ### 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

  - ### isScanUntilRemovable

    public boolean isScanUntilRemovable()
    Tests 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

  - ### 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

  - ### moveToMRU

    protected void moveToMRU(AbstractLinkedMap.LinkEntry<K,V> 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

  - ### removeLRU

    protected boolean removeLRU(AbstractLinkedMap.LinkEntry<K,V> 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
    Returns:
      `true`

  - ### reuseMapping

    protected void reuseMapping(AbstractLinkedMap.LinkEntry<K,V> entry, int hashIndex, int hashCode, K key, V 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.collections4.map.AbstractHashedMap.HashEntry<K, V>, int, org.apache.commons.collections4.map.AbstractHashedMap.HashEntry<K, V>)`, `AbstractHashedMap.reuseEntry(org.apache.commons.collections4.map.AbstractHashedMap.HashEntry<K, V>, int, int, K, V)` and `AbstractLinkedMap.addEntry(org.apache.commons.collections4.map.AbstractHashedMap.HashEntry<K, V>, 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

  - ### updateEntry

    protected void updateEntry(AbstractHashedMap.HashEntry<K,V> entry, V newValue)
    Updates an existing key-value mapping.

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

    Overrides:
      `updateEntry` in class `AbstractHashedMap<K,V>`
    Parameters:
      `entry` - the entry to update
      `newValue` - the new value to store