# Package org.apache.commons.collections4.map

This package contains implementations of the `Map`, `IterableMap`, `OrderedMap` and `SortedMap` interfaces.

See: Description

- Interface Summary
  Interface | Description
  --- | ---
  CompositeMap.MapMutator<K,V> | This interface allows definition for all of the indeterminate mutators in a CompositeMap, as well as providing a hook for callbacks on key collisions.
  PassiveExpiringMap.ExpirationPolicy<K,V> | A policy to determine the expiration time for key-value entries.

- Class Summary
  Class | Description
  --- | ---
  AbstractHashedMap<K,V> | An abstract implementation of a hash-based map which provides numerous points for subclasses to override.
  AbstractHashedMap.EntrySet<K,V> | EntrySet implementation.
  AbstractHashedMap.EntrySetIterator<K,V> | EntrySet iterator.
  AbstractHashedMap.HashEntry<K,V> | HashEntry used to store the data.
  AbstractHashedMap.HashIterator<K,V> | Base Iterator
  AbstractHashedMap.HashMapIterator<K,V> | MapIterator implementation.
  AbstractHashedMap.KeySet<K> | KeySet implementation.
  AbstractHashedMap.KeySetIterator<K> | KeySet iterator.
  AbstractHashedMap.Values<V> | Values implementation.
  AbstractHashedMap.ValuesIterator<V> | Values iterator.
  AbstractIterableMap<K,V> | Provide a basic `IterableMap` implementation.
  AbstractLinkedMap<K,V> | An abstract implementation of a hash-based map that links entries to create an ordered map and which provides numerous points for subclasses to override.
  AbstractLinkedMap.EntrySetIterator<K,V> | EntrySet iterator.
  AbstractLinkedMap.KeySetIterator<K> | KeySet iterator.
  AbstractLinkedMap.LinkEntry<K,V> | LinkEntry that stores the data.
  AbstractLinkedMap.LinkIterator<K,V> | Base Iterator that iterates in link order.
  AbstractLinkedMap.LinkMapIterator<K,V> | MapIterator implementation.
  AbstractLinkedMap.ValuesIterator<V> | Values iterator.
  AbstractMapDecorator<K,V> | Provides a base decorator that enables additional functionality to be added to a Map via decoration.
  AbstractOrderedMapDecorator<K,V> | Provides a base decorator that enables additional functionality to be added to an OrderedMap via decoration.
  AbstractReferenceMap<K,V> | An abstract implementation of a hash-based map that allows the entries to be removed by the garbage collector.
  AbstractReferenceMap.ReferenceEntry<K,V> | A MapEntry implementation for the map.
  AbstractSortedMapDecorator<K,V> | Provides a base decorator that enables additional functionality to be added to a Map via decoration.
  AbstractSortedMapDecorator.SortedMapIterator<K,V> | OrderedMapIterator implementation.
  CaseInsensitiveMap<K,V> | A case-insensitive `Map`.
  CompositeMap<K,V> | Decorates a map of other maps to provide a single unified view.
  DefaultedMap<K,V> | Decorates another `Map` returning a default value if the map does not contain the requested key.
  EntrySetToMapIteratorAdapter<K,V> | Adapts a Map entrySet to the MapIterator interface.
  FixedSizeMap<K,V> | Decorates another `Map` to fix the size, preventing add/remove.
  FixedSizeSortedMap<K,V> | Decorates another `SortedMap` to fix the size blocking add/remove.
  Flat3Map<K,V> | A `Map` implementation that stores data in simple fields until the size is greater than 3.
  HashedMap<K,V> | A `Map` implementation that is a general purpose alternative to `HashMap`.
  LazyMap<K,V> | Decorates another `Map` to create objects in the map on demand.
  LazySortedMap<K,V> | Decorates another `SortedMap` to create objects in the map on demand.
  LinkedMap<K,V> | A `Map` implementation that maintains the order of the entries.
  ListOrderedMap<K,V> | Decorates a `Map` to ensure that the order of addition is retained using a `List` to maintain order.
  LRUMap<K,V> | A `Map` implementation with a fixed maximum size which removes the least recently used entry if an entry is added when full.
  MultiKeyMap<K,V> | A `Map` implementation that uses multiple keys to map the value.
  MultiValueMap<K,V> | Deprecated since 4.1, use `MultiValuedMap` instead
  PassiveExpiringMap<K,V> | Decorates a `Map` to evict expired entries once their expiration time has been reached.
  PassiveExpiringMap.ConstantTimeToLiveExpirationPolicy<K,V> | A `ExpirationPolicy` that returns a expiration time that is a constant about of time in the future from the current time.
  PredicatedMap<K,V> | Decorates another `Map` to validate that additions match a specified predicate.
  PredicatedSortedMap<K,V> | Decorates another `SortedMap` to validate that additions match a specified predicate.
  ReferenceIdentityMap<K,V> | A `Map` implementation that allows mappings to be removed by the garbage collector and matches keys and values based on `==` not `equals()`.
  ReferenceMap<K,V> | A `Map` implementation that allows mappings to be removed by the garbage collector.
  SingletonMap<K,V> | A `Map` implementation that holds a single item and is fixed size.
  StaticBucketMap<K,V> | A StaticBucketMap is an efficient, thread-safe implementation of `java.util.Map` that performs well in in a highly thread-contentious environment.
  TransformedMap<K,V> | Decorates another `Map` to transform objects that are added.
  TransformedSortedMap<K,V> | Decorates another `SortedMap` to transform objects that are added.
  UnmodifiableEntrySet<K,V> | Decorates a map entry `Set` to ensure it can't be altered.
  UnmodifiableMap<K,V> | Decorates another `Map` to ensure it can't be altered.
  UnmodifiableOrderedMap<K,V> | Decorates another `OrderedMap` to ensure it can't be altered.
  UnmodifiableSortedMap<K,V> | Decorates another `SortedMap` to ensure it can't be altered.

- Enum Summary
  Enum | Description
  --- | ---
  AbstractReferenceMap.ReferenceStrength | Reference type enum.

## Package org.apache.commons.collections4.map Description

This package contains implementations of the `Map`, `IterableMap`, `OrderedMap` and `SortedMap` interfaces. A Map provides a lookup from a key to a value. A number of implementations also support the new MapIterator interface that enables simple iteration of map keys and values.

The following implementations are provided:

- CaseInsensitiveMap - map that compares keys in a case insensitive way
- CompositeMap - map that combines multiple maps into a single view
- HashedMap - general purpose HashMap replacement supporting MapIterator
- Flat3Map - designed for good performance at size 3 or less
- LinkedMap - a hash map that maintains insertion order, supporting OrderedMapIterator
- LRUMap - a hash map that maintains a maximum size by removing the least recently used entries
- MultiKeyMap - map that provides special methods for using more than one key to access the value
- ReferenceMap - allows the garbage collector to collect keys and values using equals() for comparison
- ReferenceIdentityMap - allows the garbage collector to collect keys and values using == for comparison
- SingletonMap - a fully featured map to hold one key-value pair
- StaticBucketMap - internally synchronized and designed for thread-contentious environments

The following decorators are provided:

- Unmodifiable - ensures the collection cannot be altered
- Predicated - ensures that only elements that are valid according to a predicate can be added
- Transformed - transforms each element added
- FixedSize - ensures that the size of the map cannot change
- Defaulted - provides default values for non-existing keys
- Lazy - creates objects in the map on demand
- ListOrdered - ensures that insertion order is retained

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