org.apache.commons.collections4.map

## Class LazyMap<K,V>

- java.lang.Object
- - org.apache.commons.collections4.map.AbstractIterableMap<K,V>
  - - org.apache.commons.collections4.map.AbstractMapDecorator<K,V>
    - - org.apache.commons.collections4.map.LazyMap<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, Map<K,V>, Get<K,V>, IterableGet<K,V>, IterableMap<K,V>, Put<K,V>
  Direct Known Subclasses:
    LazySortedMap

      public class LazyMap<K,V>
      extends AbstractMapDecorator<K,V>
      implements Serializable

  Decorates another `Map` to create objects in the map on demand.

  When the `get(Object)` method is called with a key that does not exist in the map, the factory is used to create the object. The created object will be added to the map using the requested key.

  For instance:

       Factory<Date> factory = new Factory<Date>() {
           public Date create() {
               return new Date();
           }
       }
       Map<String, Date> lazy = LazyMap.lazyMap(new HashMap<String, Date>(), factory);
       Date date = lazy.get("NOW");

  After the above code is executed, `date` will refer to a new `Date` instance. Furthermore, that `Date` instance is mapped to the "NOW" key in the map.

  Note that LazyMap 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 exceptions when accessed by concurrent threads without synchronization.

  This class is Serializable from Commons Collections 3.1.

  Since:
    3.0
  See Also:
    Serialized Form

- - ### Nested Class Summary

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

      `Map.Entry<K,V>`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected Transformer<? super K,? extends V>` | `factory` The factory to use to construct elements

  - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `LazyMap(Map<K,V> map, Factory<? extends V> factory)` Constructor that wraps (not copies).
    `protected` | `LazyMap(Map<K,V> map, Transformer<? super K,? extends V> factory)` Constructor that wraps (not copies).

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `V` | `get(Object key)`
    `static <K,V> LazyMap<K,V>` | `lazyMap(Map<K,V> map, Factory<? extends V> factory)` Factory method to create a lazily instantiated map.
    `static <V,K> LazyMap<K,V>` | `lazyMap(Map<K,V> map, Transformer<? super K,? extends V> factory)` Factory method to create a lazily instantiated map.

    - ### Methods inherited from class org.apache.commons.collections4.map.AbstractMapDecorator

      `clear, containsKey, containsValue, decorated, entrySet, equals, hashCode, isEmpty, keySet, put, putAll, remove, size, toString, values`

    - ### Methods inherited from class org.apache.commons.collections4.map.AbstractIterableMap

      `mapIterator`

    - ### Methods inherited from class java.lang.Object

      `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

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

      `compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll`

- - ### Field Detail

    - #### factory

          protected final Transformer<? super K,? extends V> factory

      The factory to use to construct elements

  - ### Constructor Detail

    - #### LazyMap

          protected LazyMap(Map<K,V> map,
                            Factory<? extends V> factory)

      Constructor that wraps (not copies).
      Parameters:
        `map` - the map to decorate, must not be null
        `factory` - the factory to use, must not be null
      Throws:
        `NullPointerException` - if map or factory is null

    - #### LazyMap

          protected LazyMap(Map<K,V> map,
                            Transformer<? super K,? extends V> factory)

      Constructor that wraps (not copies).
      Parameters:
        `map` - the map to decorate, must not be null
        `factory` - the factory to use, must not be null
      Throws:
        `NullPointerException` - if map or factory is null

  - ### Method Detail

    - #### lazyMap

          public static <K,V> LazyMap<K,V> lazyMap(Map<K,V> map,
                                                   Factory<? extends V> factory)

      Factory method to create a lazily instantiated map.
      Type Parameters:
        `K` - the key type
        `V` - the value type
      Parameters:
        `map` - the map to decorate, must not be null
        `factory` - the factory to use, must not be null
      Returns:
        a new lazy map
      Throws:
        `NullPointerException` - if map or factory is null
      Since:
        4.0

    - #### lazyMap

          public static <V,K> LazyMap<K,V> lazyMap(Map<K,V> map,
                                                   Transformer<? super K,? extends V> factory)

      Factory method to create a lazily instantiated map.
      Type Parameters:
        `K` - the key type
        `V` - the value type
      Parameters:
        `map` - the map to decorate, must not be null
        `factory` - the factory to use, must not be null
      Returns:
        a new lazy map
      Throws:
        `NullPointerException` - if map or factory is null
      Since:
        4.0

    - #### get

          public V get(Object key)

      Specified by:
        `get` in interface `Map<K,V>`
      Specified by:
        `get` in interface `Get<K,V>`
      Overrides:
        `get` in class `AbstractMapDecorator<K,V>`
      Parameters:
        `key` - the key whose associated value is to be returned
      Returns:
        the value to which the specified key is mapped, or `null` if this map contains no mapping for the key
      See Also:
        `Map.get(Object)`

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