# 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
  Description
  `protected final Transformer<? super K,? extends V>`
  `factory`
  The factory to use to construct elements

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  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

  Modifier and Type
  Method
  Description
  `V`
  `get(Object key)`
  Gets a value at a given 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 Details

  - ### factory

    protected final Transformer<? super K,? extends V> factory
    The factory to use to construct elements

- ## Constructor Details

  - ### 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 Details

  - ### 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:
      `V` - the value type
      `K` - the key 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)
    Description copied from interface: `Get`
    Gets a value at a given key.
    Specified by:
      `get` in interface `Get<K,V>`
    Specified by:
      `get` in interface `Map<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)`