Package org.apache.commons.collections4.map

Class 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

    • 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 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)