Package org.apache.commons.collections.list

Class LazyList

  • All Implemented Interfaces:
    Serializable, Iterable, Collection, List
    public class LazyList
extends AbstractSerializableListDecorator
    Decorates another List to create objects in the list on demand.

    When the get(int) method is called with an index greater than the size of the list, the list will automatically grow in size and return a new object from the specified factory. The gaps will be filled by null. If a get method call encounters a null, it will be replaced with a new object from the factory. Thus this list is unsuitable for storing null objects.

    For instance: 

     Factory factory = new Factory() {
     public Object create() {
         return new Date();
     }
 }
 List lazy = LazyList.decorate(new ArrayList(), factory);
 Object obj = lazy.get(3);
    After the above code is executed, obj will contain a new Date instance. Furthermore, that Date instance is the fourth element in the list. The first, second, and third element are all set to null.

    This class differs from GrowthList because here growth occurs on get, where GrowthList grows on set and add. However, they could easily be used together by decorating twice.

    This class is Serializable from Commons Collections 3.1.

    Since:
    Commons Collections 3.0
    Version:
    $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
    Author:
    Stephen Colebourne, Arron Bates, Paul Jack
    See Also:
    GrowthList, Serialized Form

    • Field Detail

      • factory

        protected final Factory factory
        The factory to use to lazily instantiate the objects

    • Constructor Detail

      • LazyList

        protected LazyList​(List list,
                   Factory factory)
        Constructor that wraps (not copies).
        Parameters:
        list - the list to decorate, must not be null
        factory - the factory to use for creation, must not be null
        Throws:
        IllegalArgumentException - if list or factory is null

    • Method Detail

      • decorate

        public static List decorate​(List list,
                            Factory factory)
        Factory method to create a lazily instantiating list.
        Parameters:
        list - the list to decorate, must not be null
        factory - the factory to use for creation, must not be null
        Throws:
        IllegalArgumentException - if list or factory is null

      • get

        public Object get​(int index)
        Decorate the get method to perform the lazy behaviour.

        If the requested index is greater than the current size, the list will grow to the new size and a new object will be returned from the factory. Indexes in-between the old size and the requested size are left with a placeholder that is replaced with a factory object when requested.

        Specified by:
        get in interface List
        Overrides:
        get in class AbstractListDecorator
        Parameters:
        index - the index to retrieve