Package org.apache.commons.collections4.list

Class LazyList<E>

  • All Implemented Interfaces:
    java.io.Serializable, java.lang.Iterable<E>, java.util.Collection<E>, java.util.List<E>
    public class LazyList<E>
extends AbstractSerializableListDecorator<E>
    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 or transformer. 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<Date> factory = new Factory<Date>() {
     public Date create() {
         return new Date();
     }
 }
 List<Date> lazy = LazyList.decorate(new ArrayList<Date>(), factory);
 Date date = lazy.get(3);

    After the above code is executed, date 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:
    3.0
    See Also:
    GrowthList, Serialized Form

    • Method Detail

      • lazyList

        public static <E> LazyList<E> lazyList​(java.util.List<E> list,
                                       Factory<? extends E> factory)
        Factory method to create a lazily instantiating list.
        Type Parameters:
        E - the type of the elements in the list
        Parameters:
        list - the list to decorate, must not be null
        factory - the factory to use for creation, must not be null
        Returns:
        a new lazy list
        Throws:
        java.lang.NullPointerException - if list or factory is null
        Since:
        4.0

      • lazyList

        public static <E> LazyList<E> lazyList​(java.util.List<E> list,
                                       Transformer<java.lang.Integer,​? extends E> transformer)
        Transformer method to create a lazily instantiating list.
        Type Parameters:
        E - the type of the elements in the list
        Parameters:
        list - the list to decorate, must not be null
        transformer - the transformer to use for creation, must not be null
        Returns:
        a new lazy list
        Throws:
        java.lang.NullPointerException - if list or transformer is null
        Since:
        4.4

      • get

        public E 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 or transformer. Indexes in-between the old size and the requested size are left with a placeholder that is replaced with a factory or transformer object when requested.

        Specified by:
        get in interface java.util.List<E>
        Overrides:
        get in class AbstractListDecorator<E>
        Parameters:
        index - the index to retrieve
        Returns:
        the element at the given index

      • subList

        public java.util.List<E> subList​(int fromIndex,
                                 int toIndex)
        Specified by:
        subList in interface java.util.List<E>
        Overrides:
        subList in class AbstractListDecorator<E>