Class LazyList

  • All Implemented Interfaces:, java.lang.Iterable, java.util.Collection, java.util.List

    public class LazyList
    extends AbstractSerializableListDecorator
    Commons Collections 3 is in maintenance mode. Commons Collections 4 should be used instead.
    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.

    Commons Collections 3.0
    See Also:
    GrowthList, Serialized Form
    • Method Detail

      • decorate

        public static java.util.List decorate​(java.util.List list,
                                              Factory factory)
        Factory method to create a lazily instantiating list.
        list - the list to decorate, must not be null
        factory - the factory to use for creation, must not be null
        java.lang.IllegalArgumentException - if list or factory is null
      • get

        public java.lang.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 java.util.List
        get in class AbstractListDecorator
        index - the index to retrieve
      • subList

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