org.apache.commons.collections
Class ListUtils

java.lang.Object
  |
  +--org.apache.commons.collections.ListUtils

public class ListUtils

extends Object

Contains static utility methods and decorators for List instances.

Since:

1.0

Author:

Federico Barbieri, Peter Donald, Paul Jack, Stephen Colebourne

Constructor Summary

ListUtils()
Please don't ever instantiate a ListUtils.

Method Summary

static List	fixedSizeList(List list) Returns a fixed-sized list backed by the given list.
static List	intersection(List list1, List list2) Returns a new list containing all elements that are contained in both given lists.
static List	lazyList(List list, Factory factory) Returns a "lazy" list whose elements will be created on demand.
static List	predicatedList(List list, Predicate predicate) Returns a predicated list backed by the given list.
static List	subtract(List list1, List list2) Subtracts all elements in the second list from the first list, placing the results in a new list.
static List	sum(List list1, List list2) Returns the sum of the given lists.
static List	union(List list1, List list2) Returns a new list containing the second list appended to the first list.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ListUtils

public ListUtils()

Please don't ever instantiate a ListUtils.

Method Detail

intersection

public static List intersection(List list1,
                                List list2)

Returns a new list containing all elements that are contained in both given lists.

Parameters:

list1 - the first list

list2 - the second list

Returns:

the intersection of those two lists

Throws:

NullPointerException - if either list is null

subtract

public static List subtract(List list1,
                            List list2)

Subtracts all elements in the second list from the first list, placing the results in a new list. This differs from List.removeAll(Collection) in that cardinality is respected; if list1 contains two occurrences of null and list2 only contains one occurrence, then the returned list will still contain one occurrence.

Parameters:

list1 - the list to subtract from

list2 - the lsit to subtract

Returns:

a new list containing the results

Throws:

NullPointerException - if either list is null

sum

public static List sum(List list1,
                       List list2)

Returns the sum of the given lists. This is their intersection subtracted from their union.

Parameters:

list1 - the first list

list2 - the second list

Returns:

a new list containing the sum of those lists

Throws:

NullPointerException - if either list is null

union

public static List union(List list1,
                         List list2)

Returns a new list containing the second list appended to the first list. The List.addAll(Collection) operation is used to append the two given lists into a new list.

Parameters:

list1 - the first list

list2 - the second list

Returns:

a new list containing the union of those lists

Throws:

NullPointerException - if either list is null

predicatedList

public static List predicatedList(List list,
                                  Predicate predicate)

Returns a predicated list backed by the given list. Only objects that pass the test in the given predicate can be added to the list. It is important not to use the original list after invoking this method, as it is a backdoor for adding unvalidated objects.

Parameters:

list - the list to predicate, must not be null

predicate - the predicate for the list, must not be null

Returns:

a predicated list backed by the given list

Throws:

IllegalArgumentException - if the List or Predicate is null

lazyList

public static List lazyList(List list,
                            Factory factory)

Returns a "lazy" list whose elements will be created on demand.

When the index passed to the returned list's get method is greater than the list's size, then the factory will be used to create a new object and that object will be inserted at that index.

For instance:

 Factory factory = new Factory() {
     public Object create() {
         return new Date();
     }
 }
 List lazy = ListUtils.lazyList(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.

Parameters:

list - the list to make lazy, must not be null

factory - the factory for creating new objects, must not be null

Returns:

a lazy list backed by the given list

Throws:

IllegalArgumentException - if the List or Factory is null

fixedSizeList

public static List fixedSizeList(List list)

Returns a fixed-sized list backed by the given list. Elements may not be added or removed from the returned list, but existing elements can be changed (for instance, via the List.set(int,Object) method).

Parameters:

list - the list whose size to fix, must not be null

Returns:

a fixed-size list backed by that list

Throws:

IllegalArgumentException - if the List is null