org.apache.commons.collections

Class ListUtils

public class ListUtils extends Object

Provides utility methods and decorators for List instances.

Since: Commons Collections 1.0

Version: $Revision: 348013 $ $Date: 2005-11-21 23:24:45 +0000 (Mon, 21 Nov 2005) $

Author: Federico Barbieri Peter Donald Paul Jack Stephen Colebourne Neil O'Toole Matthew Hawthorne

Field Summary
static ListEMPTY_LIST
An empty unmodifiable list.
Constructor Summary
ListUtils()
ListUtils should not normally be instantiated.
Method Summary
static ListfixedSizeList(List list)
Returns a fixed-sized list backed by the given list.
static inthashCodeForList(Collection list)
Generates a hash code using the algorithm specified in java.util.List#hashCode().
static Listintersection(List list1, List list2)
Returns a new list containing all elements that are contained in both given lists.
static booleanisEqualList(Collection list1, Collection list2)
Tests two lists for value-equality as per the equality contract in java.util.List#equals(java.lang.Object).
static ListlazyList(List list, Factory factory)
Returns a "lazy" list whose elements will be created on demand.
static ListpredicatedList(List list, Predicate predicate)
Returns a predicated (validating) list backed by the given list.
static ListremoveAll(Collection collection, Collection remove)
Removes the elements in remove from collection.
static ListretainAll(Collection collection, Collection retain)
Returns a List containing all the elements in collection that are also in retain.
static Listsubtract(List list1, List list2)
Subtracts all elements in the second list from the first list, placing the results in a new list.
static Listsum(List list1, List list2)
Returns the sum of the given lists.
static ListsynchronizedList(List list)
Returns a synchronized list backed by the given list.
static ListtransformedList(List list, Transformer transformer)
Returns a transformed list backed by the given list.
static ListtypedList(List list, Class type)
Returns a typed list backed by the given list.
static Listunion(List list1, List list2)
Returns a new list containing the second list appended to the first list.
static ListunmodifiableList(List list)
Returns an unmodifiable list backed by the given list.

Field Detail

EMPTY_LIST

public static final List EMPTY_LIST
An empty unmodifiable list. This uses the Collections Collections implementation and is provided for completeness.

Constructor Detail

ListUtils

public ListUtils()
ListUtils should not normally be instantiated.

Method Detail

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

hashCodeForList

public static int hashCodeForList(Collection list)
Generates a hash code using the algorithm specified in java.util.List#hashCode().

This method is useful for implementing List when you cannot extend AbstractList. The method takes Collection instances to enable other collection types to use the List implementation algorithm.

Parameters: list the list to generate the hashCode for, may be null

Returns: the hash code

See Also: java.util.List#hashCode()

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

isEqualList

public static boolean isEqualList(Collection list1, Collection list2)
Tests two lists for value-equality as per the equality contract in java.util.List#equals(java.lang.Object).

This method is useful for implementing List when you cannot extend AbstractList. The method takes Collection instances to enable other collection types to use the List implementation algorithm.

The relevant text (slightly paraphrased as this is a static method) is:

Compares the two list objects for equality. Returns true if and only if both lists have the same size, and all corresponding pairs of elements in the two lists are equal. (Two elements e1 and e2 are equal if (e1==null ? e2==null : e1.equals(e2)).) In other words, two lists are defined to be equal if they contain the same elements in the same order. This definition ensures that the equals method works properly across different implementations of the List interface.
Note: The behaviour of this method is undefined if the lists are modified during the equals comparison.

Parameters: list1 the first list, may be null list2 the second list, may be null

Returns: whether the lists are equal by value comparison

See Also: java.util.List

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 List#get(int) 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

predicatedList

public static List predicatedList(List list, Predicate predicate)
Returns a predicated (validating) list backed by the given list.

Only objects that pass the test in the given predicate can be added to the list. Trying to add an invalid object results in an IllegalArgumentException. It is important not to use the original list after invoking this method, as it is a backdoor for adding invalid 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

removeAll

public static List removeAll(Collection collection, Collection remove)
Removes the elements in remove from collection. That is, this method returns a list containing all the elements in c that are not in remove. The cardinality of an element e in the returned collection is the same as the cardinality of e in collection unless remove contains e, in which case the cardinality is zero. This method is useful if you do not wish to modify collection and thus cannot call collection.removeAll(remove);.

Parameters: collection the collection from which items are removed (in the returned collection) remove the items to be removed from the returned collection

Returns: a List containing all the elements of c except any elements that also occur in remove.

Throws: NullPointerException if either parameter is null

Since: Commons Collections 3.2

retainAll

public static List retainAll(Collection collection, Collection retain)
Returns a List containing all the elements in collection that are also in retain. The cardinality of an element e in the returned list is the same as the cardinality of e in collection unless retain does not contain e, in which case the cardinality is zero. This method is useful if you do not wish to modify the collection c and thus cannot call collection.retainAll(retain);.

Parameters: collection the collection whose contents are the target of the #retailAll operation retain the collection containing the elements to be retained in the returned collection

Returns: a List containing all the elements of c that occur at least once in retain.

Throws: NullPointerException if either parameter is null

Since: Commons Collections 3.2

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 list 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

synchronizedList

public static List synchronizedList(List list)
Returns a synchronized list backed by the given list.

You must manually synchronize on the returned buffer's iterator to avoid non-deterministic behavior:

 List list = ListUtils.synchronizedList(myList);
 synchronized (list) {
     Iterator i = list.iterator();
     while (i.hasNext()) {
         process (i.next());
     }
 }
 
This method uses the implementation in the decorators subpackage.

Parameters: list the list to synchronize, must not be null

Returns: a synchronized list backed by the given list

Throws: IllegalArgumentException if the list is null

transformedList

public static List transformedList(List list, Transformer transformer)
Returns a transformed list backed by the given list.

Each object is passed through the transformer as it is added to the List. It is important not to use the original list after invoking this method, as it is a backdoor for adding untransformed objects.

Parameters: list the list to predicate, must not be null transformer the transformer for the list, must not be null

Returns: a transformed list backed by the given list

Throws: IllegalArgumentException if the List or Transformer is null

typedList

public static List typedList(List list, Class type)
Returns a typed list backed by the given list.

Only objects of the specified type can be added to the list.

Parameters: list the list to limit to a specific type, must not be null type the type of objects which may be added to the list

Returns: a typed list backed by the specified list

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

unmodifiableList

public static List unmodifiableList(List list)
Returns an unmodifiable list backed by the given list.

This method uses the implementation in the decorators subpackage.

Parameters: list the list to make unmodifiable, must not be null

Returns: an unmodifiable list backed by the given list

Throws: IllegalArgumentException if the list is null

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