org.apache.commons.collections

Class ListUtils


public class ListUtils
extends java.lang.Object

Provides utility methods and decorators for List instances.
Version:
$Revision: 1.28 $ $Date: 2004/04/01 20:12:00 $
Authors:
Federico Barbieri
Peter Donald
Paul Jack
Stephen Colebourne
Neil O'Toole
Matthew Hawthorne
Since:
Commons Collections 1.0

Field Summary

static List
EMPTY_LIST
An empty unmodifiable list.

Constructor Summary

ListUtils()
ListUtils should not normally be instantiated.

Method Summary

static List
fixedSizeList(List list)
Returns a fixed-sized list backed by the given list.
static int
hashCodeForList(Collection list)
Generates a hash code using the algorithm specified in java.util.List.hashCode().
static List
intersection(List list1, List list2)
Returns a new list containing all elements that are contained in both given lists.
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).
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 (validating) 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
synchronizedList(List list)
Returns a synchronized list backed by the given list.
static List
transformedList(List list, Transformer transformer)
Returns a transformed list backed by the given list.
static List
typedList(List list, Class type)
Returns a typed list backed by the given list.
static List
union(List list1, List list2)
Returns a new list containing the second list appended to the first list.
static List
unmodifiableList(List list)
Returns an unmodifiable list backed by the given list.

Field Details

EMPTY_LIST

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

Constructor Details

ListUtils

public ListUtils()
ListUtils should not normally be instantiated.

Method Details

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

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

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 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

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

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

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

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

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

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

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

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