org.apache.commons.collections
public class ListUtils extends Object
Since: Commons Collections 1.0
Version: $Revision: 348013 $ $Date: 2005-11-21 23:24:45 +0000 (Mon, 21 Nov 2005) $
Field Summary | |
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static List | EMPTY_LIST
An empty unmodifiable list.
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Constructor Summary | |
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ListUtils()ListUtils should not normally be instantiated. |
Method Summary | |
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static List | fixedSizeList(List list)
Returns a fixed-sized list backed by the given list.
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static int | hashCodeForList(Collection list)
Generates a hash code using the algorithm specified in
java.util.List#hashCode().
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static List | intersection(List list1, List list2)
Returns a new list containing all elements that are contained in
both given lists.
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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).
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static List | lazyList(List list, Factory factory)
Returns a "lazy" list whose elements will be created on demand.
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static List | predicatedList(List list, Predicate predicate)
Returns a predicated (validating) list backed by the given list.
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static List | removeAll(Collection collection, Collection remove)
Removes the elements in remove from collection . |
static List | retainAll(Collection collection, Collection retain)
Returns a List containing all the elements in collection
that are also in retain . |
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.
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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.
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static List | transformedList(List list, Transformer transformer)
Returns a transformed list backed by the given list.
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static List | typedList(List list, Class type)
Returns a typed list backed by the given list.
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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.
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Collections Collections
implementation
and is provided for completeness.ListUtils
should not normally be instantiated.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
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()
Parameters: list1 the first list list2 the second list
Returns: the intersection of those two lists
Throws: NullPointerException if either list is null
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
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
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
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
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
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
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
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
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
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
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
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