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java.lang.Object
org.apache.commons.collections.ListUtils
public class ListUtils
extends java.lang.Object
List
instances.
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static List |
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public static final List EMPTY_LIST
An empty unmodifiable list. This uses theCollections
implementation and is provided for completeness.
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 theList.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
public static int hashCodeForList(Collection list)
Generates a hash code using the algorithm specified injava.util.List.hashCode()
. This method is useful for implementingList
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()
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 listlist2
- the second list
- Returns:
- the intersection of those two lists
public static boolean isEqualList(Collection list1, Collection list2)
Tests two lists for value-equality as per the equality contract injava.util.List.equals(java.lang.Object)
. This method is useful for implementingList
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 nulllist2
- the second list, may be null
- Returns:
- whether the lists are equal by value comparison
- See Also:
java.util.List
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'sget
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 newDate
instance. Furthermore, thatDate
instance is the fourth element in the list. The first, second, and third element are all set tonull
.
- Parameters:
list
- the list to make lazy, must not be nullfactory
- the factory for creating new objects, must not be null
- Returns:
- a lazy list backed by the given list
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 nullpredicate
- the predicate for the list, must not be null
- Returns:
- a predicated list backed by the given list
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 fromList.removeAll(Collection)
in that cardinality is respected; iflist1
contains two occurrences ofnull
andlist2
only contains one occurrence, then the returned list will still contain one occurrence.
- Parameters:
list1
- the list to subtract fromlist2
- the list to subtract
- Returns:
- a new list containing the results
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 listlist2
- the second list
- Returns:
- a new list containing the sum of those lists
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
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 nulltransformer
- the transformer for the list, must not be null
- Returns:
- a transformed list backed by the given list
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 nulltype
- the type of objects which may be added to the list
- Returns:
- a typed list backed by the specified list
public static List union(List list1, List list2)
Returns a new list containing the second list appended to the first list. TheList.addAll(Collection)
operation is used to append the two given lists into a new list.
- Parameters:
list1
- the first listlist2
- the second list
- Returns:
- a new list containing the union of those lists
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