Package org.apache.commons.collections4

Class ListUtils

java.lang.Object

org.apache.commons.collections4.ListUtils

public class ListUtils
extends Object

Provides utility methods and decorators for List instances.

Since:

1.0

Method Summary

Modifier and Type	Method	Description
static <T> List<T>	defaultIfNull(List<T> list, List<T> defaultList)	Returns either the passed in list, or if the list is null, the value of defaultList.
static <T> List<T>	emptyIfNull(List<T> list)	Returns an immutable empty list if the argument is null, or the argument itself otherwise.
static <E> List<E>	fixedSizeList(List<E> 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 List.hashCode().
static <E> int	indexOf(List<E> list, Predicate<E> predicate)	Finds the first index in the given List which matches the given predicate.
static <E> List<E>	intersection(List<? extends E> list1, List<? extends E> 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 List.equals(java.lang.Object).
static <E> List<E>	lazyList(List<E> list, Factory<? extends E> factory)	Returns a "lazy" list whose elements will be created on demand.
static <E> List<E>	lazyList(List<E> list, Transformer<Integer,? extends E> transformer)	Returns a "lazy" list whose elements will be created on demand.
static String	longestCommonSubsequence(CharSequence a, CharSequence b)	Returns the longest common subsequence (LCS) of two CharSequence objects.
static <E> List<E>	longestCommonSubsequence(List<E> a, List<E> b)	Returns the longest common subsequence (LCS) of two sequences (lists).
static <E> List<E>	longestCommonSubsequence(List<E> a, List<E> b, Equator<? super E> equator)	Returns the longest common subsequence (LCS) of two sequences (lists).
static <T> List<List<T>>	partition(List<T> list, int size)	Returns consecutive sublists of a list, each of the same size (the final list may be smaller).
static <E> List<E>	predicatedList(List<E> list, Predicate<E> predicate)	Returns a predicated (validating) list backed by the given list.
static <E> List<E>	removeAll(Collection<E> collection, Collection<?> remove)	Removes the elements in remove from collection.
static <E> List<E>	retainAll(Collection<E> collection, Collection<?> retain)	Returns a List containing all the elements in collection that are also in retain.
static <E> List<E>	select(Collection<? extends E> inputCollection, Predicate<? super E> predicate)	Selects all elements from input collection which match the given predicate into an output list.
static <E> List<E>	selectRejected(Collection<? extends E> inputCollection, Predicate<? super E> predicate)	Selects all elements from inputCollection which don't match the given predicate into an output collection.
static <E> List<E>	subtract(List<E> list1, List<? extends E> list2)	Subtracts all elements in the second list from the first list, placing the results in a new list.
static <E> List<E>	sum(List<? extends E> list1, List<? extends E> list2)	Returns the sum of the given lists.
static <E> List<E>	synchronizedList(List<E> list)	Returns a synchronized list backed by the given list.
static <E> List<E>	transformedList(List<E> list, Transformer<? super E,? extends E> transformer)	Returns a transformed list backed by the given list.
static <E> List<E>	union(List<? extends E> list1, List<? extends E> list2)	Returns a new list containing the second list appended to the first list.
static <E> List<E>	unmodifiableList(List<? extends E> list)	Returns an unmodifiable list backed by the given list.

Methods inherited from class java.lang.Object

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

Method Detail

emptyIfNull

public static <T> List<T> emptyIfNull(List<T> list)

Returns an immutable empty list if the argument is null, or the argument itself otherwise.

Type Parameters:

T - the element type

Parameters:

list - the list, possibly null

Returns:

an empty list if the argument is null

defaultIfNull

public static <T> List<T> defaultIfNull(List<T> list,
                                        List<T> defaultList)

Returns either the passed in list, or if the list is null, the value of defaultList.

Type Parameters:

T - the element type

Parameters:

list - the list, possibly null

defaultList - the returned values if list is null

Returns:

an empty list if the argument is null

Since:

4.0

intersection

public static <E> List<E> intersection(List<? extends E> list1,
                                       List<? extends E> list2)

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

Type Parameters:

E - the element type

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 <E> List<E> subtract(List<E> list1,
                                   List<? extends E> 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.

Type Parameters:

E - the element type

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 <E> List<E> sum(List<? extends E> list1,
                              List<? extends E> list2)

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

Type Parameters:

E - the element type

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 <E> List<E> union(List<? extends E> list1,
                                List<? extends E> 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.

Type Parameters:

E - the element type

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

select

public static <E> List<E> select(Collection<? extends E> inputCollection,
                                 Predicate<? super E> predicate)

Selects all elements from input collection which match the given predicate into an output list.

A null predicate matches no elements.

Type Parameters:

E - the element type

Parameters:

inputCollection - the collection to get the input from, may not be null

predicate - the predicate to use, may be null

Returns:

the elements matching the predicate (new list)

Throws:

NullPointerException - if the input list is null

Since:

4.0

See Also:

CollectionUtils.select(Iterable, Predicate)

selectRejected

public static <E> List<E> selectRejected(Collection<? extends E> inputCollection,
                                         Predicate<? super E> predicate)

Selects all elements from inputCollection which don't match the given predicate into an output collection.

If the input predicate is null, the result is an empty list.

Type Parameters:

E - the element type

Parameters:

inputCollection - the collection to get the input from, may not be null

predicate - the predicate to use, may be null

Returns:

the elements not matching the predicate (new list)

Throws:

NullPointerException - if the input collection is null

Since:

4.0

See Also:

CollectionUtils.selectRejected(Iterable, Predicate)

isEqualList

public static boolean isEqualList(Collection<?> list1,
                                  Collection<?> list2)

Tests two lists for value-equality as per the equality contract in 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:

List

hashCodeForList

public static int hashCodeForList(Collection<?> list)

Generates a hash code using the algorithm specified in 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:

List.hashCode()

retainAll

public static <E> List<E> retainAll(Collection<E> 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);.

This implementation iterates over collection, checking each element in turn to see if it's contained in retain. If it's contained, it's added to the returned list. As a consequence, it is advised to use a collection type for retain that provides a fast (e.g. O(1)) implementation of Collection.contains(Object).

Type Parameters:

E - the element type

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:

3.2

removeAll

public static <E> List<E> removeAll(Collection<E> collection,
                                    Collection<?> remove)

Removes the elements in remove from collection. That is, this method returns a list containing all the elements in collection 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);.

This implementation iterates over collection, checking each element in turn to see if it's contained in remove. If it's not contained, it's added to the returned list. As a consequence, it is advised to use a collection type for remove that provides a fast (e.g. O(1)) implementation of Collection.contains(Object).

Type Parameters:

E - the element type

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:

3.2

synchronizedList

public static <E> List<E> synchronizedList(List<E> list)

Returns a synchronized list backed by the given list.

You must manually synchronize on the returned list'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 is just a wrapper for Collections.synchronizedList(List).

Type Parameters:

E - the element type

Parameters:

list - the list to synchronize, must not be null

Returns:

a synchronized list backed by the given list

Throws:

NullPointerException - if the list is null

unmodifiableList

public static <E> List<E> unmodifiableList(List<? extends E> list)

Returns an unmodifiable list backed by the given list.

This method uses the implementation in the decorators subpackage.

Type Parameters:

E - the element type

Parameters:

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

Returns:

an unmodifiable list backed by the given list

Throws:

NullPointerException - if the list is null

predicatedList

public static <E> List<E> predicatedList(List<E> list,
                                         Predicate<E> 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.

Type Parameters:

E - the element type

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:

NullPointerException - if the List or Predicate is null

transformedList

public static <E> List<E> transformedList(List<E> list,
                                          Transformer<? super E,? extends E> transformer)

Returns a transformed list backed by the given list.

This method returns a new list (decorating the specified list) that will transform any new entries added to it. Existing entries in the specified list will not be transformed.

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.

Existing entries in the specified list will not be transformed. If you want that behaviour, see TransformedList.transformedList(java.util.List<E>, org.apache.commons.collections4.Transformer<? super E, ? extends E>).

Type Parameters:

E - the element type

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:

NullPointerException - if the List or Transformer is null

lazyList

public static <E> List<E> lazyList(List<E> list,
                                   Factory<? extends E> 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<Date> factory = new Factory<Date>() {
     public Date create() {
         return new Date();
     }
 }
 List<Date> lazy = ListUtils.lazyList(new ArrayList<Date>(), factory);
 Date date = lazy.get(3);
 

After the above code is executed, date will refer to 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.

Type Parameters:

E - the element type

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:

NullPointerException - if the List or Factory is null

lazyList

public static <E> List<E> lazyList(List<E> list,
                                   Transformer<Integer,? extends E> transformer)

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 transformer will be used to create a new object and that object will be inserted at that index.

For instance:

 List<Integer> hours = Arrays.asList(7, 5, 8, 2);
 Transformer<Integer,Date> transformer = input -> LocalDateTime.now().withHour(hours.get(input));
 List<LocalDateTime> lazy = ListUtils.lazyList(new ArrayList<LocalDateTime>(), transformer);
 Date date = lazy.get(3);
 

After the above code is executed, date will refer to 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.

Type Parameters:

E - the element type

Parameters:

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

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

Returns:

a lazy list backed by the given list

Throws:

NullPointerException - if the List or Transformer is null

fixedSizeList

public static <E> List<E> fixedSizeList(List<E> 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).

Type Parameters:

E - the element type

Parameters:

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

Returns:

a fixed-size list backed by that list

Throws:

NullPointerException - if the List is null

indexOf

public static <E> int indexOf(List<E> list,
                              Predicate<E> predicate)

Finds the first index in the given List which matches the given predicate.

If the input List or predicate is null, or no element of the List matches the predicate, -1 is returned.

Type Parameters:

E - the element type

Parameters:

list - the List to search, may be null

predicate - the predicate to use, may be null

Returns:

the first index of an Object in the List which matches the predicate or -1 if none could be found

longestCommonSubsequence

public static <E> List<E> longestCommonSubsequence(List<E> a,
                                                   List<E> b)

Returns the longest common subsequence (LCS) of two sequences (lists).

Type Parameters:

E - the element type

Parameters:

a - the first list

b - the second list

Returns:

the longest common subsequence

Throws:

NullPointerException - if either list is null

Since:

4.0

longestCommonSubsequence

public static <E> List<E> longestCommonSubsequence(List<E> a,
                                                   List<E> b,
                                                   Equator<? super E> equator)

Returns the longest common subsequence (LCS) of two sequences (lists).

Type Parameters:

E - the element type

Parameters:

a - the first list

b - the second list

equator - the equator used to test object equality

Returns:

the longest common subsequence

Throws:

NullPointerException - if either list or the equator is null

Since:

4.0

longestCommonSubsequence

public static String longestCommonSubsequence(CharSequence a,
                                              CharSequence b)

Returns the longest common subsequence (LCS) of two CharSequence objects.

This is a convenience method for using longestCommonSubsequence(List, List) with CharSequence instances.

Parameters:

a - the first sequence

b - the second sequence

Returns:

the longest common subsequence as String

Throws:

NullPointerException - if either sequence is null

Since:

4.0

partition

public static <T> List<List<T>> partition(List<T> list,
                                          int size)

Returns consecutive sublists of a list, each of the same size (the final list may be smaller). For example, partitioning a list containing [a, b, c, d, e] with a partition size of 3 yields [[a, b, c], [d, e]] -- an outer list containing two inner lists of three and two elements, all in the original order.

The outer list is unmodifiable, but reflects the latest state of the source list. The inner lists are sublist views of the original list, produced on demand using List.subList(int, int), and are subject to all the usual caveats about modification as explained in that API.

Adapted from http://code.google.com/p/guava-libraries/

Type Parameters:

T - the element type

Parameters:

list - the list to return consecutive sublists of

size - the desired size of each sublist (the last may be smaller)

Returns:

a list of consecutive sublists

Throws:

NullPointerException - if list is null

IllegalArgumentException - if size is not strictly positive

Since:

4.0