Class Collections2
 java.lang.Object

 com.google.common.collect.Collections2

@GwtCompatible public final class Collections2 extends java.lang.Object
Provides static methods for working withCollection
instances.Java 8+ users: several common uses for this class are now more comprehensively addressed by the new
Stream
library. Read the method documentation below for comparisons. These methods are not being deprecated, but we gently encourage you to migrate to streams. Since:
 2.0
 Author:
 Chris Povirk, Mike Bostock, Jared Levy


Method Summary
All Methods Static Methods Concrete Methods Modifier and Type Method Description static <E extends @Nullable java.lang.Object>
java.util.Collection<E>filter(java.util.Collection<E> unfiltered, Predicate<? super E> predicate)
Returns the elements ofunfiltered
that satisfy a predicate.static <E extends java.lang.Comparable<? super E>>
java.util.Collection<java.util.List<E>>orderedPermutations(java.lang.Iterable<E> elements)
Returns aCollection
of all the permutations of the specifiedIterable
.static <E> java.util.Collection<java.util.List<E>>
orderedPermutations(java.lang.Iterable<E> elements, java.util.Comparator<? super E> comparator)
Returns aCollection
of all the permutations of the specifiedIterable
using the specifiedComparator
for establishing the lexicographical ordering.static <E> java.util.Collection<java.util.List<E>>
permutations(java.util.Collection<E> elements)
Returns aCollection
of all the permutations of the specifiedCollection
.static <F extends @Nullable java.lang.Object,T extends @Nullable java.lang.Object>
java.util.Collection<T>transform(java.util.Collection<F> fromCollection, Function<? super F,T> function)
Returns a collection that appliesfunction
to each element offromCollection
.



Method Detail

filter
public static <E extends @Nullable java.lang.Object> java.util.Collection<E> filter(java.util.Collection<E> unfiltered, Predicate<? super E> predicate)
Returns the elements ofunfiltered
that satisfy a predicate. The returned collection is a live view ofunfiltered
; changes to one affect the other.The resulting collection's iterator does not support
remove()
, but all other collection methods are supported. When given an element that doesn't satisfy the predicate, the collection'sadd()
andaddAll()
methods throw anIllegalArgumentException
. When methods such asremoveAll()
andclear()
are called on the filtered collection, only elements that satisfy the filter will be removed from the underlying collection.The returned collection isn't threadsafe or serializable, even if
unfiltered
is.Many of the filtered collection's methods, such as
size()
, iterate across every element in the underlying collection and determine which elements satisfy the filter. When a live view is not needed, it may be faster to copyIterables.filter(unfiltered, predicate)
and use the copy.Warning:
predicate
must be consistent with equals, as documented atPredicate.apply(T)
. Do not provide a predicate such asPredicates.instanceOf(ArrayList.class)
, which is inconsistent with equals. (SeeIterables.filter(Iterable, Class)
for related functionality.)Stream
equivalent:Stream.filter
.

transform
public static <F extends @Nullable java.lang.Object,T extends @Nullable java.lang.Object> java.util.Collection<T> transform(java.util.Collection<F> fromCollection, Function<? super F,T> function)
Returns a collection that appliesfunction
to each element offromCollection
. The returned collection is a live view offromCollection
; changes to one affect the other.The returned collection's
add()
andaddAll()
methods throw anUnsupportedOperationException
. All other collection methods are supported, as long asfromCollection
supports them.The returned collection isn't threadsafe or serializable, even if
fromCollection
is.When a live view is not needed, it may be faster to copy the transformed collection and use the copy.
If the input
Collection
is known to be aList
, considerLists.transform(java.util.List<F>, com.google.common.base.Function<? super F, ? extends T>)
. If only anIterable
is available, useIterables.transform(java.lang.Iterable<F>, com.google.common.base.Function<? super F, ? extends T>)
.Stream
equivalent:Stream.map
.

orderedPermutations
public static <E extends java.lang.Comparable<? super E>> java.util.Collection<java.util.List<E>> orderedPermutations(java.lang.Iterable<E> elements)
Returns aCollection
of all the permutations of the specifiedIterable
.Notes: This is an implementation of the algorithm for Lexicographical Permutations Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7, Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the first permutation will be in ascending order, and the last will be in descending order.
Duplicate elements are considered equal. For example, the list [1, 1] will have only one permutation, instead of two. This is why the elements have to implement
Comparable
.An empty iterable has only one permutation, which is an empty list.
This method is equivalent to
Collections2.orderedPermutations(list, Ordering.natural())
. Parameters:
elements
 the original iterable whose elements have to be permuted. Returns:
 an immutable
Collection
containing all the different permutations of the original iterable.  Throws:
java.lang.NullPointerException
 if the specified iterable is null or has any null elements. Since:
 12.0

orderedPermutations
public static <E> java.util.Collection<java.util.List<E>> orderedPermutations(java.lang.Iterable<E> elements, java.util.Comparator<? super E> comparator)
Returns aCollection
of all the permutations of the specifiedIterable
using the specifiedComparator
for establishing the lexicographical ordering.Examples:
for (List<String> perm : orderedPermutations(asList("b", "c", "a"))) { println(perm); } // > ["a", "b", "c"] // > ["a", "c", "b"] // > ["b", "a", "c"] // > ["b", "c", "a"] // > ["c", "a", "b"] // > ["c", "b", "a"] for (List<Integer> perm : orderedPermutations(asList(1, 2, 2, 1))) { println(perm); } // > [1, 1, 2, 2] // > [1, 2, 1, 2] // > [1, 2, 2, 1] // > [2, 1, 1, 2] // > [2, 1, 2, 1] // > [2, 2, 1, 1]
Notes: This is an implementation of the algorithm for Lexicographical Permutations Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7, Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the first permutation will be in ascending order, and the last will be in descending order.
Elements that compare equal are considered equal and no new permutations are created by swapping them.
An empty iterable has only one permutation, which is an empty list.
 Parameters:
elements
 the original iterable whose elements have to be permuted.comparator
 a comparator for the iterable's elements. Returns:
 an immutable
Collection
containing all the different permutations of the original iterable.  Throws:
java.lang.NullPointerException
 If the specified iterable is null, has any null elements, or if the specified comparator is null. Since:
 12.0

permutations
public static <E> java.util.Collection<java.util.List<E>> permutations(java.util.Collection<E> elements)
Returns aCollection
of all the permutations of the specifiedCollection
.Notes: This is an implementation of the Plain Changes algorithm for permutations generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7, Section 7.2.1.2.
If the input list contains equal elements, some of the generated permutations will be equal.
An empty collection has only one permutation, which is an empty list.
 Parameters:
elements
 the original collection whose elements have to be permuted. Returns:
 an immutable
Collection
containing all the different permutations of the original collection.  Throws:
java.lang.NullPointerException
 if the specified collection is null or has any null elements. Since:
 12.0

