@GwtCompatible public final class Collections2 extends Object
Collection
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.
Modifier and Type | Method and Description |
---|---|
static <E> Collection<E> |
filter(Collection<E> unfiltered,
Predicate<? super E> predicate)
Returns the elements of
unfiltered that satisfy a predicate. |
static <E extends Comparable<? super E>> |
orderedPermutations(Iterable<E> elements)
Returns a
Collection of all the permutations of the specified
Iterable . |
static <E> Collection<List<E>> |
orderedPermutations(Iterable<E> elements,
Comparator<? super E> comparator)
Returns a
Collection of all the permutations of the specified
Iterable using the specified Comparator for establishing
the lexicographical ordering. |
static <E> Collection<List<E>> |
permutations(Collection<E> elements)
Returns a
Collection of all the permutations of the specified
Collection . |
static <F,T> Collection<T> |
transform(Collection<F> fromCollection,
Function<? super F,T> function)
Returns a collection that applies
function to each element of
fromCollection . |
public static <E> Collection<E> filter(Collection<E> unfiltered, Predicate<? super E> predicate)
unfiltered
that satisfy a predicate. The
returned collection is a live view of unfiltered
; 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's add()
and addAll()
methods throw an IllegalArgumentException
. When methods
such as removeAll()
and clear()
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 copy Iterables.filter(unfiltered, predicate)
and use the copy.
Warning: predicate
must be consistent with equals,
as documented at Predicate.apply(T)
. Do not provide a predicate such
as Predicates.instanceOf(ArrayList.class)
, which is inconsistent
with equals. (See Iterables.filter(Iterable, Class)
for related
functionality.)
Stream
equivalent: Stream.filter
.
public static <F,T> Collection<T> transform(Collection<F> fromCollection, Function<? super F,T> function)
function
to each element of
fromCollection
. The returned collection is a live view of fromCollection
; changes to one affect the other.
The returned collection's add()
and addAll()
methods
throw an UnsupportedOperationException
. All other collection
methods are supported, as long as fromCollection
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 a List
, consider
Lists.transform(java.util.List<F>, com.google.common.base.Function<? super F, ? extends T>)
. If only an Iterable
is available, use
Iterables.transform(java.lang.Iterable<F>, com.google.common.base.Function<? super F, ? extends T>)
.
Stream
equivalent: Stream.map
.
@Beta public static <E extends Comparable<? super E>> Collection<List<E>> orderedPermutations(Iterable<E> elements)
Collection
of all the permutations of the specified
Iterable
.
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())
.
elements
- the original iterable whose elements have to be permuted.Collection
containing all the different
permutations of the original iterable.NullPointerException
- if the specified iterable is null or has any
null elements.@Beta public static <E> Collection<List<E>> orderedPermutations(Iterable<E> elements, Comparator<? super E> comparator)
Collection
of all the permutations of the specified
Iterable
using the specified Comparator
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.
elements
- the original iterable whose elements have to be permuted.comparator
- a comparator for the iterable's elements.Collection
containing all the different
permutations of the original iterable.NullPointerException
- If the specified iterable is null, has any
null elements, or if the specified comparator is null.@Beta public static <E> Collection<List<E>> permutations(Collection<E> elements)
Collection
of all the permutations of the specified
Collection
.
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.
elements
- the original collection whose elements have to be permuted.Collection
containing all the different
permutations of the original collection.NullPointerException
- if the specified collection is null or has any
null elements.Copyright © 2010-2017. All Rights Reserved.