Class Collections2

java.lang.Object
com.google.common.collect.Collections2
@GwtCompatible public final class Collections2 extends Object
Provides static methods for working with 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.

Since:
2.0
Author:
Chris Povirk, Mike Bostock, Jared Levy

  • Method Details

    • filter

      public static <E extends @Nullable Object> Collection<E> filter(Collection<E> unfiltered, Predicate<? super E> predicate)
      Returns the elements of 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.

    • transform

      public static <F extends @Nullable Object, T extends @Nullable Object> Collection<T> transform(Collection<F> fromCollection, Function<? super F, T> function)
      Returns a collection that applies 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.

    • orderedPermutations

      public static <E extends Comparable<? super E>> Collection<List<E>> orderedPermutations(Iterable<E> elements)
      Returns a 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()).

      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:
      NullPointerException - if the specified iterable is null or has any null elements.
      Since:
      12.0

    • orderedPermutations

      public 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.

      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:
      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> Collection<List<E>> permutations(Collection<E> elements)
      Returns a 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.

      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:
      NullPointerException - if the specified collection is null or has any null elements.
      Since:
      12.0