com.google.common.collect
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.

Since:

2.0 (imported from Google Collections Library)

Author:

Chris Povirk, Mike Bostock, Jared Levy

Method Summary

static <E> Collection<E>	filter(Collection<E> unfiltered, Predicate<? super E> predicate) Returns the elements of unfiltered that satisfy a predicate.
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.

Methods inherited from class java.lang.Object

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

Method Detail

filter

public static <E> 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.)

transform

public 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. 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, com.google.common.base.Function). If only an Iterable is available, use Iterables.transform(java.lang.Iterable, com.google.common.base.Function).