Java tutorial
/* * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements. See the NOTICE file distributed with * this work for additional information regarding copyright ownership. * The ASF licenses this file to You under the Apache License, Version 2.0 * (the "License"); you may not use this file except in compliance with * the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.commons.collections4; import java.util.ArrayList; import java.util.Collection; import java.util.Collections; import java.util.Comparator; import java.util.Iterator; import java.util.List; import java.util.Set; import org.apache.commons.collections4.functors.EqualPredicate; import org.apache.commons.collections4.iterators.LazyIteratorChain; import org.apache.commons.collections4.iterators.ReverseListIterator; import org.apache.commons.collections4.iterators.UniqueFilterIterator; /** * Provides utility methods and decorators for {@link Iterable} instances. * <p> * <b>Note</b>: this util class has been designed for fail-fast argument checking. * <ul> * <li> * all decorator methods are <b>NOT</b> null-safe wrt the provided Iterable argument, i.e. * they will throw a {@link NullPointerException} if a null Iterable is passed as argument. * <li> * all other utility methods are null-safe wrt the provided Iterable argument, i.e. they will * treat a null Iterable the same way as an empty one. Other arguments which are null, * e.g. a {@link Predicate}, will result in a {@link NullPointerException}. Exception: passing * a null {@link Comparator} is equivalent to a Comparator with natural ordering. * </ul> * * @since 4.1 * @version $Id: IterableUtils.java 1716538 2015-11-25 20:27:49Z tn $ */ public class IterableUtils { /** * An empty iterable. */ @SuppressWarnings("rawtypes") static final FluentIterable EMPTY_ITERABLE = new FluentIterable<Object>() { @Override public Iterator<Object> iterator() { return IteratorUtils.emptyIterator(); } }; // Empty // ---------------------------------------------------------------------- /** * Gets an empty iterable. * <p> * This iterable does not contain any elements. * * @param <E> the element type * @return an empty iterable */ @SuppressWarnings("unchecked") // OK, empty collection is compatible with any type public static <E> Iterable<E> emptyIterable() { return EMPTY_ITERABLE; } // Chained // ---------------------------------------------------------------------- /** * Combines two iterables into a single iterable. * <p> * The returned iterable has an iterator that traverses the elements in {@code a}, * followed by the elements in {@code b}. The source iterators are not polled until * necessary. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <E> the element type * @param a the first iterable, may not be null * @param b the second iterable, may not be null * @return a new iterable, combining the provided iterables * @throws NullPointerException if either a or b is null */ @SuppressWarnings("unchecked") public static <E> Iterable<E> chainedIterable(final Iterable<? extends E> a, final Iterable<? extends E> b) { return chainedIterable(new Iterable[] { a, b }); } /** * Combines three iterables into a single iterable. * <p> * The returned iterable has an iterator that traverses the elements in {@code a}, * followed by the elements in {@code b} and {@code c}. The source iterators are * not polled until necessary. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <E> the element type * @param a the first iterable, may not be null * @param b the second iterable, may not be null * @param c the third iterable, may not be null * @return a new iterable, combining the provided iterables * @throws NullPointerException if either of the provided iterables is null */ @SuppressWarnings("unchecked") public static <E> Iterable<E> chainedIterable(final Iterable<? extends E> a, final Iterable<? extends E> b, final Iterable<? extends E> c) { return chainedIterable(new Iterable[] { a, b, c }); } /** * Combines four iterables into a single iterable. * <p> * The returned iterable has an iterator that traverses the elements in {@code a}, * followed by the elements in {@code b}, {@code c} and {@code d}. The source * iterators are not polled until necessary. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <E> the element type * @param a the first iterable, may not be null * @param b the second iterable, may not be null * @param c the third iterable, may not be null * @param d the fourth iterable, may not be null * @return a new iterable, combining the provided iterables * @throws NullPointerException if either of the provided iterables is null */ @SuppressWarnings("unchecked") public static <E> Iterable<E> chainedIterable(final Iterable<? extends E> a, final Iterable<? extends E> b, final Iterable<? extends E> c, final Iterable<? extends E> d) { return chainedIterable(new Iterable[] { a, b, c, d }); } /** * Combines the provided iterables into a single iterable. * <p> * The returned iterable has an iterator that traverses the elements in the order * of the arguments, i.e. iterables[0], iterables[1], .... The source iterators * are not polled until necessary. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <E> the element type * @param iterables the iterables to combine, may not be null * @return a new iterable, combining the provided iterables * @throws NullPointerException if either of the provided iterables is null */ public static <E> Iterable<E> chainedIterable(final Iterable<? extends E>... iterables) { checkNotNull(iterables); return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return new LazyIteratorChain<E>() { @Override protected Iterator<? extends E> nextIterator(int count) { if (count > iterables.length) { return null; } else { return iterables[count - 1].iterator(); } } }; } }; } // Collated // ---------------------------------------------------------------------- /** * Combines the two provided iterables into an ordered iterable using * natural ordering. * <p> * The returned iterable's iterator supports {@code remove()} when the * corresponding input iterator supports it. * * @param <E> the element type * @param a the first iterable, may not be null * @param b the second iterable, may not be null * @return a filtered view on the specified iterable * @throws NullPointerException if either of the provided iterables is null */ public static <E> Iterable<E> collatedIterable(final Iterable<? extends E> a, final Iterable<? extends E> b) { checkNotNull(a, b); return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return IteratorUtils.collatedIterator(null, a.iterator(), b.iterator()); } }; } /** * Combines the two provided iterables into an ordered iterable using the * provided comparator. If the comparator is null, natural ordering will be * used. * <p> * The returned iterable's iterator supports {@code remove()} when the * corresponding input iterator supports it. * * @param <E> the element type * @param comparator the comparator defining an ordering over the elements, * may be null, in which case natural ordering will be used * @param a the first iterable, may not be null * @param b the second iterable, may not be null * @return a filtered view on the specified iterable * @throws NullPointerException if either of the provided iterables is null */ public static <E> Iterable<E> collatedIterable(final Comparator<? super E> comparator, final Iterable<? extends E> a, final Iterable<? extends E> b) { checkNotNull(a, b); return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return IteratorUtils.collatedIterator(comparator, a.iterator(), b.iterator()); } }; } // Filtered // ---------------------------------------------------------------------- /** * Returns a view of the given iterable that only contains elements matching * the provided predicate. * <p> * The returned iterable's iterator supports {@code remove()} when the * corresponding input iterator supports it. * * @param <E> the element type * @param iterable the iterable to filter, may not be null * @param predicate the predicate used to filter elements, may not be null * @return a filtered view on the specified iterable * @throws NullPointerException if either iterable or predicate is null */ public static <E> Iterable<E> filteredIterable(final Iterable<E> iterable, final Predicate<? super E> predicate) { checkNotNull(iterable); if (predicate == null) { throw new NullPointerException("Predicate must not be null."); } return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return IteratorUtils.filteredIterator(emptyIteratorIfNull(iterable), predicate); } }; } // Bounded // ---------------------------------------------------------------------- /** * Returns a view of the given iterable that contains at most the given number * of elements. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <E> the element type * @param iterable the iterable to limit, may not be null * @param maxSize the maximum number of elements, must not be negative * @return a bounded view on the specified iterable * @throws IllegalArgumentException if maxSize is negative * @throws NullPointerException if iterable is null */ public static <E> Iterable<E> boundedIterable(final Iterable<E> iterable, final long maxSize) { checkNotNull(iterable); if (maxSize < 0) { throw new IllegalArgumentException("MaxSize parameter must not be negative."); } return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return IteratorUtils.boundedIterator(iterable.iterator(), maxSize); } }; } // Looping // ---------------------------------------------------------------------- /** * Returns a view of the given iterable which will cycle infinitely over * its elements. * <p> * The returned iterable's iterator supports {@code remove()} if * {@code iterable.iterator()} does. After {@code remove()} is called, subsequent * cycles omit the removed element, which is no longer in {@code iterable}. The * iterator's {@code hasNext()} method returns {@code true} until {@code iterable} * is empty. * * @param <E> the element type * @param iterable the iterable to loop, may not be null * @return a view of the iterable, providing an infinite loop over its elements * @throws NullPointerException if iterable is null */ public static <E> Iterable<E> loopingIterable(final Iterable<E> iterable) { checkNotNull(iterable); return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return new LazyIteratorChain<E>() { @Override protected Iterator<? extends E> nextIterator(int count) { if (IterableUtils.isEmpty(iterable)) { return null; } else { return iterable.iterator(); } } }; } }; } // Reversed // ---------------------------------------------------------------------- /** * Returns a reversed view of the given iterable. * <p> * In case the provided iterable is a {@link List} instance, a * {@link ReverseListIterator} will be used to reverse the traversal * order, otherwise an intermediate {@link List} needs to be created. * <p> * The returned iterable's iterator supports {@code remove()} if the * provided iterable is a {@link List} instance. * * @param <E> the element type * @param iterable the iterable to use, may not be null * @return a reversed view of the specified iterable * @throws NullPointerException if iterable is null * @see ReverseListIterator */ public static <E> Iterable<E> reversedIterable(final Iterable<E> iterable) { checkNotNull(iterable); return new FluentIterable<E>() { @Override public Iterator<E> iterator() { final List<E> list = (iterable instanceof List<?>) ? (List<E>) iterable : IteratorUtils.toList(iterable.iterator()); return new ReverseListIterator<E>(list); } }; } // Skipping // ---------------------------------------------------------------------- /** * Returns a view of the given iterable that skips the first N elements. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <E> the element type * @param iterable the iterable to use, may not be null * @param elementsToSkip the number of elements to skip from the start, must not be negative * @return a view of the specified iterable, skipping the first N elements * @throws IllegalArgumentException if elementsToSkip is negative * @throws NullPointerException if iterable is null */ public static <E> Iterable<E> skippingIterable(final Iterable<E> iterable, final long elementsToSkip) { checkNotNull(iterable); if (elementsToSkip < 0) { throw new IllegalArgumentException("ElementsToSkip parameter must not be negative."); } return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return IteratorUtils.skippingIterator(iterable.iterator(), elementsToSkip); } }; } // Transformed // ---------------------------------------------------------------------- /** * Returns a transformed view of the given iterable where all of its elements * have been transformed by the provided transformer. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <I> the input element type * @param <O> the output element type * @param iterable the iterable to transform, may not be null * @param transformer the transformer, must not be null * @return a transformed view of the specified iterable * @throws NullPointerException if either iterable or transformer is null */ public static <I, O> Iterable<O> transformedIterable(final Iterable<I> iterable, final Transformer<? super I, ? extends O> transformer) { checkNotNull(iterable); if (transformer == null) { throw new NullPointerException("Transformer must not be null."); } return new FluentIterable<O>() { @Override public Iterator<O> iterator() { return IteratorUtils.transformedIterator(iterable.iterator(), transformer); } }; } // Unique // ---------------------------------------------------------------------- /** * Returns a unique view of the given iterable. * <p> * The returned iterable's iterator supports {@code remove()} when the * corresponding input iterator supports it. Calling {@code remove()} * will only remove a single element from the underlying iterator. * * @param <E> the element type * @param iterable the iterable to use, may not be null * @return a unique view of the specified iterable * @throws NullPointerException if iterable is null */ public static <E> Iterable<E> uniqueIterable(final Iterable<E> iterable) { checkNotNull(iterable); return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return new UniqueFilterIterator<E>(iterable.iterator()); } }; } // Unmodifiable // ---------------------------------------------------------------------- /** * Returns an unmodifiable view of the given iterable. * <p> * The returned iterable's iterator does not support {@code remove()}. * * @param <E> the element type * @param iterable the iterable to use, may not be null * @return an unmodifiable view of the specified iterable * @throws NullPointerException if iterable is null */ public static <E> Iterable<E> unmodifiableIterable(final Iterable<E> iterable) { checkNotNull(iterable); if (iterable instanceof UnmodifiableIterable<?>) { return iterable; } return new UnmodifiableIterable<E>(iterable); } /** * Inner class to distinguish unmodifiable instances. */ private static final class UnmodifiableIterable<E> extends FluentIterable<E> { private final Iterable<E> unmodifiable; public UnmodifiableIterable(final Iterable<E> iterable) { super(); this.unmodifiable = iterable; } @Override public Iterator<E> iterator() { return IteratorUtils.unmodifiableIterator(unmodifiable.iterator()); } } // Zipping // ---------------------------------------------------------------------- /** * Interleaves two iterables into a single iterable. * <p> * The returned iterable has an iterator that traverses the elements in {@code a} * and {@code b} in alternating order. The source iterators are not polled until * necessary. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <E> the element type * @param a the first iterable, may not be null * @param b the second iterable, may not be null * @return a new iterable, interleaving the provided iterables * @throws NullPointerException if either a or b is null */ public static <E> Iterable<E> zippingIterable(final Iterable<? extends E> a, final Iterable<? extends E> b) { checkNotNull(a); checkNotNull(b); return new FluentIterable<E>() { @Override public Iterator<E> iterator() { return IteratorUtils.zippingIterator(a.iterator(), b.iterator()); } }; } /** * Interleaves two iterables into a single iterable. * <p> * The returned iterable has an iterator that traverses the elements in {@code a} * and {@code b} in alternating order. The source iterators are not polled until * necessary. * <p> * The returned iterable's iterator supports {@code remove()} when the corresponding * input iterator supports it. * * @param <E> the element type * @param first the first iterable, may not be null * @param others the array of iterables to interleave, may not be null * @return a new iterable, interleaving the provided iterables * @throws NullPointerException if either of the provided iterables is null */ public static <E> Iterable<E> zippingIterable(final Iterable<? extends E> first, final Iterable<? extends E>... others) { checkNotNull(first); checkNotNull(others); return new FluentIterable<E>() { @Override public Iterator<E> iterator() { @SuppressWarnings("unchecked") // safe Iterator<? extends E>[] iterators = new Iterator[others.length + 1]; iterators[0] = first.iterator(); for (int i = 0; i < others.length; i++) { iterators[i + 1] = others[i].iterator(); } return IteratorUtils.zippingIterator(iterators); } }; } // Utility methods // ---------------------------------------------------------------------- /** * Returns an immutable empty iterable if the argument is null, * or the argument itself otherwise. * * @param <E> the element type * @param iterable the iterable, may be null * @return an empty iterable if the argument is null */ public static <E> Iterable<E> emptyIfNull(final Iterable<E> iterable) { return iterable == null ? IterableUtils.<E>emptyIterable() : iterable; } /** * Applies the closure to each element of the provided iterable. * * @param <E> the element type * @param iterable the iterator to use, may be null * @param closure the closure to apply to each element, may not be null * @throws NullPointerException if closure is null */ public static <E> void forEach(final Iterable<E> iterable, final Closure<? super E> closure) { IteratorUtils.forEach(emptyIteratorIfNull(iterable), closure); } /** * Executes the given closure on each but the last element in the iterable. * <p> * If the input iterable is null no change is made. * * @param <E> the type of object the {@link Iterable} contains * @param iterable the iterable to get the input from, may be null * @param closure the closure to perform, may not be null * @return the last element in the iterable, or null if iterable is null or empty */ public static <E> E forEachButLast(final Iterable<E> iterable, final Closure<? super E> closure) { return IteratorUtils.forEachButLast(emptyIteratorIfNull(iterable), closure); } /** * Finds the first element in the given iterable which matches the given predicate. * <p> * A <code>null</code> or empty iterator returns null. * * @param <E> the element type * @param iterable the iterable to search, may be null * @param predicate the predicate to use, may not be null * @return the first element of the iterable which matches the predicate or null if none could be found * @throws NullPointerException if predicate is null */ public static <E> E find(final Iterable<E> iterable, final Predicate<? super E> predicate) { return IteratorUtils.find(emptyIteratorIfNull(iterable), predicate); } /** * Returns the index of the first element in the specified iterable that * matches the given predicate. * <p> * A <code>null</code> or empty iterable returns -1. * * @param <E> the element type * @param iterable the iterable to search, may be null * @param predicate the predicate to use, may not be null * @return the index of the first element which matches the predicate or -1 if none matches * @throws NullPointerException if predicate is null */ public static <E> int indexOf(final Iterable<E> iterable, final Predicate<? super E> predicate) { return IteratorUtils.indexOf(emptyIteratorIfNull(iterable), predicate); } /** * Answers true if a predicate is true for every element of an iterable. * <p> * A <code>null</code> or empty iterable returns true. * * @param <E> the type of object the {@link Iterable} contains * @param iterable the {@link Iterable} to use, may be null * @param predicate the predicate to use, may not be null * @return true if every element of the collection matches the predicate or if the * collection is empty, false otherwise * @throws NullPointerException if predicate is null */ public static <E> boolean matchesAll(final Iterable<E> iterable, final Predicate<? super E> predicate) { return IteratorUtils.matchesAll(emptyIteratorIfNull(iterable), predicate); } /** * Answers true if a predicate is true for any element of the iterable. * <p> * A <code>null</code> or empty iterable returns false. * * @param <E> the type of object the {@link Iterable} contains * @param iterable the {@link Iterable} to use, may be null * @param predicate the predicate to use, may not be null * @return true if any element of the collection matches the predicate, false otherwise * @throws NullPointerException if predicate is null */ public static <E> boolean matchesAny(final Iterable<E> iterable, final Predicate<? super E> predicate) { return IteratorUtils.matchesAny(emptyIteratorIfNull(iterable), predicate); } /** * Counts the number of elements in the input iterable that match the predicate. * <p> * A <code>null</code> iterable matches no elements. * * @param <E> the type of object the {@link Iterable} contains * @param input the {@link Iterable} to get the input from, may be null * @param predicate the predicate to use, may not be null * @return the number of matches for the predicate in the collection * @throws NullPointerException if predicate is null */ public static <E> long countMatches(final Iterable<E> input, final Predicate<? super E> predicate) { if (predicate == null) { throw new NullPointerException("Predicate must not be null."); } return size(filteredIterable(emptyIfNull(input), predicate)); } /** * Answers true if the provided iterable is empty. * <p> * A <code>null</code> iterable returns true. * * @param iterable the {@link Iterable to use}, may be null * @return true if the iterable is null or empty, false otherwise */ public static boolean isEmpty(final Iterable<?> iterable) { if (iterable instanceof Collection<?>) { return ((Collection<?>) iterable).isEmpty(); } else { return IteratorUtils.isEmpty(emptyIteratorIfNull(iterable)); } } /** * Checks if the object is contained in the given iterable. * <p> * A <code>null</code> or empty iterable returns false. * * @param <E> the type of object the {@link Iterable} contains * @param iterable the iterable to check, may be null * @param object the object to check * @return true if the object is contained in the iterable, false otherwise */ public static <E> boolean contains(final Iterable<E> iterable, final Object object) { if (iterable instanceof Collection<?>) { return ((Collection<E>) iterable).contains(object); } else { return IteratorUtils.contains(emptyIteratorIfNull(iterable), object); } } /** * Checks if the object is contained in the given iterable. Object equality * is tested with an {@code equator} unlike {@link #contains(Iterable, Object)} * which uses {@link Object#equals(Object)}. * <p> * A <code>null</code> or empty iterable returns false. * A <code>null</code> object will not be passed to the equator, instead a * {@link org.apache.commons.collections4.functors.NullPredicate NullPredicate} * will be used. * * @param <E> the type of object the {@link Iterable} contains * @param iterable the iterable to check, may be null * @param object the object to check * @param equator the equator to use to check, may not be null * @return true if the object is contained in the iterable, false otherwise * @throws NullPointerException if equator is null */ public static <E> boolean contains(final Iterable<? extends E> iterable, final E object, final Equator<? super E> equator) { if (equator == null) { throw new NullPointerException("Equator must not be null."); } return matchesAny(iterable, EqualPredicate.equalPredicate(object, equator)); } /** * Returns the number of occurrences of the provided object in the iterable. * * @param <E> the element type that the {@link Iterable} may contain * @param <T> the element type of the object to find * @param iterable the {@link Iterable} to search * @param obj the object to find the cardinality of * @return the the number of occurrences of obj in iterable */ public static <E, T extends E> int frequency(final Iterable<E> iterable, final T obj) { if (iterable instanceof Set<?>) { return ((Set<E>) iterable).contains(obj) ? 1 : 0; } if (iterable instanceof Bag<?>) { return ((Bag<E>) iterable).getCount(obj); } return size(filteredIterable(emptyIfNull(iterable), EqualPredicate.<E>equalPredicate(obj))); } /** * Returns the <code>index</code>-th value in the <code>iterable</code>'s {@link Iterator}, throwing * <code>IndexOutOfBoundsException</code> if there is no such element. * <p> * If the {@link Iterable} is a {@link List}, then it will use {@link List#get(int)}. * * @param <T> the type of object in the {@link Iterable}. * @param iterable the {@link Iterable} to get a value from, may be null * @param index the index to get * @return the object at the specified index * @throws IndexOutOfBoundsException if the index is invalid */ public static <T> T get(final Iterable<T> iterable, final int index) { CollectionUtils.checkIndexBounds(index); if (iterable instanceof List<?>) { return ((List<T>) iterable).get(index); } return IteratorUtils.get(emptyIteratorIfNull(iterable), index); } /** * Returns the number of elements contained in the given iterator. * <p> * A <code>null</code> or empty iterator returns {@code 0}. * * @param iterable the iterable to check, may be null * @return the number of elements contained in the iterable */ public static int size(final Iterable<?> iterable) { if (iterable instanceof Collection<?>) { return ((Collection<?>) iterable).size(); } else { return IteratorUtils.size(emptyIteratorIfNull(iterable)); } } /** * Partitions all elements from iterable into separate output collections, * based on the evaluation of the given predicate. * <p> * For each predicate, the result will contain a list holding all elements of the * input iterable matching the predicate. The last list will hold all elements * which didn't match any predicate: * <pre> * [C1, R] = partition(I, P1) with * I = input * P1 = first predicate * C1 = collection of elements matching P1 * R = collection of elements rejected by all predicates * </pre> * <p> * If the input iterable is <code>null</code>, the same is returned as for an * empty iterable. * <p> * Example: for an input list [1, 2, 3, 4, 5] calling partition with a predicate [x < 3] * will result in the following output: [[1, 2], [3, 4, 5]]. * * @param <O> the type of object the {@link Iterable} contains * @param iterable the iterable to partition, may be null * @param predicate the predicate to use, may not be null * @return a list containing the output collections * @throws NullPointerException if predicate is null */ public static <O> List<List<O>> partition(final Iterable<? extends O> iterable, final Predicate<? super O> predicate) { if (predicate == null) { throw new NullPointerException("Predicate must not be null."); } @SuppressWarnings({ "unchecked", "rawtypes" }) // safe final Factory<List<O>> factory = FactoryUtils.instantiateFactory((Class) ArrayList.class); @SuppressWarnings("unchecked") // safe final Predicate<? super O>[] predicates = new Predicate[] { predicate }; return partition(iterable, factory, predicates); } /** * Partitions all elements from iterable into separate output collections, * based on the evaluation of the given predicates. * <p> * For each predicate, the result will contain a list holding all elements of the * input iterable matching the predicate. The last list will hold all elements * which didn't match any predicate: * <pre> * [C1, C2, R] = partition(I, P1, P2) with * I = input * P1 = first predicate * P2 = second predicate * C1 = collection of elements matching P1 * C2 = collection of elements matching P2 * R = collection of elements rejected by all predicates * </pre> * <p> * <b>Note</b>: elements are only added to the output collection of the first matching * predicate, determined by the order of arguments. * <p> * If the input iterable is <code>null</code>, the same is returned as for an * empty iterable. * <p> * Example: for an input list [1, 2, 3, 4, 5] calling partition with predicates [x < 3] * and [x < 5] will result in the following output: [[1, 2], [3, 4], [5]]. * * @param <O> the type of object the {@link Iterable} contains * @param iterable the collection to get the input from, may be null * @param predicates the predicates to use, may not be null * @return a list containing the output collections * @throws NullPointerException if any predicate is null */ public static <O> List<List<O>> partition(final Iterable<? extends O> iterable, final Predicate<? super O>... predicates) { @SuppressWarnings({ "unchecked", "rawtypes" }) // safe final Factory<List<O>> factory = FactoryUtils.instantiateFactory((Class) ArrayList.class); return partition(iterable, factory, predicates); } /** * Partitions all elements from iterable into separate output collections, * based on the evaluation of the given predicates. * <p> * For each predicate, the returned list will contain a collection holding * all elements of the input iterable matching the predicate. The last collection * contained in the list will hold all elements which didn't match any predicate: * <pre> * [C1, C2, R] = partition(I, P1, P2) with * I = input * P1 = first predicate * P2 = second predicate * C1 = collection of elements matching P1 * C2 = collection of elements matching P2 * R = collection of elements rejected by all predicates * </pre> * <p> * <b>Note</b>: elements are only added to the output collection of the first matching * predicate, determined by the order of arguments. * <p> * If the input iterable is <code>null</code>, the same is returned as for an * empty iterable. * If no predicates have been provided, all elements of the input collection * will be added to the rejected collection. * <p> * Example: for an input list [1, 2, 3, 4, 5] calling partition with predicates [x < 3] * and [x < 5] will result in the following output: [[1, 2], [3, 4], [5]]. * * @param <O> the type of object the {@link Iterable} contains * @param <R> the type of the output {@link Collection} * @param iterable the collection to get the input from, may be null * @param partitionFactory the factory used to create the output collections * @param predicates the predicates to use, may not be null * @return a list containing the output collections * @throws NullPointerException if any predicate is null */ public static <O, R extends Collection<O>> List<R> partition(final Iterable<? extends O> iterable, final Factory<R> partitionFactory, final Predicate<? super O>... predicates) { if (iterable == null) { final Iterable<O> empty = emptyIterable(); return partition(empty, partitionFactory, predicates); } if (predicates == null) { throw new NullPointerException("Predicates must not be null."); } for (Predicate<?> p : predicates) { if (p == null) { throw new NullPointerException("Predicate must not be null."); } } if (predicates.length < 1) { // return the entire input collection as a single partition final R singlePartition = partitionFactory.create(); CollectionUtils.addAll(singlePartition, iterable); return Collections.singletonList(singlePartition); } // create the empty partitions final int numberOfPredicates = predicates.length; final int numberOfPartitions = numberOfPredicates + 1; final List<R> partitions = new ArrayList<R>(numberOfPartitions); for (int i = 0; i < numberOfPartitions; ++i) { partitions.add(partitionFactory.create()); } // for each element in inputCollection: // find the first predicate that evaluates to true. // if there is a predicate, add the element to the corresponding partition. // if there is no predicate, add it to the last, catch-all partition. for (final O element : iterable) { boolean elementAssigned = false; for (int i = 0; i < numberOfPredicates; ++i) { if (predicates[i].evaluate(element)) { partitions.get(i).add(element); elementAssigned = true; break; } } if (!elementAssigned) { // no predicates evaluated to true // add element to last partition partitions.get(numberOfPredicates).add(element); } } return partitions; } /** * Gets a new list with the contents of the provided iterable. * * @param <E> the element type * @param iterable the iterable to use, may be null * @return a list of the iterator contents */ public static <E> List<E> toList(final Iterable<E> iterable) { return IteratorUtils.toList(emptyIteratorIfNull(iterable)); } /** * Returns a string representation of the elements of the specified iterable. * <p> * The string representation consists of a list of the iterable's elements, * enclosed in square brackets ({@code "[]"}). Adjacent elements are separated * by the characters {@code ", "} (a comma followed by a space). Elements are * converted to strings as by {@code String.valueOf(Object)}. * * @param <E> the element type * @param iterable the iterable to convert to a string, may be null * @return a string representation of {@code iterable} */ public static <E> String toString(final Iterable<E> iterable) { return IteratorUtils.toString(emptyIteratorIfNull(iterable)); } /** * Returns a string representation of the elements of the specified iterable. * <p> * The string representation consists of a list of the iterable's elements, * enclosed in square brackets ({@code "[]"}). Adjacent elements are separated * by the characters {@code ", "} (a comma followed by a space). Elements are * converted to strings as by using the provided {@code transformer}. * * @param <E> the element type * @param iterable the iterable to convert to a string, may be null * @param transformer the transformer used to get a string representation of an element * @return a string representation of {@code iterable} * @throws NullPointerException if {@code transformer} is null */ public static <E> String toString(final Iterable<E> iterable, final Transformer<? super E, String> transformer) { if (transformer == null) { throw new NullPointerException("Transformer must not be null."); } return IteratorUtils.toString(emptyIteratorIfNull(iterable), transformer); } /** * Returns a string representation of the elements of the specified iterable. * <p> * The string representation consists of a list of the iterable's elements, * enclosed by the provided {@code prefix} and {@code suffix}. Adjacent elements * are separated by the provided {@code delimiter}. Elements are converted to * strings as by using the provided {@code transformer}. * * @param <E> the element type * @param iterable the iterable to convert to a string, may be null * @param transformer the transformer used to get a string representation of an element * @param delimiter the string to delimit elements * @param prefix the prefix, prepended to the string representation * @param suffix the suffix, appended to the string representation * @return a string representation of {@code iterable} * @throws NullPointerException if either transformer, delimiter, prefix or suffix is null */ public static <E> String toString(final Iterable<E> iterable, final Transformer<? super E, String> transformer, final String delimiter, final String prefix, final String suffix) { return IteratorUtils.toString(emptyIteratorIfNull(iterable), transformer, delimiter, prefix, suffix); } // Helper methods // ---------------------------------------------------------------------- /** * Fail-fast check for null arguments. * * @param iterable the iterable to check * @throws NullPointerException if iterable is null */ static void checkNotNull(final Iterable<?> iterable) { if (iterable == null) { throw new NullPointerException("Iterable must not be null."); } } /** * Fail-fast check for null arguments. * * @param iterable the iterable to check * @throws NullPointerException if the argument or any of its contents is null */ static void checkNotNull(final Iterable<?>... iterables) { if (iterables == null) { throw new NullPointerException("Iterables must not be null."); } for (final Iterable<?> iterable : iterables) { checkNotNull(iterable); } } /** * Returns an empty iterator if the argument is <code>null</code>, * or {@code iterable.iterator()} otherwise. * * @param <E> the element type * @param iterable the iterable, possibly <code>null</code> * @return an empty iterator if the argument is <code>null</code> */ private static <E> Iterator<E> emptyIteratorIfNull(final Iterable<E> iterable) { return iterable != null ? iterable.iterator() : IteratorUtils.<E>emptyIterator(); } }