Iterables.java

/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed 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 com.google.inject.internal.util;

import java.util.Arrays;
import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * This class contains static utility methods that operate on or return objects
 * of type {@code Iterable}. Also see the parallel implementations in {@link
 * Iterators}.
 *
 * @author Kevin Bourrillion
 * @author Scott Bonneau
 */
public final class Iterables {
  private Iterables() {}

  /**
   * Returns a string representation of {@code iterable}, with the format
   * {@code [e1, e2, ..., en]}.
   */
  public static String toString(Iterable<?> iterable) {
    return Iterators.toString(iterable.iterator());
  }

  /**
   * Returns the single element contained in {@code iterable}.
   *
   * @throws NoSuchElementException if the iterable is empty
   * @throws IllegalArgumentException if the iterable contains multiple
   *     elements
   */
  public static <T> T getOnlyElement(Iterable<T> iterable) {
    return Iterators.getOnlyElement(iterable.iterator());
  }

  /**
   * Combines two iterables into a single iterable. 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.
   */
  @SuppressWarnings("unchecked")
  public static <T> Iterable<T> concat(
      Iterable<? extends T> a, Iterable<? extends T> b) {
    Preconditions.checkNotNull(a);
    Preconditions.checkNotNull(b);
    return concat(Arrays.asList(a, b));
  }

  /**
   * Combines multiple iterables into a single iterable. The returned iterable
   * has an iterator that traverses the elements of each iterable in
   * {@code inputs}. The input iterators are not polled until necessary.
   *
   * <p>The returned iterable's iterator supports {@code remove()} when the
   * corresponding input iterator supports it. The methods of the returned
   * iterable may throw {@code NullPointerException} if any of the input
   * iterators are null.
   */
  public static <T> Iterable<T> concat(
      Iterable<? extends Iterable<? extends T>> inputs) {
    /*
     * Hint: if you let A represent Iterable<? extends T> and B represent
     * Iterator<? extends T>, then this Function would look simply like:
     *
     *   Function<A, B> function = new Function<A, B> {
     *     public B apply(A from) {
     *       return from.iterator();
     *     }
     *   }
     *
     * TODO: there may be a better way to do this.
     */

    Function<Iterable<? extends T>, Iterator<? extends T>> function
        = new Function<Iterable<? extends T>, Iterator<? extends T>>() {
      public Iterator<? extends T> apply(Iterable<? extends T> from) {
        return from.iterator();
      }
    };
    final Iterable<Iterator<? extends T>> iterators
        = transform(inputs, function);
    return new IterableWithToString<T>() {
      public Iterator<T> iterator() {
        return Iterators.concat(iterators.iterator());
      }
    };
  }

  /**
   * Returns an iterable that applies {@code function} to each element of {@code
   * fromIterable}.
   *
   * <p>The returned iterable's iterator supports {@code remove()} if the
   * provided iterator does. After a successful {@code remove()} call,
   * {@code fromIterable} no longer contains the corresponding element.
   */
  public static <F, T> Iterable<T> transform(final Iterable<F> fromIterable,
      final Function<? super F, ? extends T> function) {
    Preconditions.checkNotNull(fromIterable);
    Preconditions.checkNotNull(function);
    return new IterableWithToString<T>() {
      public Iterator<T> iterator() {
        return Iterators.transform(fromIterable.iterator(), function);
      }
    };
  }

  // Methods only in Iterables, not in Iterators

  abstract static class IterableWithToString<E> implements Iterable<E> {
    @Override public String toString() {
      return Iterables.toString(this);
    }
  }
}