guava-tests/test/com/google/common/collect/PeekingIteratorTest.java

/*
 * Copyright (C) 2008 The Guava Authors
 *
 * 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.common.collect;

import static com.google.common.collect.Iterators.peekingIterator;
import static com.google.common.collect.testing.IteratorFeature.MODIFIABLE;
import static com.google.common.collect.testing.IteratorFeature.UNMODIFIABLE;
import static java.util.Collections.emptyList;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.collect.testing.IteratorTester;

import junit.framework.TestCase;

import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

/**
  * Unit test for {@link PeekingIterator}.
  *
  * @author Mick Killianey
  */
@SuppressWarnings("serial") // No serialization is used in this test
@GwtCompatible(emulated = true)
public class PeekingIteratorTest extends TestCase {

  /**
   * Version of {@link IteratorTester} that compares an iterator over
   * a given collection of elements (used as the reference iterator)
   * against a {@code PeekingIterator} that *wraps* such an iterator
   * (used as the target iterator).
   *
   * <p>This IteratorTester makes copies of the master so that it can
   * later verify that {@link PeekingIterator#remove()} removes the
   * same elements as the reference's iterator {@code #remove()}.
   */
  private static class PeekingIteratorTester<T> extends IteratorTester<T> {
    private Iterable<T> master;
    private List<T> targetList;

    public PeekingIteratorTester(Collection<T> master) {
      super(master.size() + 3, MODIFIABLE, master,
          IteratorTester.KnownOrder.KNOWN_ORDER);
      this.master = master;
    }
    @Override protected Iterator<T> newTargetIterator() {
      // make copy from master to verify later
      targetList = Lists.newArrayList(master);
      Iterator<T> iterator = targetList.iterator();
      return Iterators.peekingIterator(iterator);
    }
    @Override protected void verify(List<T> elements) {
      // verify same objects were removed from reference and target
      assertEquals(elements, targetList);
    }
  }

  private <T> void actsLikeIteratorHelper(final List<T> list) {
    // Check with modifiable copies of the list
    new PeekingIteratorTester<T>(list).test();

    // Check with unmodifiable lists
    new IteratorTester<T>(list.size() * 2 + 2, UNMODIFIABLE, list,
        IteratorTester.KnownOrder.KNOWN_ORDER) {
      @Override protected Iterator<T> newTargetIterator() {
        Iterator<T> iterator = Collections.unmodifiableList(list).iterator();
        return Iterators.peekingIterator(iterator);
      }
    }.test();
  }

  public void testPeekingIteratorBehavesLikeIteratorOnEmptyIterable() {
    actsLikeIteratorHelper(Collections.emptyList());
  }

  public void testPeekingIteratorBehavesLikeIteratorOnSingletonIterable() {
    actsLikeIteratorHelper(Collections.singletonList(new Object()));
  }

  // TODO(cpovirk): instead of skipping, use a smaller number of steps
  @GwtIncompatible("works but takes 5 minutes to run")
  public void testPeekingIteratorBehavesLikeIteratorOnThreeElementIterable() {
    actsLikeIteratorHelper(Lists.newArrayList("A", "B", "C"));
  }

  @GwtIncompatible("works but takes 5 minutes to run")
  public void testPeekingIteratorAcceptsNullElements() {
    actsLikeIteratorHelper(Lists.newArrayList(null, "A", null));
  }

  public void testPeekOnEmptyList() {
    List<?> list = Collections.emptyList();
    Iterator<?> iterator = list.iterator();
    PeekingIterator<?> peekingIterator = Iterators.peekingIterator(iterator);

    try {
      peekingIterator.peek();
      fail("Should throw NoSuchElementException if nothing to peek()");
    } catch (NoSuchElementException e) { /* expected */ }
  }

  public void testPeekDoesntChangeIteration() {
    List<?> list = Lists.newArrayList("A", "B", "C");
    Iterator<?> iterator = list.iterator();
    PeekingIterator<?> peekingIterator =
        Iterators.peekingIterator(iterator);

    assertEquals("Should be able to peek() at first element",
        "A", peekingIterator.peek());
    assertEquals("Should be able to peek() first element multiple times",
        "A", peekingIterator.peek());
    assertEquals("next() should still return first element after peeking",
        "A", peekingIterator.next());

    assertEquals("Should be able to peek() at middle element",
        "B", peekingIterator.peek());
    assertEquals("Should be able to peek() middle element multiple times",
        "B", peekingIterator.peek());
    assertEquals("next() should still return middle element after peeking",
        "B", peekingIterator.next());

    assertEquals("Should be able to peek() at last element",
        "C", peekingIterator.peek());
    assertEquals("Should be able to peek() last element multiple times",
        "C", peekingIterator.peek());
    assertEquals("next() should still return last element after peeking",
        "C", peekingIterator.next());

    try {
      peekingIterator.peek();
      fail("Should throw exception if no next to peek()");
    } catch (NoSuchElementException e) { /* expected */ }
    try {
      peekingIterator.peek();
      fail("Should continue to throw exception if no next to peek()");
    } catch (NoSuchElementException e) { /* expected */ }
    try {
      peekingIterator.next();
      fail("next() should still throw exception after the end of iteration");
    } catch (NoSuchElementException e) { /* expected */ }
  }

  public void testCantRemoveAfterPeek() {
    List<String> list = Lists.newArrayList("A", "B", "C");
    Iterator<String> iterator = list.iterator();
    PeekingIterator<?> peekingIterator = Iterators.peekingIterator(iterator);

    assertEquals("A", peekingIterator.next());
    assertEquals("B", peekingIterator.peek());

    /* Should complain on attempt to remove() after peek(). */
    try {
      peekingIterator.remove();
      fail("remove() should throw IllegalStateException after a peek()");
    } catch (IllegalStateException e) { /* expected */ }

    assertEquals("After remove() throws exception, peek should still be ok",
        "B", peekingIterator.peek());

    /* Should recover to be able to remove() after next(). */
    assertEquals("B", peekingIterator.next());
    peekingIterator.remove();
    assertEquals("Should have removed an element", 2, list.size());
    assertFalse("Second element should be gone", list.contains("B"));
  }

  static class ThrowsAtEndException extends RuntimeException { /* nothing */ }

  /**
    * This Iterator claims to have more elements than the underlying
    * iterable, but when you try to fetch the extra elements, it throws
    * an unchecked exception.
    */
  static class ThrowsAtEndIterator<E> implements Iterator<E> {
    Iterator<E> iterator;
    public ThrowsAtEndIterator(Iterable<E> iterable) {
      this.iterator = iterable.iterator();
    }
    @Override
    public boolean hasNext() {
      return true;  // pretend that you have more...
    }
    @Override
    public E next() {
      // ...but throw an unchecked exception when you ask for it.
      if (!iterator.hasNext()) {
        throw new ThrowsAtEndException();
      }
      return iterator.next();
    }
    @Override
    public void remove() {
      iterator.remove();
    }
  }

  public void testPeekingIteratorDoesntAdvancePrematurely() throws Exception {
    /*
     * This test will catch problems where the underlying iterator
     * throws a RuntimeException when retrieving the nth element.
     *
     * If the PeekingIterator is caching elements too aggressively,
     * it may throw the exception on the (n-1)th element (oops!).
     */

    /* Checks the case where the first element throws an exception. */

    List<Integer> list = emptyList();
    Iterator<Integer> iterator =
        peekingIterator(new ThrowsAtEndIterator<Integer>(list));
    assertNextThrows(iterator);

    /* Checks the case where a later element throws an exception. */

    list = Lists.newArrayList(1, 2);
    iterator = peekingIterator(new ThrowsAtEndIterator<Integer>(list));
    assertTrue(iterator.hasNext());
    iterator.next();
    assertTrue(iterator.hasNext());
    iterator.next();
    assertNextThrows(iterator);
  }

  private void assertNextThrows(Iterator<?> iterator) {
    try {
      iterator.next();
      fail();
    } catch (ThrowsAtEndException expected) {
    }
  }
}