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[COLLECTIONS-462] Added PeekingIterator. Thanks to Andy Seaborne 

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1479760 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Thomas Neidhart 2013-05-07 05:12:06 +00:00
parent 278eed3e1f
commit 73dcea01b1
5 changed files with 326 additions and 0 deletions
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1
RELEASE-NOTES.txt
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@ -62,6 +62,7 @@ Removed classes
New classes
-----------
o [COLLECTIONS-462] PeekingIterator - supports one-element lookahead during iteration. Thanks to Andy Seaborne, Claude Warren.
o [COLLECTIONS-432] CircularFifoQueue - analogous class to CircularFifoBuffer for the Queue interface
PredicatedQueue - analogous class to PredicatedBuffer
TransformedQueue - analogous class to TransformedBuffer

3
src/changes/changes.xml
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@ -22,6 +22,9 @@
<body>
<release version="4.0" date="TBA" description="Next release">
<action issue="COLLECTIONS-462" dev="tn" type="add" due-to="Andy Seaborne, Claude Warren">
Added "PeekingIterator" decorator to support one-element lookahead during iteration.
</action>
<action issue="COLLECTIONS-458" dev="sebb" type="remove">
Removed unused class "AbstractUntypedCollectionDecorator&lt;E, D&gt;".
</action>

16
src/main/java/org/apache/commons/collections4/IteratorUtils.java
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@ -50,6 +50,7 @@ import org.apache.commons.collections4.iterators.NodeListIterator;
import org.apache.commons.collections4.iterators.ObjectArrayIterator;
import org.apache.commons.collections4.iterators.ObjectArrayListIterator;
import org.apache.commons.collections4.iterators.ObjectGraphIterator;
import org.apache.commons.collections4.iterators.PeekingIterator;
import org.apache.commons.collections4.iterators.SingletonIterator;
import org.apache.commons.collections4.iterators.SingletonListIterator;
import org.apache.commons.collections4.iterators.TransformIterator;
@ -801,6 +802,21 @@ public class IteratorUtils {
return new NodeListIterator(node);
}
// Peeking
//-----------------------------------------------------------------------
/**
* Gets an iterator that supports one-element lookahead.
*
* @param <E> the element type
* @param iterator the iterator to decorate, not null
* @return a peeking iterator
* @throws NullPointerException if the iterator is null
*/
public static <E> Iterator<E> peekingIterator(final Iterator<? extends E> iterator) {
return PeekingIterator.peekingIterator(iterator);
}
// Views
//-----------------------------------------------------------------------
/**

160
src/main/java/org/apache/commons/collections4/iterators/PeekingIterator.java Normal file
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@ -0,0 +1,160 @@
/*
* 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.iterators;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* Decorates an iterator to support one-element lookahead while iterating.
* <p>
* The decorator supports the removal operation, but an {@link IllegalStateException}
* will be thrown if {@link #remove()} is called directly after a call to
* {@link #peek()} or {@link #element()}.
*
* @since 4.0
* @version $Id$
*/
public class PeekingIterator<E> implements Iterator<E> {
/** The iterator being decorated. */
private final Iterator<? extends E> iterator;
/** Indicates that the decorated iterator is exhausted. */
private boolean exhausted = false;
/** Indicates if the lookahead slot is filled. */
private boolean slotFilled = false;
/** The current slot for lookahead. */
private E slot;
//-----------------------------------------------------------------------
/**
* Decorates the specified iterator to support one-element lookahead.
* <p>
* If the iterator is already a {@link PeekingIterator} it is returned directly.
*
* @param <E> the element type
* @param iterator the iterator to decorate
* @return a new peeking iterator
* @throws IllegalArgumentException if the iterator is null
*/
public static <E> PeekingIterator<E> peekingIterator(final Iterator<? extends E> iterator) {
if (iterator == null) {
throw new IllegalArgumentException("Iterator must not be null");
}
if (iterator instanceof PeekingIterator<?>) {
@SuppressWarnings("unchecked") // safe cast
final PeekingIterator<E> it = (PeekingIterator<E>) iterator;
return it;
}
return new PeekingIterator<E>(iterator);
}
//-----------------------------------------------------------------------
/**
* Constructor.
*
* @param iterator the iterator to decorate
*/
public PeekingIterator(final Iterator<? extends E> iterator) {
this.iterator = iterator;
}
private void fill() {
if (exhausted || slotFilled) {
return;
}
if (iterator.hasNext()) {
slot = iterator.next();
slotFilled = true;
} else {
exhausted = true;
slot = null;
slotFilled = false;
}
}
//-----------------------------------------------------------------------
public boolean hasNext() {
if (exhausted) {
return false;
}
return slotFilled ? true : iterator.hasNext();
}
/**
* Returns the next element in iteration without advancing the underlying iterator.
* If the iterator is already exhausted, null will be returned.
* <p>
* Note: this method does not throw a {@link NoSuchElementException} if the iterator
* is already exhausted. If you want such a behavior, use {@link #element()} instead.
* <p>
* The rationale behind this is to follow the {@link java.util.Queue} interface
* which uses the same terminology.
*
* @return the next element from the iterator
*/
public E peek() {
fill();
return exhausted ? null : slot;
}
/**
* Returns the next element in iteration without advancing the underlying iterator.
* If the iterator is already exhausted, null will be returned.
*
* @return the next element from the iterator
* @throws NoSuchElementException if the iterator is already exhausted according to {@link #hasNext()}
*/
public E element() {
fill();
if (exhausted) {
throw new NoSuchElementException();
}
return slot;
}
public E next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
final E x = slotFilled ? slot : iterator.next();
// reset the lookahead slot
slot = null;
slotFilled = false;
return x;
}
/**
* {@inheritDoc}
*
* @throws IllegalStateException if {@link #peek()} or {@link #element()} has been called
* prior to the call to {@link #remove()}
*/
public void remove() {
if (slotFilled) {
throw new IllegalStateException();
} else {
iterator.remove();
}
}
}

146
src/test/java/org/apache/commons/collections4/iterators/PeekingIteratorTest.java Normal file
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@ -0,0 +1,146 @@
/*
* 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.iterators;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import org.junit.Test;
/**
* Tests the PeekingIterator.
*
* @version $Id$
*/
public class PeekingIteratorTest<E> extends AbstractIteratorTest<E> {
private String[] testArray = { "a", "b", "c" };
private List<E> testList;
public PeekingIteratorTest(final String testName) {
super(testName);
}
/**
* {@inheritDoc}
*/
@SuppressWarnings("unchecked")
@Override
protected void setUp() throws Exception {
super.setUp();
testList = new ArrayList<E>(Arrays.asList((E[]) testArray));
}
@Override
public Iterator<E> makeEmptyIterator() {
return PeekingIterator.peekingIterator(Collections.<E>emptyList().iterator());
}
@Override
public PeekingIterator<E> makeObject() {
return PeekingIterator.peekingIterator(testList.iterator());
}
@Override
public boolean supportsRemove() {
return true;
}
//-----------------------------------------------------------------------
@Test
public void testEmpty() {
Iterator<E> it = makeEmptyIterator();
assertFalse(it.hasNext());
}
@Test
@SuppressWarnings("unchecked")
public void testSinglePeek() {
PeekingIterator<E> it = makeObject();
assertEquals("a", it.peek());
assertEquals("a", it.element());
validate(it, (E[]) testArray);
}
@Test
public void testMultiplePeek() {
PeekingIterator<E> it = makeObject();
assertEquals("a", it.peek());
assertEquals("a", it.peek());
assertEquals("a", it.next());
assertTrue(it.hasNext());
assertEquals("b", it.peek());
assertEquals("b", it.peek());
assertEquals("b", it.next());
assertTrue(it.hasNext());
assertEquals("c", it.peek());
assertEquals("c", it.peek());
assertEquals("c", it.next());
assertFalse(it.hasNext());
}
@Test
public void testIteratorExhausted() {
PeekingIterator<E> it = makeObject();
it.next();
it.next();
it.next();
assertFalse(it.hasNext());
assertNull(it.peek());
try {
it.element();
fail();
} catch (NoSuchElementException e) {
// expected
}
}
@Test
public void testIllegalRemove() {
PeekingIterator<E> it = makeObject();
it.next();
it.remove(); // supported
assertTrue(it.hasNext());
assertEquals("b", it.peek());
try {
it.remove();
fail();
} catch (IllegalStateException e) {
// expected
}
}
private void validate(Iterator<E> iter, E... items) {
for (E x : items) {
assertTrue(iter.hasNext());
assertEquals(x, iter.next());
}
assertFalse(iter.hasNext());
}
}