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/*******************************************************************************
* Copyright (c) 2005, 2009 Oracle. All rights reserved.
* This program and the accompanying materials are made available under the
* terms of the Eclipse Public License v1.0, which accompanies this distribution
* and is available at http://www.eclipse.org/legal/epl-v10.html.
*
* Contributors:
* Oracle - initial API and implementation
******************************************************************************/
package org.eclipse.jpt.common.utility.internal.iterators;
import java.util.Iterator;
import java.util.NoSuchElementException;
import org.eclipse.jpt.common.utility.internal.StringTools;
/**
* A <code>PeekableIterator</code> wraps another {@link Iterator}
* and allows a {@link #peek()} at the next element to be
* returned by {@link #next()}.
* <p>
* One, possibly undesirable, side-effect of using this iterator is that
* the nested iterator's <code>next()</code> method will be invoked
* <em>before</em> the peekable iterator's {@link #next()}
* method is invoked. This is because the "next" element must be
* pre-loaded for the {@link #peek()} method.
* This also prevents a peekable iterator from supporting the optional
* {@link #remove()} method.
*
* @param <E> the type of elements returned by the iterator
*
* @see org.eclipse.jpt.common.utility.internal.iterables.PeekableIterable
*/
public class PeekableIterator<E>
implements Iterator<E>
{
private final Iterator<? extends E> iterator;
private E next;
private boolean done;
/**
* Construct a peekable iterator that wraps the specified
* iterable.
*/
public PeekableIterator(Iterable<? extends E> iterable) {
this(iterable.iterator());
}
/**
* Construct a peekable iterator that wraps the specified nested
* iterator.
*/
public PeekableIterator(Iterator<? extends E> iterator) {
super();
this.iterator = iterator;
this.done = false;
this.loadNext();
}
public boolean hasNext() {
return ! this.done;
}
public E next() {
if (this.done) {
throw new NoSuchElementException();
}
E result = this.next;
this.loadNext();
return result;
}
/**
* Return the element that will be returned by the next call to the
* {@link #next()} method, without advancing past it.
*/
public E peek() {
if (this.done) {
throw new NoSuchElementException();
}
return this.next;
}
/**
* Because we need to pre-load the next element
* to be returned, we cannot support the {@link #remove()}
* method.
*/
public void remove() {
throw new UnsupportedOperationException();
}
/**
* Load next with the next entry from the nested
* iterator. If there are none, {@link #next} is set to <code>null</code>
* and {@link #done} is set to <code>true</code>.
*/
private void loadNext() {
if (this.iterator.hasNext()) {
this.next = this.iterator.next();
} else {
this.next = null;
this.done = true;
}
}
@Override
public String toString() {
return StringTools.buildToStringFor(this, this.iterator);
}
}
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