PeekableIterator.java

/*******************************************************************************
 * Copyright (c) 1998, 2015 Oracle and/or its affiliates. All rights reserved.
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *     Oracle - initial API and implementation from Oracle TopLink
******************************************************************************/
package org.eclipse.persistence.tools.workbench.utility.iterators;

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

import org.eclipse.persistence.tools.workbench.utility.ClassTools;


/**
 * A <code>PeekableIterator</code> wraps another <code>Iterator</code>
 * and allows a <code>peek()</code> at the next element to be
 * returned by <code>next()</code>.
 * <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 <code>next()</code>
 * method is invoked. This is because the "next" element must be
 * pre-loaded for the <code>peek()</code> method.
 * This also prevents a peekable iterator from supporting the optional
 * <code>remove()</code> method.
 */

public class PeekableIterator implements Iterator {
    private Iterator nestedIterator;
    private Object next;

    private static final Object END = new Object();

    /**
     * Construct a peekable iterator that wraps the specified nested
     * iterator.
     */
    public PeekableIterator(Iterator nestedIterator) {
        super();
        this.nestedIterator = nestedIterator;
        this.loadNext();
    }

    /**
     * @see java.util.Iterator#hasNext()
     */
    public boolean hasNext() {
        return this.next != END;
    }

    /**
     * @see java.util.Iterator#next()
     */
    public Object next() {
        if (this.next == END) {
            throw new NoSuchElementException();
        }
        Object result = this.next;
        this.loadNext();
        return result;
    }

    /**
     * Return the element that will be returned by the next call to the
     * <code>next()</code> method, without advancing past it.
     */
    public Object peek() {
        if (this.next == END) {
            throw new NoSuchElementException();
        }
        return this.next;
    }

    /**
     * Because we need to pre-load the next element
     * to be returned, we cannot support the <code>remove()</code>
     * method.
     * @see java.util.Iterator#remove()
     */
    public void remove() {
        throw new UnsupportedOperationException();
    }

    /**
     * Load next with the next entry from the nested
     * iterator. If there are none, next is set to <code>END</code>.
     */
    private void loadNext() {
        if (this.nestedIterator.hasNext()) {
            this.next = this.nestedIterator.next();
        } else {
            this.next = END;
        }
    }

    /**
     * @see Object#toString()
     */
    public String toString() {
        return ClassTools.shortClassNameForObject(this) + '(' + this.nestedIterator + ')';
    }

}