TreeIterator.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.Collection;
import java.util.Iterator;
import java.util.LinkedList;

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


/**
 * A <code>TreeIterator</code> simplifies the traversal of a
 * tree of objects, where the objects' protocol(s) provides
 * a method for getting the immediate children of the given
 * node but does not provide a method for getting all the
 * descendants (children, grandchildren, etc.) of the given node.
 * <p>
 * To use, supply:<ul>
 * <li> either the root element of the tree or, if the tree has
 * multiple roots, an <code>Iterator</code> over the set of roots
 * <li> a <code>Midwife</code> that delivers the children
 * of each child
 * (alternatively, subclass <code>TreeIterator</code>
 * and override the <code>children(Object)</code> method)
 * </ul>
 * <p>
 */
public class TreeIterator implements Iterator {
    private Iterator currentIterator;
    private Collection iterators;
    private Midwife midwife;


    /**
     * Construct an iterator with the specified collection of roots
     * and a midwife that simply returns an empty iterator
     * for each of the roots.
     * Use this constructor if you want to override the
     * <code>children(Object)</code> method instead of building
     * a <code>Midwife</code>.
     */
    public TreeIterator(Iterator roots) {
        this(roots, Midwife.NULL_INSTANCE);
    }

    /**
     * Construct an iterator with the specified root
     * and a midwife that simply returns an empty iterator
     * for the root.
     * Use this constructor if you want to override the
     * <code>children(Object)</code> method instead of building
     * a <code>Midwife</code>.
     */
    public TreeIterator(Object root) {
        this(root, Midwife.NULL_INSTANCE);
    }

    /**
     * Construct an iterator with the specified root
     * and midwife.
     */
    public TreeIterator(Object root, Midwife midwife) {
        this(new SingleElementIterator(root), midwife);
    }

    /**
     * Construct an iterator with the specified roots
     * and midwife.
     */
    public TreeIterator(Iterator roots, Midwife midwife) {
        super();
        this.currentIterator = roots;
        // use a LinkedList since we will be pulling off the front and adding to the end
        this.iterators = new LinkedList();
        this.midwife = midwife;
    }

    /**
     * @see java.util.Iterator#hasNext()
     */
    public boolean hasNext() {
        if (this.currentIterator.hasNext()) {
            return true;
        }
        for (Iterator stream = this.iterators.iterator(); stream.hasNext(); ) {
            Iterator iterator = (Iterator) stream.next();
            if (iterator.hasNext()) {
                return true;
            }
        }
        return false;
    }

    /**
     * @see java.util.Iterator#next()
     */
    public Object next() {
        if (this.currentIterator.hasNext()) {
            return this.nextInternal();
        }
        for (Iterator stream = this.iterators.iterator(); stream.hasNext(); ) {
            this.currentIterator = (Iterator) stream.next();
            if (this.currentIterator.hasNext()) {
                break;
            }
            stream.remove();
        }
        return this.nextInternal();
    }

    /**
     * Fetch the children of the next node before returning it.
     */
    private Object nextInternal() {
        Object next = this.currentIterator.next();
        this.iterators.add(this.children(next));
        return next;
    }

    /**
     * @see java.util.Iterator#remove()
     */
    public void remove() {
        this.currentIterator.remove();
    }

    /**
     * Return the immediate children of the specified object.
     */
    protected Iterator children(Object next) {
        return this.midwife.children(next);
    }

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


//********** inner classes **********

/**
 * Used by <code>TreeIterator</code> to retrieve
 * the immediate children of a node in the tree.
 */
public interface Midwife {

    /**
     * Return the immediate children of the specified object.
     */
    Iterator children(Object next);


    Midwife NULL_INSTANCE =
        new Midwife() {
            // return no children
            public Iterator children(Object next) {
                return NullIterator.instance();
            }
        };

}

}