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Commons Collections example source code file (IteratorChain.java)

This example Commons Collections source code file (IteratorChain.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons Collections tags/keywords

arraylist, indexoutofboundsexception, iterator, iterator, iteratorchain, iteratorchain, list, list, nullpointerexception, nullpointerexception, object, unsupportedoperationexception, util

The Commons Collections IteratorChain.java source code

/*
 *  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.collections.iterators;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;

import org.apache.commons.collections.list.UnmodifiableList;

/**
 * An IteratorChain is an Iterator that wraps a number of Iterators.
 * <p>
 * This class makes multiple iterators look like one to the caller
 * When any method from the Iterator interface is called, the IteratorChain
 * will delegate to a single underlying Iterator. The IteratorChain will
 * invoke the Iterators in sequence until all Iterators are exhausted.
 * <p>
 * Under many circumstances, linking Iterators together in this manner is
 * more efficient (and convenient) than reading out the contents of each
 * Iterator into a List and creating a new Iterator.
 * <p>
 * Calling a method that adds new Iterator<i>after a method in the Iterator
 * interface has been called</i> will result in an UnsupportedOperationException.
 * Subclasses should <i>take care to not alter the underlying List of Iterators.
 * <p>
 * NOTE: As from version 3.0, the IteratorChain may contain no
 * iterators. In this case the class will function as an empty iterator.
 *
 * @since Commons Collections 2.1
 * @version $Revision: 647116 $ $Date: 2008-04-11 12:23:08 +0100 (Fri, 11 Apr 2008) $
 *
 * @author Morgan Delagrange
 * @author Stephen Colebourne
 */
public class IteratorChain implements Iterator {

    /** The chain of iterators */
    protected final List iteratorChain = new ArrayList();
    /** The index of the current iterator */
    protected int currentIteratorIndex = 0;
    /** The current iterator */
    protected Iterator currentIterator = null;
    /**
     * The "last used" Iterator is the Iterator upon which
     * next() or hasNext() was most recently called
     * used for the remove() operation only
     */
    protected Iterator lastUsedIterator = null;
    /**
     * ComparatorChain is "locked" after the first time
     * compare(Object,Object) is called
     */
    protected boolean isLocked = false;

    //-----------------------------------------------------------------------
    /**
     * Construct an IteratorChain with no Iterators.
     * <p>
     * You will normally use {@link #addIterator(Iterator)} to add
     * some iterators after using this constructor.
     */
    public IteratorChain() {
        super();
    }

    /**
     * Construct an IteratorChain with a single Iterator.
     *
     * @param iterator first Iterator in the IteratorChain
     * @throws NullPointerException if the iterator is null
     */
    public IteratorChain(Iterator iterator) {
        super();
        addIterator(iterator);
    }

    /**
     * Constructs a new <code>IteratorChain over the two
     * given iterators.
     *
     * @param a  the first child iterator
     * @param b  the second child iterator
     * @throws NullPointerException if either iterator is null
     */
    public IteratorChain(Iterator a, Iterator b) {
        super();
        addIterator(a);
        addIterator(b);
    }

    /**
     * Constructs a new <code>IteratorChain over the array
     * of iterators.
     *
     * @param iterators  the array of iterators
     * @throws NullPointerException if iterators array is or contains null
     */
    public IteratorChain(Iterator[] iterators) {
        super();
        for (int i = 0; i < iterators.length; i++) {
            addIterator(iterators[i]);
        }
    }

    /**
     * Constructs a new <code>IteratorChain over the collection
     * of iterators.
     *
     * @param iterators  the collection of iterators
     * @throws NullPointerException if iterators collection is or contains null
     * @throws ClassCastException if iterators collection doesn't contain an iterator
     */
    public IteratorChain(Collection iterators) {
        super();
        for (Iterator it = iterators.iterator(); it.hasNext();) {
            Iterator item = (Iterator) it.next();
            addIterator(item);
        }
    }
    
    //-----------------------------------------------------------------------
    /**
     * Add an Iterator to the end of the chain
     *
     * @param iterator Iterator to add
     * @throws IllegalStateException if I've already started iterating
     * @throws NullPointerException if the iterator is null
     */
    public void addIterator(Iterator iterator) {
        checkLocked();
        if (iterator == null) {
            throw new NullPointerException("Iterator must not be null");
        }
        iteratorChain.add(iterator);
    }

    /**
     * Set the Iterator at the given index
     *
     * @param index      index of the Iterator to replace
     * @param iterator   Iterator to place at the given index
     * @throws IndexOutOfBoundsException if index < 0 or index > size()
     * @throws IllegalStateException if I've already started iterating
     * @throws NullPointerException if the iterator is null
     */
    public void setIterator(int index, Iterator iterator) throws IndexOutOfBoundsException {
        checkLocked();
        if (iterator == null) {
            throw new NullPointerException("Iterator must not be null");
        }
        iteratorChain.set(index, iterator);
    }

    /**
     * Get the list of Iterators (unmodifiable)
     *
     * @return the unmodifiable list of iterators added
     */
    public List getIterators() {
        return UnmodifiableList.decorate(iteratorChain);
    }

    /**
     * Number of Iterators in the current IteratorChain.
     *
     * @return Iterator count
     */
    public int size() {
        return iteratorChain.size();
    }

    /**
     * Determine if modifications can still be made to the IteratorChain.
     * IteratorChains cannot be modified once they have executed a method
     * from the Iterator interface.
     *
     * @return true if IteratorChain cannot be modified, false if it can
     */
    public boolean isLocked() {
        return isLocked;
    }

    /**
     * Checks whether the iterator chain is now locked and in use.
     */
    private void checkLocked() {
        if (isLocked == true) {
            throw new UnsupportedOperationException("IteratorChain cannot be changed after the first use of a method from the Iterator interface");
        }
    }

    /**
     * Lock the chain so no more iterators can be added.
     * This must be called from all Iterator interface methods.
     */
    private void lockChain() {
        if (isLocked == false) {
            isLocked = true;
        }
    }

    /**
     * Updates the current iterator field to ensure that the current Iterator
     * is not exhausted
     */
    protected void updateCurrentIterator() {
        if (currentIterator == null) {
            if (iteratorChain.isEmpty()) {
                currentIterator = EmptyIterator.INSTANCE;
            } else {
                currentIterator = (Iterator) iteratorChain.get(0);
            }
            // set last used iterator here, in case the user calls remove
            // before calling hasNext() or next() (although they shouldn't)
            lastUsedIterator = currentIterator;
        }

        while (currentIterator.hasNext() == false && currentIteratorIndex < iteratorChain.size() - 1) {
            currentIteratorIndex++;
            currentIterator = (Iterator) iteratorChain.get(currentIteratorIndex);
        }
    }

    //-----------------------------------------------------------------------
    /**
     * Return true if any Iterator in the IteratorChain has a remaining element.
     *
     * @return true if elements remain
     */
    public boolean hasNext() {
        lockChain();
        updateCurrentIterator();
        lastUsedIterator = currentIterator;

        return currentIterator.hasNext();
    }

    /**
     * Returns the next Object of the current Iterator
     *
     * @return Object from the current Iterator
     * @throws java.util.NoSuchElementException if all the Iterators are exhausted
     */
    public Object next() {
        lockChain();
        updateCurrentIterator();
        lastUsedIterator = currentIterator;

        return currentIterator.next();
    }

    /**
     * Removes from the underlying collection the last element
     * returned by the Iterator.  As with next() and hasNext(),
     * this method calls remove() on the underlying Iterator.
     * Therefore, this method may throw an
     * UnsupportedOperationException if the underlying
     * Iterator does not support this method.
     *
     * @throws UnsupportedOperationException
     *   if the remove operator is not supported by the underlying Iterator
     * @throws IllegalStateException
     *   if the next method has not yet been called, or the remove method has
     *   already been called after the last call to the next method.
     */
    public void remove() {
        lockChain();
        if (currentIterator == null) {
            updateCurrentIterator();
        }
        lastUsedIterator.remove();
    }

}

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