CompositeIterator.java

/*
 * Copyright 2002-2012 the original author or authors.
 *
 * Licensed 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 com.xiongyingqi.util;

import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.NoSuchElementException;

/**
 * Iterator that combines multiple other iterators.
 * This implementation maintains a list of iterators which are invoked in sequence until all iterators are exhausted.
 *
 * @author Erwin Vervaet
 */
public class CompositeIterator<E> implements Iterator<E> {

    private List<Iterator<E>> iterators = new LinkedList<Iterator<E>>();

    private boolean inUse = false;

    /**
     * Create a new composite iterator. Add iterators using the {@link #add(java.util.Iterator)} method.
     */
    public CompositeIterator() {
    }

    /**
     * Add given iterator to this composite.
     */
    public void add(Iterator<E> iterator) {
        Assert.state(!inUse, "You can no longer add iterator to a composite iterator that's already in use");
        if (iterators.contains(iterator)) {
            throw new IllegalArgumentException("You cannot add the same iterator twice");
        }
        iterators.add(iterator);
    }

    @Override
    public boolean hasNext() {
        inUse = true;
        for (Iterator<Iterator<E>> it = iterators.iterator(); it.hasNext(); ) {
            if (it.next().hasNext()) {
                return true;
            }
        }
        return false;
    }

    @Override
    public E next() {
        inUse = true;
        for (Iterator<Iterator<E>> it = iterators.iterator(); it.hasNext(); ) {
            Iterator<E> iterator = it.next();
            if (iterator.hasNext()) {
                return iterator.next();
            }
        }
        throw new NoSuchElementException("Exhausted all iterators");
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException("Remove is not supported");
    }
}