content: /**
   * 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.solr.common.util;
  
  import java.util.ArrayList;
  import java.util.Iterator;
  import java.util.List;
  
  /** Chain several Iterators, so that this iterates
   *  over all of them in sequence.
   */
  
  public class IteratorChain<E> implements Iterator<E> {
  
    private final List<Iterator<E>> iterators = new ArrayList<Iterator<E>>();
    private Iterator<Iterator<E>> itit;
    private Iterator<E> current;
   
    public void addIterator(Iterator<E> it) {
      if(itit!=null) throw new RuntimeException("all Iterators must be added before calling hasNext()");
      iterators.add(it);
    }
    
    public boolean hasNext() {
      if(itit==null) itit = iterators.iterator();
      return recursiveHasNext();
    }
    
    /** test if current iterator hasNext(), and if not try the next
     *  one in sequence, recursively
     */
    private boolean recursiveHasNext() {
      // return false if we have no more iterators
      if(current==null) {
        if(itit.hasNext()) {
          current=itit.next();
        } else {
          return false;
        }
      }
      
      boolean result = current.hasNext();
      if(!result) {
        current = null;
        result = recursiveHasNext();
      }
      
      return result;
    }
  
    /** hasNext() must ALWAYS be called before calling this
     *  otherwise it's a bit hard to keep track of what's happening
     */
    public E next() {
      if(current==null) { 
        throw new RuntimeException("For an IteratorChain, hasNext() MUST be called before calling next()");
      }
      return current.next();
    }
  
    public void remove() {
      // we just need this class 
      // to iterate in readonly mode
      throw new UnsupportedOperationException();
    }
    
  }
  