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The Berkeley parser analyzes the grammatical structure of natural language using probabilistic context-free grammars (PCFGs).

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package edu.berkeley.nlp.util;

import java.util.*;

/**
 * Concatenates an iterator over iterators into one long iterator.
 *
 * @author Dan Klein
 */
public class ConcatenationIterator implements Iterator {

  Iterator> sourceIterators;
  Iterator currentIterator;
  Iterator lastIteratorToReturn;
  
  public boolean hasNext() {
    if (currentIterator.hasNext())
      return true;
    return false;
  }

  public E next() {
    if (currentIterator.hasNext()) {
      E e = currentIterator.next();
      lastIteratorToReturn = currentIterator;
      advance();
      return e;
    }
    throw new NoSuchElementException();
  }

  private void advance() {
    while (! currentIterator.hasNext() && sourceIterators.hasNext()) {
      currentIterator = sourceIterators.next();
    }
  }

  public void remove() {
    if (lastIteratorToReturn == null)
      throw new IllegalStateException();
    currentIterator.remove();
  }

  public ConcatenationIterator(Iterator> sourceIterators) {
    this.sourceIterators = sourceIterators;
    this.currentIterator = (new ArrayList()).iterator();
    this.lastIteratorToReturn = null;
    advance();
  }

  public ConcatenationIterator(Collection> iteratorCollection) {
    this(iteratorCollection.iterator());
  }

  public static void main(String[] args) {
    List list0 = Collections.emptyList();
    List list1 = Arrays.asList("a b c d".split(" "));
    List list2 = Arrays.asList("e f".split(" "));
    List> iterators = new ArrayList>();
    iterators.add(list1.iterator());
    iterators.add(list0.iterator());
    iterators.add(list2.iterator());
    iterators.add(list0.iterator());
    Iterator iterator = new ConcatenationIterator(iterators);
    while (iterator.hasNext()) {
      System.out.println(iterator.next());
    }
  }
}