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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.AbstractList;
import java.util.ArrayList;
import java.util.List;

/**
 * List which returns special boundary symbols hwen get() is called outside the
 * range of the list.
 *
 * @author Dan Klein
 */
public class BoundedList  extends AbstractList {
  private E leftBoundary;
  private E rightBoundary;
  private List list;

  /**
   * Returns the object at the given index, provided the index is between 0
   * (inclusive) and size() (exclusive).  If the index is < 0, then a left
   * boundary object is returned.  If the index is >= size(), a right boundary
   * object is returned.  The default boundary objects are both null, unless
   * other objects are specified on construction.
   */
  @Override
public E get(int index) {
    if (index < 0) return leftBoundary;
    if (index >= list.size()) return rightBoundary;
    return list.get(index);
  }

  @Override
public int size() {
    return list.size();
  }

  public BoundedList(List list, E leftBoundary, E rightBoundary) {
    this.list = list;
    this.leftBoundary = leftBoundary;
    this.rightBoundary = rightBoundary;
  }

  public BoundedList(List list, E boundary) {
    this(list, boundary, boundary);
  }

  public BoundedList(List list) {
    this(list, null);
  }
  
  @Override
	public List subList(int fromIndex, int toIndex)
	{
		List retVal = new ArrayList(toIndex - fromIndex);
		for (int i = fromIndex; i < toIndex; ++i)
		{
			retVal.add(get(i));
		}
		return retVal;
	}
  
}