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Stanford CoreNLP provides a set of natural language analysis tools which can take raw English language text input and give the base forms of words, their parts of speech, whether they are names of companies, people, etc., normalize dates, times, and numeric quantities, mark up the structure of sentences in terms of phrases and word dependencies, and indicate which noun phrases refer to the same entities. It provides the foundational building blocks for higher level text understanding applications.

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package edu.stanford.nlp.util;

import java.util.AbstractList;
import java.util.List;
import java.io.Serializable;

/**
 * A PaddedList wraps another list, presenting an apparently infinite
 * list by padding outside the real confines of the list with a default
 * value.  Note that size() returns the true size, but
 * get() works for any number.
 *
 * @author Christopher Manning
 */
public class PaddedList extends AbstractList implements Serializable {

  private final List l;
  private final E padding;

  public E getPad() {
    return padding;
  }

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

  @Override
  public E get(int i) {
    if (i < 0 || i >= size()) {
      return padding;
    }
    return l.get(i);
  }

  @Override
  public String toString() {
    return l.toString();
  }

  /** With this constructor, get() will return null for
   *  elements outside the real list.
   */
  public PaddedList(List l) {
    this(l, null);
  }

  public PaddedList(List l, E padding) {
    this.l = l;
    this.padding = padding;
  }

  /** This returns the inner list that was wrapped.
   *  Use of this method should be avoided.  There's currently only
   *  one use.
   *
   *  @return The inner list of the PaddedList.
   */
  @Deprecated
  public List getWrappedList() {
    return l;
  }

  /** A static method that provides an easy way to create a list of a
   *  certain parametric type.
   *  This static constructor works better with generics.
   *
   *  @param list The list to pad
   *  @param padding The padding element (may be null)
   *  @return The padded list
   */
  public static  PaddedList valueOf(List list, IN padding) {
    return new PaddedList<>(list, padding);
  }

  /** Returns true if this PaddedList and another are wrapping the
   *  same list.  This is tested as ==. Kinda yucky, but sometimes you
   *  want to know.
   */
  public boolean sameInnerList(PaddedList p) {
    return p != null && l == p.l;
  }

  private static final long serialVersionUID = 2064775966439971729L;

}