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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.trees; 
import edu.stanford.nlp.util.logging.Redwood;

import edu.stanford.nlp.ling.Label;

import java.util.List;

/**
 * A SimpleTree is a minimal concrete implementation of an
 * unlabeled, unscored Tree.  It has a tree structure, but
 * nothing is stored at a node (no label or score).
 * So, most of the time, this is the wrong class to use!
 * Look at {@code LabeledScoredTreeNode}.
 *
 * @author Christopher Manning
 */
public class SimpleTree extends Tree  {

  /** A logger for this class */
  private static Redwood.RedwoodChannels log = Redwood.channels(SimpleTree.class);

  private static final long serialVersionUID = -8075763706877132926L;

  /**
   * Daughters of the parse tree.
   */
  private Tree[] daughterTrees;

  /**
   * Create an empty parse tree.
   */
  public SimpleTree() {
    daughterTrees = EMPTY_TREE_ARRAY;
  }

  /**
   * Create parse tree with given root and null daughters.
   *
   * @param label root label of new tree to construct.  For a SimpleTree
   *              this parameter is ignored.
   */
  public SimpleTree(Label label) {
    this();
  }

  /**
   * Create parse tree with given root and array of daughter trees.
   *
   * @param label             root label of tree to construct.  For a SimpleTree
   *                          this parameter is ignored
   * @param daughterTreesList list of daughter trees to construct.
   */
  public SimpleTree(Label label, List daughterTreesList) {
    setChildren(daughterTreesList);
  }


  /**
   * Returns an array of children for the current node, or null
   * if it is a leaf.
   */
  @Override
  public Tree[] children() {
    return daughterTrees;
  }

  /**
   * Sets the children of this Tree.  If given
   * null, this method sets the Tree's children to a
   * unique zero-length Tree[] array.
   *
   * @param children An array of child trees
   */
  @Override
  public void setChildren(Tree[] children) {
    if (children == null) {
      log.info("Warning -- you tried to set the children of a SimpleTree to null.\nYou should be really using a zero-length array instead.");
      daughterTrees = EMPTY_TREE_ARRAY;
    } else {
      daughterTrees = children;
    }
  }


  // extra class guarantees correct lazy loading (Bloch p.194)
  private static class TreeFactoryHolder {
    static final TreeFactory tf = new SimpleTreeFactory();
  }


  /**
   * Return a TreeFactory that produces trees of the
   * SimpleTree type.
   * The factory returned is always the same one (a singleton).
   *
   * @return a factory to produce simple (unlabelled) trees
   */
  @Override
  public TreeFactory treeFactory() {
    return TreeFactoryHolder.tf;
  }


  /**
   * Return a TreeFactory that produces trees of the
   * SimpleTree type.
   * The factory returned is always the same one (a singleton).
   *
   * @return a factory to produce simple (unlabelled) trees
   */
  public static TreeFactory factory() {
    return TreeFactoryHolder.tf;
  }

}