Null Object 模式

有时候，我们可以定义一个"空对象"来代替null，这样可以更加优雅的处理对象为null的情况，避免空指针异常。例如定义二叉树结点的时候，可以用一个NullNode结点来定义空结点。那么一个叶子结点的左子树和右子树就都可以指向NullNode，从而避免了可能引起空指针异常的null。

类图：

代码：

/**
 * 
 * Interface for binary tree node.
 *
 */
public interface Node {

  String getName();

  int getTreeSize();

  Node getLeft();

  Node getRight();

  void walk();
}

/**
 * 
 * Implementation for binary tree's normal nodes.
 *
 */
public class NodeImpl implements Node {

  private static final Logger LOGGER = LoggerFactory.getLogger(NodeImpl.class);

  private final String name;
  private final Node left;
  private final Node right;

  /**
   * Constructor
   */
  public NodeImpl(String name, Node left, Node right) {
    this.name = name;
    this.left = left;
    this.right = right;
  }

  @Override
  public int getTreeSize() {
    return 1 + left.getTreeSize() + right.getTreeSize();
  }

  @Override
  public Node getLeft() {
    return left;
  }

  @Override
  public Node getRight() {
    return right;
  }

  @Override
  public String getName() {
    return name;
  }

  @Override
  public void walk() {
    LOGGER.info(name);
    if (left.getTreeSize() > 0) {
      left.walk();
    }
    if (right.getTreeSize() > 0) {
      right.walk();
    }
  }
}

/**
 * 
 * Null Object implementation for binary tree node.
 * <p>
 * Implemented as Singleton, since all the NullNodes are the same.
 *
 */
public final class NullNode implements Node {

  private static NullNode instance = new NullNode();

  private NullNode() {}

  public static NullNode getInstance() {
    return instance;
  }

  @Override
  public int getTreeSize() {
    return 0;
  }

  @Override
  public Node getLeft() {
    return null;
  }

  @Override
  public Node getRight() {
    return null;
  }

  @Override
  public String getName() {
    return null;
  }

  @Override
  public void walk() {
    // Do nothing
  }
}