public class Stack<E> extends Vector<E> {

  public Stack() {//创建空栈
  }

  public E push(E item) {//入栈
    addElement(item);
    return item;
  }
  //出栈
  public synchronized E pop() {
    E    obj;
    int   len = size();
    obj = peek();
    removeElementAt(len - 1);
    return obj;
  }
  //返回栈顶元素，但并不出栈
  public synchronized E peek() {
    int   len = size();

    if (len == 0)
      throw new EmptyStackException();
    return elementAt(len - 1);
  }
  //判断栈是否为空
  public boolean empty() {
    return size() == 0;
  }
  //查找元素并返回栈深
  public synchronized int search(Object o) {
    int i = lastIndexOf(o);

    if (i >= 0) {
      return size() - i;
    }
    return -1;
  }

  //序列版本号
  private static final long serialVersionUID = 1224463164541339165L;
}