执行一个步骤之前运行：

Do one more step before running:

Right Click(Project) -> Properties -> Java Build Path -> Libraries -> 
                                          Add Library -> Select the Java Library

点击确定，然后清洁构建（项目 - ＆GT;清洁＆安培; 项目 - ＆GT;全部构建或按Ctrl + B ）的项目。它应该是罚款。

Click OK and then clean-build (Project -> Clean & Project -> Build All or Ctrl+B) your project. It should be fine.