如果您按照页面上的常见问题解答
http://introcs.cs.princeton.edu/java/stdlib/

Q. If I use a named package to structure my code,the compiler can no
longer access the libraries in stdlib.jar. Why not?
A. The libraries
in stdlib.jar are in the “default” package. In Java,you can’t access
classes in the default package from a named package. If you need to
use our libraries with a named package,you can use the packaged
version stdlib-package.jar.

下载包jar文件：
http://introcs.cs.princeton.edu/java/stdlib/stdlib-package.jar

右键单击项目文件夹并添加外部JAR.

import edu.princeton.cs.introcs.*;

将上面的行添加到需要引用类的类中.
第一行引用正确的包名称,*通配符会导入其中的所有类.

希望有帮助.

//编辑 – 如果您右键单击项目文件夹并使用“组织导入”将会更快,因此您不必手动添加到每个类.