Questions and Exercises: The Platform Environment问题和练习：平台环境

Questions问题

1. A programmer installs a new library contained in a .jar file. 程序员安装一个包含在.jar文件中的新库。In order to access the library from his code, he sets the CLASSPATH environment variable to point to the new .jar file. 为了从代码中访问库，他将CLASSPATH环境变量设置为指向新的.jar文件。Now he finds that he gets an error message when he tries to launch simple applications:现在他发现，当他尝试启动简单应用程序时，会收到一条错误消息：

java Hello
Exception in thread "main" java.lang.NoClassDefFoundError: Hello

In this case, the Hello class is compiled into a .class file in the current directory — yet the java command can't seem to find it. What's going wrong?

Exercises练习

1. Write an application, PersistentEcho, with the following features:编写具有以下功能的应用程序PersistentEcho：

• If PersistentEcho is run with command line arguments, it prints out those arguments. It also saves the string printed out to a property, and saves the property to a file called PersistentEcho.txt
• If PersistentEcho is run with no command line arguments, it looks for an environment variable called PERSISTENTECHO. If that variable exists, PersistentEcho prints out its value, and also saves the value in the same way it does for command line arguments.
• If PersistentEcho is run with no command line arguments, and the PERSISTENTECHO environment variable is not defined, it retrieves the property value from PersistentEcho.txt and prints that out.

Check your answers.检查你的答案。