运行程序时，首先由浏览器创建MouseApplet的applet对象。applet对象执行init()方法过程中创建监听对象ma，并由this把对applet自身的引用传送给监听对象，这样监听对象就可以通过这个引用操作applet对象，即在响应程序中改变小应用程序对象的背景色。-Run the program, the first from the browser to create MouseApplet applet object. ma of the process of implementation of the applet object init () method to create a listener object, the applet s own reference to transmission to the listener object, so that the listener object can be referenced through this operation applet object by this response procedures to change a small application objects the background color