程序功能：在Applet 上使用鼠标在屏幕上随意画直线和画点。用户点击“画线”按钮可画直线，点击“画点”按钮可画连续点，点击“清除”按钮可清除画面上的所有内容。-The program features: the Applet, use the mouse to draw a straight line anywhere on the screen and draw points. The user clicks on the " drawing a line" button