The Java program keeps track of the images on 7 different layers, (background scene plus 6 small objects).  When a small object is selected, it is moved to the frontmost layer, and any objects which were previously layered on top of it are moved back one layer. 
  
This program uses double-buffering to avoid on-screen flicker when the image is being redrawn. An updated image is first prepared in a background buffer and the Graphics update method is overridden to paint from this buffer when a repaint is requested. 
  
Java makes no distinction between Left and Right Mouse Buttons, and either the Left or Right Mouse Button can be pressed for a Mouse-Click event.  (Note that the Mouse on a Macintosh computer has only one Button, whereas those on PC's have both Left and Right Buttons, and sometimes a Center Button as well.) 

Note:  When I wrote and tested this Applet using the Applet Viewer provided with the Java Development Kit from Sun Microsystems, the mouse responses were immediate, but when I view this page on my computer using the popular Web Browsers, the Java interpreters which are provided with these Browsers seem to suffer from performance problems.