No, you cannot. As soon as you wrote " where Stage inherits the PApplet" I felt there was something went wrong...
Indeed, the main class of a sketch is generated by Processing. A library can extend the main PGraphics used by the sketch, but no more.

It seems to me that you have only used Netbeans to replace the Processing IDE to create a Processing application. If that is true then It still might be possible to create a library from your project but as PhiLho pointed out the difficulty is the Stage class.

I stress the might because it will very much depend on what the Stage class does and how much of the PApplet class is overridden. It would mean removing the inheritance from PApplet and creating a PApplet attribute instead. This attribute will be set by passing a reference to the sketch when it is created e.g.

1. public class Stage {
2.       private final PApplet app;
3. 
   
4.       public Stage(PApplet sketch){
5.             app = sketch;
6.       }
7. //rest of the class
   

Now in the Stage class all calls to PApplet methods should be prefixed with app. so
fill(255);   becomes     app.fill(255);
You will also have to prefix static methods with PApplet e.g.
a = constrain(value, low, high);     becomes     a = PApplet.constrain(value, low, high);

Obviously the current Stage class will have methods for setup, draw etc and these will no longer be executed because the class does not inherit from PApplet. The solution is two stage
perform all the setup actions in the constructor
register the methods for mouse handling and drawing with the applet

both of these in the constructor e.g.

1. public class Stage {
2.       private final PApplet app;
3. 
   
4.       public Stage(PApplet sketch){
5.             app = sketch;
6.             // setup code here
7.             // ...
   
8.             // Register the draw and mouse methods (2.0b6 syntax)
   
9.             app.registerMethod("draw", this);
10.             app.registerMethod("mouseEvent");
    
11.       }
12. 
    
13.       public void draw(){
14.             // drawing code
15.       }
16. 
    
17.       public void mouseEvent(processing.event.MouseEvent event){
18.             // mouse handling code
    
19.       }
20. //rest of the class

This means that these methods will be automatically executed by the sketch at the appropriate time for instance the draw method will be executed immediately at the end of the sketch draw method.

To use Stage then your skecth code would look something like

1. // import statement for your library
2. Stage stage;
3. 
   
4. public void setup(){
5.       size(400,400); // or whatever
6.       shape = new Shape(this);
7. }
8. 
   
9. public void draw(){
10.       //various drawing statements
11.      
12.    // after this method is executed Processing will execute
13.    // the drawing method from Stage
14. }

This is not a trivial task and as I said may not be possible with your project. If you decide to attempt it I suggest that you create a new class called StageLib for the eventual replacement of Stage but leave Stage unaltered so if it all becomes a disaster you can delete the StageLib class and still have your original code..

Hello PhiLho,

Oops It should have been

stage = new Stage(this);

also I just noticed line 10 should have been

app.registerMethod("mouseEvent", this);

I am new to library development for processing.org

If you haven't done so already I suggest you look at the Processing docs on creating libraries.

Although not essential I suggest that you consider using Eclipse rather than NetBeans as there  will be more support on the forum with Eclipse (you can see that in the documentation).

Quarks,