Any global variables you define in the main sketch should be accessible to the napplets.  I should make an example sketch to show this, but the process is simple enough: just declare variables in your main sketch and then modify them however you like in the napplets.

Here's a modified version of the last example sketch to show how this works:
Code:

import napplet.*;

NAppletManager nappletManager;
PFont mainFont, codeFont;

// These will be changed by the napplets:
int nicePressTotal = 0;
int meanPressTotal = 0;

String titleText = "NApplets can be created in their own windows.";

String button1Text = 
  "Click Here\nto Create a\nNice Window";

String button2Text = 
  "Click Here\nto Create a\nMean Window";

void setup() {
  size(400, 200);
  mainFont = loadFont("ArialMT-18.vlw");
  codeFont = loadFont("CourierNewPS-BoldMT-14.vlw");
  textMode(SCREEN);
  textAlign(CENTER, CENTER);
  
  nappletManager = new NAppletManager(this);
}

void draw() {
  background(0);
  stroke(255);
  fill(100);
  
  rect(width/12, height/4, width/3, height/2);
  rect(7*width/12, height/4, width/3, height/2);
  
  textFont(mainFont);
  fill(255);
  text(titleText, width/2, height/8);
  text(button1Text, width/4, height/2);
  text(button2Text, 3*width/4, height/2);
  
  String s;
  s = "Total clicks on\nnice windows: " + nicePressTotal;
  text(s, width/4, 7*height/8);
  s = "Total clicks on\nmean windows: " + meanPressTotal;
  text(s, 3*width/4, 7*height/8);
}

void mousePressed() {
  if (mouseY >= height/4 && mouseY <= 3*height/4) {
    if (mouseX >= width/12 && mouseX <= 5*width/12) {
	nappletManager.createWindowedNApplet("NiceWindow",
	(int) random(100, 400), (int) random(100, 400));
    }
    else if (mouseX >= 7*width/12 && mouseX <= 11*width/12) {
	nappletManager.createWindowedNApplet("MeanWindow",
	(int) random(100, 400), (int) random(100, 400));
    }
  }
}

public class NiceWindow extends NApplet {
  
  String sketchText = 
  "This window can be\n" + 
  "closed with the controls\n" + 
  "on the title bar, because\n" +
  "nappletCloseable is true.\n" +
  "(Or the NApplet will close\n" + 
  "itself if you hit the X key.)";
  
  public void setup() {
    size(250, 250);
    nappletCloseable = true;
    textMode(SCREEN);
    textAlign(CENTER, CENTER);
  }
  
  public void draw() {
    background(0, 100, 0);
    stroke(255);
    fill(255);
    
    textFont(mainFont);
    text(sketchText, width/2, height/2);
  }
  
  public void keyPressed() {
    if (key=='x' || key=='X') exit();
  }
  
  public void mousePressed() {
    nicePressTotal += 1;
  }
}


public class MeanWindow extends NApplet {
  
  String sketchText = 
  "This window can't be\n" + 
  "closed with the controls\n" + 
  "on the title bar, because\n" +
  "nappletCloseable is false.\n" +
  "(But the NApplet will close\n" + 
  "itself if you hit the X key.)";
  
  public void setup() {
    size(250, 250);
    nappletCloseable = false; // Not actually necessary, false by default.
    textMode(SCREEN);
    textAlign(CENTER, CENTER);
  }
  
  public void draw() {
    background(100, 0, 0);
    stroke(255);
    fill(255);
    
    textFont(mainFont);
    text(sketchText, width/2, height/2);
  }
  
  public void keyPressed() {
    if (key=='x' || key=='X') exit();
  }
  
  public void mousePressed() {
    meanPressTotal += 1;
  }
} 

There's always the same error. No Such Field Error..

using a library that's incompatible with your version of processing..

but I have downloaded the actual version.. Don't know what to do now.. could it be that the problem is with the computer? or what else should I try/change..

thanks

Smitty.. thanks for your answer. I worked it out. I just had to open the new processing before and open then the file with: "file, open".. maybe it's so vague because it's not my mother tongue.. I'm sorry for this.. but now it works. i think that's the main thing.

but now I've another question.. (sorry for asking so many things... )
I've programmed a little version on my own. I'm able to create napplets in a new and in the same window now. but what I can't understand -  and I've studied the files I've downloaded.. - how is it possible to link to another file.. In this example it's linked to "scrollbar" or "pattern" or something like this.. It is linked as if it was a public class but it isn't, it's another file.. so, how can I import extern files?  I think that would make more sense than a public class, or not?

but at least it's running now.. thank you all for your great help..

yes.. it is exactly what I'm talking about... but I don't know how to make these tabs.. how to link to these other files..

oh.. I didn't know that this is possible never noticed this button.. thank you.. now I see a little bit better how it works..

I changed my program so that they are in tabs. but now it's the problem with the void setup()..

Duplicate method setup() in type "THE NAME OF MY FOLDER"..

I'm not really sure what the public in front of the void means.. So I've written it and I also let it be.. but anyway there's the same error.. I've looked at the napplet-example but there are also more than one void setup() in the files...

what's my mistake?

You need one setup() method for the main sketch, and one setup() method for each NApplet subclass.  The setup() methods for the NApplets must be inside the NApplet declaration.

To make it easier to remember, each tab acts like it's part of the main tab.  What this means is that if you want to create a NApplet in a tab, you still need to wrap the NApplet with a declaration, just like if you were typing it in the main tab.  For example, you might have your main tab like this:
Code:

import napplet.*;

NAppletManager n;

void setup() {
  size(300, 300);

  n = new NAppletManager(this);
  n.createEmbeddedNApplet("MyNApplet", 50, 50);
}

void draw() {
  background(255);
} 

And then have a separate tab called "MyNApplet" like this:
Code:

public class MyNApplet extends NApplet {

  public void setup() {
    // NApplet setup code in here.
  }

  public void draw() {
    // NApplet draw code in here.
  }
} 

This should work just the same as it would if you put it all in one big tab.

As for the public keyword, it's not what's important here.  I think you need to have it on the NApplet class and on the NApplet's draw() and setup() methods, but maybe you don't.  (I don't have time to test this right now, but you can.)

On the methods for the main applet (like the main setup() and draw() methods) you can make them public or not, and it will work either way.

now it works...


but... (it couldn't be different ) ..

Code:

controlP5 = new ControlP5(this);
  controlP5.addToggle("toggle",false,5,10,50,20);
} 

void toggle(boolean but1) {
  if(but1==true) {
    
    parentPApplet.background(255);
  } else {
     button1=false;
}
} 

here I have a part of a sub-tab. There I have a controlP5 button and if I press it, I set the main background on 255 (parentPApplet.background...) this works.. but in the else-case I work with "button1" (boolean).. that is set false and in the main tab:

Code:

if(button1==false) {
  background(25);
	rect(10,10,20,20);
  } 

this code.. It works too, so I can't be my fault.. but it doesn't work immediately but when I have resized the window... why?? I think that's not a problem of controlP5 but of napplet..

I tried it like this yesterday.. but that doesn't seem to be the problem. the problem is, that only with parentPapplet it works.. otherwise you have to resize the window first.. so is there a parentPapplet-function for everything.. I know that you can use parentPapplet.background or .height, .width but could I also use it to set a function true or  something like this...
So I could take a parentPapplet function to make the things work (but how to use it to set a function true or false?)or there has to be another option how the (boolean)-variable of one napplet can influence the other napplet IMMEDIATELY and not after resizing the window..
(by the way: I know that it's "more beautiful" with the same variables and so on, this example that I've posted I've written like this to show in one code the both ways I've tried it .. )