ExampleSketch.java

import processing.core.PApplet;

public class ExampleSketch extends TransparentApplet{

    public static void main(String[] args){
            PApplet.main(java.lang.invoke.MethodHandles.lookup().lookupClass());
    }

    @Override
    public void settings(){
        size(400,400,P2D);
    }

    @Override
    public void setup(){
        super.setup();
        background(255);
    }

    @Override
    public void draw(){
        super.draw();
        rect(width/2f-25,height/2f-25,50,50);
    }
}

TransparentApplet.java

import processing.core.PApplet;

public class TransparentApplet extends PApplet {


    public void setup(){
        // do stuff here that always happens before setup() in the extending sketch
    }

    public void draw(){
        // do stuff here that always happens before draw() in the extending sketch
    }
}