Knopf knopf1;
Knopf knopf2;
int x;
public void setup()
{
  size(600,300);
  frameRate(30);
  knopf1 = new Knopf(10,10,200,50);
  knopf2 = new Knopf(10,70,200,50);
}

public void draw()
{
   background(255);
   knopf1.draw();
   knopf2.draw();
}

public void mousePressed()
{
   knopf1.clicked(mouseX,mouseY);
   knopf2.clicked(mouseX,mouseY);
}

Code 0-1: Hauptteil des Sketches

public class Knopf
{
    private int x,y,breite,hoehe;
    private boolean an;
    public Knopf(int x,int y,int breite,int hoehe)
    {
        this.an = false;
        this.x = x;
        this.y = y;
        this.breite = breite;
        this.hoehe = hoehe;
    }
    
    public void draw()
    {
        if(this.an)
            fill(0,255,0);
        else    
            fill(255,0,0);
        rect(x,y,breite,hoehe);
    }
    
    public void clicked(int x, int y)
    {
        if(x>this.x && x<this.x+this.breite && y>this.y && y<this.y+this.hoehe)
        {
             this.an=!this.an;
        }
    }       
}

Code 0-2: Klasse Knopf in extra Tab.