The screeWidth and screenHeight variables are named displayWidth and displayHeight in this release of Processing.  

Change screenWidth and screenHeight to displayWidth and displayHeight. 

2. import java.awt.Toolkit;
3. import java.awt.GraphicsEnvironment;
4. import java.awt.GraphicsDevice;
5. import java.awt.DisplayMode;
6. 
   
7. void setup() {
8.   Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();
9.   GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
10.   GraphicsDevice[] gs = ge.getScreenDevices();
11. 
    
12.   // Get size of each screen
13.   int sW = 0;
14.   int sH = 0;
15.   for (int i=0; i<gs.length; i++) {
16.       DisplayMode dm = gs[i].getDisplayMode();
17.       sW += dm.getWidth();
18.       sH += dm.getHeight();
19.   }
20.   size(sW, sH);
21. }
22. 
    
23. void draw() { 
24.  background(0);
25.  stroke(random(255));
26.  line(0, 0, displayWidth, displayHeight);
27.  line(displayWidth, 0, 0, displayHeight);
28. }
29. 
    
30. boolean sketchFullScreen() {
31.   return true;
32. }