This method forces the result of Math.random into an int in the range we require. Pay special attention to the last line. When you see a raw type in parentheses like (int) or (float) it's a cast . Casts change one value type into another. Thus here we're changing a double into an int . The cast rounds as necessary.

Casting in Java is safer than in C or other languages that allow arbitrary casting. Java only lets casts occur when they make sense, such as a cast between a float and an int . However you can't cast between an int and a String for example.

//Draw a rectangle import java.applet.Applet; import java.awt.*; public class Mondrian5 extends Applet { int Rect, Rect, RectTop, RectLeft, Applet, Applet; public void init() { Dimension d = size(); Applet = d.; Applet = d.; RectTop = Randomize(Applet); RectLeft= Randomize(Applet); Rect = Randomize(Applet - RectTop); Rect = Randomize(Applet - RectLeft); repaint(); } public void paint(Graphics g) { g.drawRect(0, 0, Applet-1, Applet-1); g.fillRect(RectLeft, RectTop, Rect-1, Rect-1); } private int Randomize(int range) { double rawResult; rawResult = Math.random(); return (int) (rawResult * range); } } Occasionally this applet does randomly produce a rectangle that's two small to see so if you don't see anything, reload it. Reload it a few times. Each time you'll see a rectangle of a different size appear in a different place.

Let's make our world a little more colorful. To do this we'll change the rectangle color to red. To do this we'll use a new methods setColor(), part of the Graphics class.

//Draw a colored rectangle import java.applet.Applet; import java.awt.*; public class Mondrian6 extends Applet { int Rect, Rect, RectTop, RectLeft, Applet, Applet; public void init() { Dimension d = size(); Applet = d.; Applet = d.; RectTop = Randomize(Applet); RectLeft= Randomize(Applet); Rect = Randomize(Applet - RectTop); Rect = Randomize(Applet - RectLeft); repaint(); } public void paint(Graphics g) { // g.setBackground(Color.white); g.setColor(Color.red); g.drawRect(0, 0, Applet-1, Applet-1); g.fillRect(RectLeft, RectTop, Rect-1, Rect-1); } private int Randomize(int range) { double rawResult; rawResult = Math.random(); return (int) (rawResult * range); } }