Flow Control - SCJP Sun Certified Programmer for Java Platform SE6 - page 223

The following example demonstrates a class invariant. A Rectangle object is not
considered valid if either its width or height is negative. Examine the following Rectangle
class, and assuming assertions are turned on, determine the output of running the main
method:
1. public class Rectangle {
2. private int width, height;
3.
4. public Rectangle(int width, int height) {
5. this.width = width;
6. this.height = height;
7. }
8.
9. public int getArea() {
10. assert isValid() : “Not a valid Rectangle”;
11. return width * height;
12. }
13.
14. private boolean isValid() {
15. return (width >= 0 && height >= 0);
16. }
17.
18. public static void main(String [] args) {
19. Rectangle one = new Rectangle(5,12);
20. Rectangle two = new Rectangle(-4,10);
21. System.out.println(“Area one = “ + one.getArea());
22. System.out.println(“Area two = “ + two.getArea());
23. }
24.}
The isValid method is an example of a class invariant. It is a private method that
tests the state of the object. Line 10 invokes isValid in an assertion statement before
computing the area. Within main , Rectangle one is valid and its area is output. Rectangle
two has a negative width so the assertion fails on line 10. The output is shown here:
Area one = 60
Exception in thread “main” java.lang.AssertionError: Not a valid Rectangle
at Rectangle.getArea(Rectangle.java:10)
at Rectangle.main(Rectangle.java:22)