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Debugging technique — every class to have a main() method A technique often used by Java developers working on large software projects is to write a main() method for every class they write. Each main() method creates one or more instance objects of the class, sends the object(s) some message(s) and displays the changing state of the object(s). In this way each individual class can be tested in isolation. For large software systems this can help reduce bugs, or mean that they are found earlier, since it can be very difficult debugging a system with many objects from many class. Although for the programs you are developing for this module you probably do not need to test each class in isolation (since it almost as easy to debug programs containing only 2 or 3 classes), an example is given here to illustrate this technique. (Note - this is only appropriate for non-abstract classes). Consider a simple applet to draw 2 triangles in its window. This applet makes use of a class MyTriangle. Let’s assume the output we want from the applet is as follows:
However, the output from the applet we have written is as follows:
The writer of our applet has made the mistake of forgetting that the origin of Java graphics is the top left (i.e. the smaller the Y co-ordinate the higher up the screen a point is). The writer of this applet can test that their MyTriangle class works in isolation as follows:
We can run the main method of the MyTriangle class using the java command:
Had the programmer used such a method, they might have remembered that for Java graphics the bottom of the triangle has a larger Y value than the top, which explains why the triangle is draw with the point to the top of the screen. Even if they cannot remember that, at least they now know the values of the variables forming arguments for the drawLine() messages and so on. By the way, the code of an applet to produce such triangles is as follows:
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