Chapter: Void methods

In Alice you met methods and functions. Alice methods correspond to void methods in Java. They are "for effect" only: they cause something to happen (in Alice, some movement perhaps; in Java, the printing of a message perhaps) and do not return any value for the caller to use. Alice functions correspond to non-void methods in Java. Their primary purpose is to compute some value that will be returned to be used by the caller. non-void methods in Java need a return statement.

Study the program in Cross.java. You will notice:

• There is only one static method. main must be static because no objects exist at the time your program starts (when the operating system invokes the main method). But main creates a Cross object by calling the constructor.
• The constructor public Cross(int size) is a very special method. It is not void. Neither does it specify a return type. It has the same name (Cross) as the class itself. These are the characteristics of constructors. When called they create an object.
• There is a public void makeCross(int size) method. It has the effect of drawing a cross. But it does not, need not, cannot return a value. That's what void means!
• The method makeCross is called from the action listener for the JButton.

Be sure to create a project workspace for lab9.

Exercise 1

and on providing a value and clicking the button should see a better face than:

Hint: Look at the documentation for the Graphics class. Look for a method that can draw a circle (remember that a circle is a special case of an ellipse). The arguments you supply are the coordinates of the TOP LEFT corner of the oval's bounding rectangle, and the width and height of the oval (the width and height will be equal for a circle, of course).

Name your class Ex1. This will require you to change all occurrences of Cross to Ex1 and to save the file as Ex1. Ask your lab instructor if you are not sure how to do this.