02115   Java Programming
Autumn 2011

Preparations for the week

• A GUI constructed by use of a tool could probably easy be made user friendly because components can be moved around by using the mouse. Such tools are useful for companies making professional software, but not in an educational context where the goal is to understand principles and gain practical training in handling the Java language. When looking at the tool generated source code it's often close to be unreadable by a human who would like to find out how things are constructed. And that's often difficult to figure out when looking at tool generated Java source code.
• Error detection and error handling should not be carried out in the same method. It's tempting to do the error detection by evaluating boolean expressions of some if ... else constructs placed in the method bodies. But in the branches to be executed when an error has occured the immediate error handling action should not be to inform about the error by applying some System.out.println's and then return from the method.

  That way of reporting errors is not well-suited when having a GUI. It's better to split the error detection and the error handling and postpone the error handling to take place at a position close to the user interface. Then depending on the kind of user interface applied in the actual run of the program one can just choose to present the error message by using either a System.out.println or a kind of message dialogue.

  In order to achieve this generality allowing the program to use a command line interface or a GUI without needing different versions of some classes exceptions could be applied. In the method body where the error is detected the printing of text should be changed into throwing a checked exception. By adding a throws clause to the method declaration the method may return with an execption object informing about the error instead of doing a normal return. It requires the method call to be placed in the try branch of a try ... catch construct and the error handling to be done by a catch.
  That first catch can either act on the exception or throw it further up to another try ... catch construct until the error situation is finally handled on a level close to the user interface

The lectures

Exercises at the PC's

The link on page 375 of BK referring to the Java Tutorial Creating a GUI with JFC/Swing has been changed into http://download.oracle.com/javase/tutorial/uiswing
Just click on this new link here:
    Creating a GUI With JFC/Swing

Instead experiment with the types of components you have in mind to use in the Mandatory Assignment based primarily on what is presented in the sections 11.8 and 11.9 of our BK textbook.

You can be introduced to and guided through some useful components by doing Exercises 11.49 - 11.63

After the lectures

For objects which are elements of a collection class you have to declare a boolean equals(Object o) method in the corresponding class. For a HashSet of elements a hashCode method should be supplied. And for a HashMap the class from which the keys originate should also declare a hashCode method. To use TreeSet<T> and TreeMap<T, ...> class T should declare a int compareTo(T o) method. Refer to our two textbooks PS, primarily section 22.8, and BK section 9.8 (Object equality: equals and hashCode)