Fun With Java Seminar

Homework #4: A Java Calculator

Background and Motivation

Java is one of the first programming languages to have a standard windowing framework as a standard part of the language. In Java, this is called the Abstract Window Toolkit (AWT). In this homework, you'll exercise the AWT and become more familiar with it.

To massage your skills with the GUI facilities of Java, you will develop a little desktop calculator - in stages, of course. All "input" to the calculator will be through the "keys" in the UI you provide.

What to Read

See chapters 7 and 10 of Just Java by van der Linden (Prentice-Hall, 1997). Another thing to read would be the Writing Applets chapter of The Java Tutorial by Mary Campione and Kathy Walrath (Addison-Wesley), but see also their notes on the AWT in JDK 1.1 (as opposed to the 1.0.2 that they usually discuss).

See also the documentation for the packages java.applet and awt for details. (The hyperlinks here are for the JDK 1.1.2 documentation.)

Directions

This homework, like the others in this seminar, will be open-ended and somewhat ill-defined. we'll try to give you some idea of the difficulty/effort involved in each part. Do what you have time for.

Bring a printout of your code to class, and be prepared to discuss it with other people in the class.

Problems

1. (easy) Develop a simple calculator (as an application) that just handles integer arithmetic (+, -, *, /). You will, of course, need to include an "=" key, along with the 10 digits. Don't worry about anything in this version except having the calculator behave correctly for "correct" input. So in this version you do not have to be concerned with overflow, underflow, zero divide, ... . Just get it so the user can enter a number on the display, enter an operation, enter another number, enter "=", and see the result.
2. Make your calculator an applet.
3. Extend the calculator so the user sees "E" in the display if any error condition has been encountered. This might occur, for example, if the user tries to perform an operation and not enough numbers have been entered. Note that Java handles overflow, underflow, and division by zero itself, in accord with the IEEE floating point standard.
4. Extend the calculator so that it handles "real" numbers. Let's assume the same sort of semantics we have for mixed-mode arithmentic expressions that we have in Java.
5. Look at the interpreter pattern and other patterns in the Design Patterns book (which is on reserve). Try to make your code more OO by using intelligent objects. Pay particular attention to the idea of the model-view-controller pattern, so that your calculator engine and its registers are separated from the user interface.
6. Let's add a key labelled EXPR to the calculator, - for "expression". When the user "clicks" on this key, they can enter an expression "template" that can contain parenthesis, operators, and "?", to represent where a value would go in the expression. So, for example, the user could enter the expression:
   
   ((? + ?) * ?) - ?
   
   Let's allow the user to store up to 10 such expressions. (You decide on how this should work for the user.) When the user wants to evaluate an expression, they are prompted for the values in the order in which they appear in the expression. When the last value is entered, the expression is evaluated.
   
   Be clever here. What can you do to help the user "edit" the expression as it is entered? The user will not be able to "see" the entire expression in the display - how can we help them "scroll back and forth" in the expression?