Princeton University |
Computer Science 441 |
This next assignment is one that I give to students in the CS2 (Data Structures) course at Williams College. The programming itself should be rather simple for you (at least if you remember how to implement a postfix expression evaluator using a stack). The point of assigning you this exercise is for you to see a somewhat more complex Java program and learn how Java supports event-driven programming.
Your next program is to implement a calculator application. The calculator should take expressions input in postfix notation and display the results of the computation. Thus to calculate 3+5, click on 3, enter, 5, +. The answer should then be displayed. To calculate 3+5*7 (which is written 3 5 7 * + in postfix notation), click on 3, enter, 5, enter, 7, *, +. Try these on the calculator below:
A state object represents the memory of the calculator. You may think of it as representing the printed circuit in the caculator. Its purpose is to keep track of the current state of the computation. For instance, it has to keep track of whether the user is entering a number, and what the number is so far. It also must keep track of the stack used in the computation and update the display window.
Objects of class DigitButtonListener and OpButtonListener respond to events generated by buttons which represent digits and operations, respectively. Each digit key has its own specialized DigitButtonListener which is responsible for knowing which number key it is listening to. Similarly each of the operation keys has its own specialized OpButtonListener which is responsible for knowing which number key it is listening to. When the user clicks on a button with a digit on it, its corresponding listener is responsible for informing the state what number has been clicked on so the state can use it to build the number being typed in. When the user clicks on an operation button (+, -, *, or /), the corresponding listener is responsible for informing the state what operation is to be performed next so that the state can perform the operation.
Notice that we only create "smart" buttons for digits and operations. This is because there are several buttons of each kind. The "unique" buttons: Enter, pop, and clear, are handled directly by the MiscButtonListener. They simply send a message to the state corresponding to the operation.
To help you get started I have provided you with the code for the main Calculator class. This relieves you of the burden of layout out the buttons and display of the calculator. I have also included the code for MiscButtonListener as an inner class of Calculator, and the code for OpButtonListener as a separate class. I have also included the skeleton of code for the State class to indicate what methods it should support. You can are to write the code for DigitButtonListener and fill in the bodies of the constructor and other methods of State. Note that there is a call of the constructor of DigitButtonListener in the constructor of Calculator. Your class should support that constructor. Note also that the state is responsible for updating the display when necessary. You are on your own with the stack, though it should support the standard Stack operations. I don't care whether you implement it with arrays or with linked lists. you can find all of the code in ~kb/COS441/StudentCalc/ on the Suns. Be sure to copy the entire subdirectory graphics as that is used in my code and must be in a subdirectory of the directory with your code (or alternatively listed in your CLASSPATH).
You might find it easier to write the code first under the assumption that the user must push the "enter" button after punching in each number. However, for full credit, it should also handle the case where the user punches in a number followed immediately by an operation. The result should be equivalent to sticking in an intervening "enter". That is punching in 5, enter, 7, + should give the same results as 5, enter, 7, enter, +.
Once you have the rest of the calculator working properly I recommend (though don't require) that you add one or more new buttons. Possibilities include a squaring button, factorial, or xy. You should add the new button so that it looks nice, add an appropriate listener (probably created from OpButtonListener), and make sure that the state knows how to handle that operation.
If you want to go all out, I suggest that you add a decimal point button so that your calculator can handle floating point numbers. Be sure to check that the user does not input an illegal number (e.g., with two decimal points). Possibilities for new buttons with this include square root, trig functions (e.g., sin, cos, tan, etc.), inverse trig functions, xy, e x,factorial, inverses, or logarithms. See the class Math in package java.lang for a list of available mathematical functions in Java.