Project 2: A Calculator

The goal of this project is to review the basics of object-oriented programming and to give you an initial try at designing an object-oriented system.

Your task is to build a program to emulate a four-function (addition, subtraction, multiplication, and division) calculator such as were common many years ago. The use will be presented a window that looks like a calculator and be able to click on the buttons to get a result. I am providing the GUI, which is analogous to the outside cover and buttons of a real-world calculator. Your task will be to write the code for the internal workings, analogous to the electronics inside a calculator.

This will give you practice using objects to represent information, as well as designing the interaction between objects. This will also give you some practice working with action listeners.

1. Basic setup

After moving into your folder for this course, and making a folder for this project, copy the starter code given for this project:

cd 245
mkdir proj2
cd proj2
cp -r ~tvandrun/Public/cs245/proj2/. .

This will give you the following files and folders:

• .classpath, a hidden file (you can see it only if you do ls -a that Eclipse will use for the project set-up; you you can ignore it
• calc, a folder for a package containing the following files:
  • CalculatorFace.java, an interface defining how the internals are to interact with the calculator's face
  • PlainCalculatorFace.java, the code for the GUI
  • NoSuchButtonException.java
  • SetUp.java, a class containing the main method
• test, a folder containing JUnit tests to help you with debugging.

Now open Eclipse and start a new project. When the new project wizard comes up, give it a reasonable project name, un-click "Use default location", and point the location to the folder you made (for example proj2), which contains the package folders calc and test.

When you hit next, make sure you see a screen like the following. If it looks different (for example, having "src" and "bin" folders), then you've done something wrong.

You can hit "Finish" now.

For more help on setting up a project in Eclipse, see this screencast demonstration of how to do it.

2. The calculator face

Open CalculatorFace. As mentioned above, this is an interface defining how your code will interact with the GUI. It defines how to attach action listeners to the buttons and how to write to the screen. Its documentation tells you everything you need to know about communicating between your code and the window.

The class PlainCalculatorFace, as mentioned above, is a class implementing CalculatorFace, that is, it makes/models the window. You do not need to modify this class. In fact, you do not need even to look at this class. Again, the interface CalculatorFace gives you everything you need to know to interact with this class or any other class that implements the interface. When we grade the project submissions we will not use PlainCalculatorFace. We will use another class that implements the interface.

If you work on this project on your own computer instead of the lab machines, then there is one change you might make to PlainCalculatorFace so that the window will appear correctly on another platform; see the documentation in the file for details.

3. SetUp, action listeners, and calculator internals

Your task in the project has three parts:

• Write a class (possibly with other helper classes) to represent the internal state of the calculator.
• Write classes that implement the ActionListener interface that will be attached to the buttons and will be a bridge between the calculator face and the calculator internals.
• Finish the method SetUp.setUpCalculator(), which should initialize the calculator including connecting the action listeners and internals to the calculator face.

If you haven't used the ActionListener interface or don't remember it well enough, you can read the relevant sections in the textbook (from chapter 17, particularly chapter 17.2), and look at the Java API. Here's a quick review/overview:

An action listener is an object that is "attached" to a GUI component-- for example, a button. Action listener classes implement the interface ActionListener in the package awt.event, which has the method actionPerformed(ActionEvent ae). For our purposes, you can ignore the parameter (but don't let that stop you from finding out how to use it, if you're curious). In this project, you may want to make a PlusListener class and attach an instance to the plus button on the calculator face. When the button is pressed by the user, the method actionPerformed() is invoked. For our present purposes, the actionPerformed() methods will cause some change to the calculator internals which will probably then be reflected on the calculator's screen.

Open SetUp. You will notice the method setUpCalculator() takes in a CalculatorFace---but not necessarily a PlainCalculatorFace. The code you add to this method presumably will

• Instantiate the class or classes representing the calculator internals
• Instantiate the action listener classes
• Attach the action listeners to appropriate buttons on the given face.

For example, if we suppose you have written a class PlusListener that implements ActionListener and handles the pressing of the plus button, you can attach an object to the button by

    face.addActionListener('+', new PlusListener());

The SetUp class has a main method that instantiates a PlainCalculatorFace and calls setUpCalculator. You will use this for testing your calculator by hand (as opposed to automated tested, described below). So, to run the program from the commandline, use

java calc.SetUp

Or hit the "run" button for SetUp in Eclipse.

You may assume that your code will be used only after being instantiated through SetUp.setUpCalculator(), but you should not assume that it is called through SetUp.main(). The method SetUp.setUpCalculator() is public and may be called from other code.

The intent is to make this program work "just like a hand-held four-function calculator." You can find a decent example for reference here. Don't try to implement anything fancy; don't use Ubuntu's built-in calculator as a reference.

Here are a few specific scenarios and what should happen (at least, what should happen in terms of the external behavior; what happens inside the calculator is up to you).

• As the user enters a first operand, the number entered so far should appear on the screen. If the decimal button is pressed, then subsequent number button presses will add digits to the right of the decimal point.
• If the decimal button is pressed a second time for the same operand, it is ignored.
• When the user pressed an operation button, the first operand remains on the screen, but as soon as the user begins to enter the second operand, the first operand disappears from the screen; the screen then shows the second operand as it is being entered.
• When the equals button is pressed (after a second operand), then the operation that had been entered between the two operands is executed and the result is displayed.
• If, after the result is displayed, the user begins entering a new number, then the earlier result is forgotten and the new number is treated like a first operand. The screen shows this new operand as it is being entered.
• If, after the result is displayed, the user presses another operation button, then the result is treated like a first operand to that new operation. It remains on the screen until the user begins to enter the second operand, at which point the screen shows the new operand as it is being entered.
• If, after entering a second operand, the user presses an operation button instead of equals (for example, 1 + 2 + instead of 1 + 2 =, then the first operation is executed, its result is shown on the screen, and that result is taken as the first operand to the new operation. Thus 1 + 2 + has the same effect as 1 + 2 = +
• Pressing the plus-minus button has the same effect if pressed before, during, or after the number is being entered. It toggles whether the number is positive or negative. If pressed twice, for example, the number will be positive.
• The clear button C resets the calculator back to its initial state.

Examples:

Whether integer results appear as plain integers (3) or with a zero after the decimal point (3.0) is up to you.

This doesn't cover all possible scenarios, of course. Part of this assignment is for you to think through what the user might do and how the calculator should behave in those circumstances. Feel free to ask me questions like "What should happen if....?"

Important: The "screen" on the calculator is only 15 characters wide. Java displays double numbers using more than 15 characters. Make sure that when you display results that you do not run off the end of the screen. You need to think about how to format your results. There are several ways to do this. One way is to use the DecimalFormat class; it's described in Absolute Java (check the index, pages differ with edition) or the Java API. You also can devise your own formatting strategy using String manipulation.

Also, be sure your code is documented according to the specifications given in the style principles.

Finally, as has been mentioned in class, this is a point where many students realize they never understood what the static parts of a class were all about---which usually stems from having not yet mastered the concepts of class and instance. So if something is wrong and Eclipse suggests you make some variable static, don't do it. Something else is wrong; ask for help if need be. Specifically, any static variable must include in its documentation an explanation why that variable is static.

5. Tips

This is the time of the semester when I get to know students in this class, because so many of them show up at office hours wanting help on this project. You are warmly invited to stop by for help, but here are things you should think about before that conversation:

There are two basic strategies for using action listeners: Either you can write very busy actionPerformed() methods, which do a lot of the actual work of the calculator themselves; or you can write simple actionPerformed() methods which merely call another method in a central object. The choice between these is up to you, though that doesn't mean they're equally good options. As we go on this semester, we'll discuss some of the trade-offs in a decision like this, and at this point you should try to use the principles and examples we've seen so far to make as good a design as possible.

Don't confuse the calculator's screen with the calculator's memory or state. The screen is used to show the user a result or the operand the user is currently entering. It is not supposed to show everything that is going on inside the calculator, and it shouldn't be used to store information. That's the job---or, one of the jobs---of the calculator internals.

Don't confuse the event of an operation button (such as plus) with the actual operation happening. When the user presses the + button, the calculator doesn't even have the second operand yet. The action listener on the plus button isn't supposed to do addition. Instead it should cause an addition to be stored as a pending operation, to be done later.

Don't try to write this entire project at once. Start implementing the number buttons first, making numbers appear on the screen. Then implement the plus button. When that's working, start doing the other operator buttons.

6. Testing

Test your calculator "by hand" by running it through calc.SetUp. Additionally, I have provided some JUnit tests that will exercise how well your calculator handles some of the scenarios described earlier. Run these tests by running test.TestCalcBasic as JUnit test.

You may inspect these tests and add your own. Here's how they are set up:

    @Test
    public void addEq() {
        testSequence("1+2=", new String[] {"3", "3.0"});
    }
 

The first parameter to testSequence(), the string "1+2=" indicates a sequence of buttons pressed on a newly initialized calculator. The second parameter is an array of strings indicating the set of acceptable results that could be on the screen after that sequence of buttons. In this case, the test will pass if either 3 or 3.0 is on the screen after the given sequence of buttons were pressed.

The test cases given do not represent all the tests your submitted project will be graded against. We will run your submission against a wider range of tests; you should test it more thoroughly too.

7. To turn in

Please turn in all the files you wrote or modified (ie, the files you didn't modify don't need to be turned in) to a turn-in directory. To turn in a specific file, do

cp (some file) /cslab.all/ubuntu/cs245/turnin/(your user id)/proj2

To turn every file in whatever directory you are currently in, do

cp * /cslab.all/ubuntu/cs245/turnin/(your user id)/proj2

DUE: 5:00 pm Wednesday, Feb 17, 2016, 5:00 pm. Note that this is also the due date of Project 3.

8. Grading

Getting your calculator to work perfectly right is actually pretty hard to do. Even the best students rarely get full marks on the first try of this project. (A later project will revisit this problem.)

Grading for this project will follow this approximate point breakdown (the right to make minor adjustments in point distribution is reserved):

• Correctness: 16 (How does your calculator behave when tested against various button-sequence scenarios?)
• Design / efficiency: 3 (How well does your code adhere to principles of good design?)
• Documentation / coding style: 1 (How well does your code adhere to the documentation and style principles?)

Design and style are weighted disproportionately low in the grading of this project, since you are just learning how to design systems. The relative weight of design and style in the grading of projects will increase as the semester goes on.