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CIS 201 – Computer Science I
Laboratory Assignment 9
Introduction
In this lab you will create a game that reads its configuration from a file.
Reading a file
Consider the process of writing a pinball game. Given a screen and a ball that bounces, different levels or pinball games frequently differ in how their bumpers (and other things) are laid out.
What we really want, then, is to build a pinball engine , a program that can simulate any pinball machine described in an appropriate way. You will begin with a simulator that has ball physics but no balls or bumpers. Your job is to extend the program so that it prompts the user for the name of a file (format described below) and then creates the appropriate sprites in the machine before setting the ball(s) loose.
Copy the files Pinball.java and PinballGame.java into a directory Lab09. You can find the files to copy in the directory
/home/student/Classes/201/Labs/Lab
Also in that directory is a file named first.pnd. We will be using the extension .pnd to stand for a “Pinball Description” file, an extension we coined. This file will have a format described later.
Download the Java and pinball files into your lab directory. Compile and run the pinball game. It should compile but it should not do anything because there is no description of a pinball game.
Look at the PinballGame class. Find all of the stub functions (they are the ones with little or no code in them). In this lab, you will replace the stubs with working code.
Modify getFileName so that it displays the prompt that is passed into it followed by a single space and then reads a string from the user, returning that value.
Reading information from the user’s keyboard is somewhat like printing information to the user’s screen. Code such as System.out.print(...) will display the expression to your screen based on the type of the expression – such as a String, an int, or whatever. Reading information from your keyboard uses System.in, which should not be surprising. But how will the program know the type of the value to be read? Here is where the notion of a Scanner comes in.
A Scanner is a Java object that takes raw input from the keyboard (or from a file, as we shall see) and interprets it as a string or integer or whatever, depending on what the program asks it to do.
How does one read a string from the keyboard, for example? Here is a snippet of code that would do so:
Scanner kbd = new Scanner(System.in); // tokenize raw input from the keyboard String str = kbd.next(); // interpret the next token from the keyboard
For a Scanner object, the term “token” means a string of input characters that does not have any whitespace (such as spaces, tabs, or newlines). The next() method skips any whitespace until it encounters a non-whitespace character and then collects all of the following non-whitespace characters into a single string that it returns to the caller.
A Scanner can also return ints using nextInt() and doubles using nextDouble(), for example. You should look at the Scanner class documentation to see all of the things that a Scanner can process.
Use a Scanner as described above to implement the getFileName method.
Checkpoint 1
Compile and run your program and show us your results.
The ensureExtension method
The ensureExtension method is passed a file name and returns that file name with a given ex- tension. For example, if ensureExtension were passed the parameters something and txt, it would return the string something.txt. The method is called ensure Extension because it should only add the extension if the file name does not already end with it. Thus calling the method with parameters something.txt and txt should also return something.txt.
Before you implement the ensureExtension method, look at the documentation for the String class and study the following methods:
equals endsWith
The ensureExtension method takes two String parameters. The first parameter is the file name (which you get using the getFileName method), and the second is the desired extension. If the file name does not end with a dot and the given extension, it needs fixing; otherwise return it
Lines in a PND file are of the form:
Ball Oval Rectangle
When using a scanner, if you know what the next item from to be read will be, you can simply call the appropriate method such as nextInt() (for an int), nextDouble() (for a double), or just next() (for a String). For example, if you have a line that begins with Oval, you will call the handleOval method that will then get a String (the color) and four doubles, create an OvalSprite object with the appropriate width and height, set its color to the given color value, locate it at the appropriate x and y values, and add it to the game canvas. (Note that you got to the handleOval method because you already saw the string Oval in the readPNDFile.) Similar remarks apply to lines that begin with Rectangle. For Ball lines, you need to create a Pinball object (look at Pinball.java to see what the constructor requires for this class!), adjust its color and location, add it to the game canvas, and add it to the pinballs list.
The word can be passed to getColor to return a color and all the rest are double values. The variables x and y are the location of the bumper (or ball); w and h are width and height. The dX and dY are the velocity of the ball. You should use the Scanner variable in to do most of the work of reading all the fields of a given bumper or ball.
0.1 Checkpoint 3
Show us your code at this point.
Write your own PND file
Create a PND file of your own choosing that describes a pinball level with two balls (starting near the left and right edges of the screen heading toward the center of the screen) and two oval sprite eyes and one rectangle sprite “mouth” to make a “smiley face”. Your PND file should end with a .pnd extension.
Checkpoint 4
Run your PinballGame file and enter your PND file as the filename. Show us your results.
Are you done?
If you have finished with the checkpoints, start working on your Assignment 5.
