Java Games tutorial #4

Before we start adding new code to the project, we're going to clean up what's already there. What do we mean by "clean up"? Simple: we're going to take the time to enforce some good programming habits. In particular, we're going to replace "hard-coded" numbers with symbolic constants.

I can already hear you asking, "Hard-coded numbers? Symbolic constants? What's he talking about?" An example should clear everything up.

If you examine the code in Battleship.java (and if you don't have the code, you might want to get it from tutorial #3), you'll find the following line in the constructor:

this.setSize(600, 500);

where 600 is the horizontal size of the game in pixels, and 500 is the vertical height, also in pixels. We're going to replace both 600 and 500 with static constants, like this:

public static final int GAME_WIDTH = 600;

public static final int GAME_HEIGHT = 500;

.

.

.

this.setSize(GAME_WIDTH, GAME_HEIGHT);

Why bother with this? There are several reasons. First off, it will save you time. We'll always define constants like GAME_WIDTH and GAME_HEIGHT at the top of our files. That way, if you ever need to change them, you just scroll to the top of the file and edit the number. Using the old method, you'd have to search through the code until you found the setSize statement and change the numbers there.

Second--and more important--this technique helps reduce bugs. Suppose that you have to refer to the game width (600) in several places throughout the code. You might have statements like:

public void paint(Graphics gfx) {

    gfx.setColor(Color.black);

    gfx.fillRect(0, 0, 600, 500);

    gfx.setColor(Color.red);

    int i = 0;

    while (i < 10) {

        gfx.drawLine(i * 600 / 10, 0, i * 600 / 10, 500);

    }

}

Now imagine that you want to change the width of the game from 600 pixels to 640 pixels. You'd have to find every instance of the number 600 and change it to 640. Even if you did this perfectly, you might still introduce bugs--especially if you have another value of 600 that doesn't represent the screen width and you change it to 640 by accident.

Now consider the same code using our symbolic constants:

public static final int GAME_WIDTH = 600;

public static final int GAME_HEIGHT = 500;

public static final int LINE_COUNT = 10;

public void paint(Graphics gfx) {

    gfx.setColor(Color.black);

    gfx.fillRect(0, 0, GAME_WIDTH, GAME_HEIGHT);

    gfx.setColor(Color.red);

    int i = 0;

    while (i < LINE_COUNT) {

        gfx.drawLine(i * GAME_WIDTH / LINE_COUNT, 0, i * GAME_WIDTH / LINE_COUNT, GAME_HEIGHT);

    }

}

Now, no matter how many times you use GAME_WIDTH and GAME_HEIGHT in the code, you can change them all simultaneously just by redefining the constants at the top of the file. Still want to change the game's width from 600 to 640? No problem--just change this:

public static final int GAME_WIDTH = 600;

to this:

public static final int GAME_WIDTH = 640;

Finally, as you've probably already noticed, using constants helps others understand your code. The original paint method above is pretty abstract--you probably can't tell what it's supposed to do. The second paint method makes much more sense--you can see that it's supposed to draw lines from the top of the screen to the bottom, evenly spaced along the x-direction.

Now that we've (hopefully) convinced you that constants are wonderous and beautiful, let's go put some in our code.