[JAVA][Beginning Java 8 Games Development]

Chapter 12 ■ Setting Boundaries for Your Action Figure in 2D: Using the Node Class LocalToParent Attribute
Setting Screen Boundaries: .setBoundaries( ) Method
The next most important thing to do for our InvinciBagel game is to make sure that the character does not disappear
off the edge of the screen by placing a .setBoundaries() method call between the .setXYLocation() method, which
evaluates arrow (or ASDW) keypress combinations, and increments the iX and iY Bagel object properties accordingly,
and the .moveInvinciBagel() method, which actually executes the movement. By placing the .setBoundaries() method
before the sprite movement is invoked, we can make sure that the sprite is not off the screen (and if he is, move him
back onto the screen) before we actually call the move function (method). The first step in writing this code is to
define the sprite size in pixels so that we can calculate this along with our WIDTH and HEIGHT Stage size constants to
determine the boundary variable values that we will need to check our iX and iY sprite location against inside of the
.setBoundaries() method and its conditional if() statement structures. As you can see in Figure 12-15, I define these
sprite pixel size constant declarations at the top of the Bagel.java class, by using the following two lines of Java code:
protected static final double SPRITE_PIXELS_X = 81;
protected static final double SPRITE_PIXELS_Y = 81;
Figure 12-15. Declare protected static final double SPRITE_PIXELS_X and SPRITE_PIXELS_Y constants at the top of class
Next, we need to calculate the four screen boundary values using the WIDTH and HEIGHT constants in the
InvinciBagel class and the SPRITE_PIXELS_X and SPRITE_PIXELS_Y constants we just defined at the top of this class.
As you may have noticed from our 0,0 initial X,Y Bagel object location coordinates putting our sprite in the center of the
screen, JavaFX is using a centered X axis and Y axis screen addressing paradigm. This means there are four quadrants,
and that negative values (which mirror positive values) move left and up, and positive values move right and down.
We can actually use this paradigm later on to quickly ascertain which quadrant of the screen the character is in. The
way we would thus calculate the boundaries is to take half of the screen width and subtract half of the sprite width to
find the right (positive) boundary value and simply take the negative of this for the value for the left boundary limit. A
similar calculation applies to the top and bottom boundary value limit, for which we will take half of the screen height
and subtract half of the sprite height to find the bottom (positive) boundary value and simply take the negative of this
for the value for the top boundary limit value. The Java code for these calculations should look like the following:
protected static final double rightBoundary = WIDTH/2 - SPRITE_PIXELS_X/2;
protected static final double leftBoundary = -(WIDTH/2 - SPRITE_PIXELS_X/2);
protected static final double bottomBoundary = HEIGHT/2 - SPRITE_PIXELS_Y/2;
protected static final double topBoundary = -(HEIGHT/2 - SPRITE_PIXELS_Y/2);
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