Java 7 for Absolute Beginners

CHAPTER 10 ■ ANIMATION

public class Firework {

private int startX, startY, burstX, burstY, burstStep, currentStep, panelHeight, steps;
Color color;

// It takes a fair bit of setup work to figure out
// the starting location
// the bursting location
// which step we're on
// the total number of steps
Firework(int x, int height, int ticks) {
panelHeight = height;
steps = ticks;
startX = x;
startY = height;
burstX = x;
burstY = height >> 1;
burstStep = ticks >> 1;
currentStep = 0;
color = new Color((int)(Math.random() * 0xFFFFFF));
}

void draw(Graphics g) {
Color drawColor = color;
g.setColor(drawColor);
int height = panelHeight - panelHeight / steps * currentStep;
// the origin is at the top of the screen, so we check for being in
// the bottom half by checking to see if the current height is larger
// than the burst height. It seems backwards, but that's how it works.
if (height > burstY) {
// if we are in the bottom half of the panel, just draw
// the line that represents the firework shell going up.
g.drawLine(startX, startY, burstX, height);
}

if(currentStep >= burstStep) {
// When we reach the bursting point, draw 12 lines in a circle.
// For that, we need a tiny bit of trigonometry.
for (int i = 0; i < 12; i++) {
double xPrime, yPrime;
double currentRadians = Math.toRadians(30 * i);

int length = burstY / 2 / steps * currentStep;
xPrime = (Math.cos(currentRadians)

• Math.sin(currentRadians)) * length;
  yPrime = (Math.sin(currentRadians)

• Math.cos(currentRadians)) * length;
  int endX = new Double(xPrime).intValue() + burstX;
  int endY = new Double(yPrime).intValue() + burstY;