Planetary Gravitation in Java

Question

I'm making a simple game that will have a space ship being gravitationally attracted to a star at the center of the screen. The code I have so far moved the rectangle (soon to be a rocket) in a way that is not smooth at all. The rectangle also will not stop when it reaches the star but rather keeps moving well beyond the star before turning back.

Here's my code:

public void move() {
    // Deals with gravity towards the center star

    if (x > game.getWidth() / 2) {
        xVel -= xa;
    }
    if (x < game.getWidth() / 2) {
        xVel += xa;
    }
    if (y > game.getHeight() / 2) {
        yVel -= ya;
    }
    if (y < game.getHeight() / 2) {
        yVel += ya;
    }

    x += xVel;
    y += yVel;
}

I start by calculating where the rocket is located on the screen relative to the center, and then change its acceleration values, xa and ya, as needed. I then add the acceleration values to the Velocity variables, xVel and yVel.

Answer 1

You have not supplied all the code but to me it sounds like you are using integers where you should be using floats or doubles.

If you decalre the variables like the following

int x;
int velX;

You will not get the precision needed for smooth animation (there is nothing after the full stop). With integers 5 / 2 == 2 and not 2.5 as integers can only store whole numbers. This is why you are getting jerky motion.

You need to use floating point variables by decalring them as such.

float x = 0.0f; 
float vecX;

or

double x = 0.0;
double vecX;

That will give you the smooth motion you want.