How can I relate the Java Timer to milliseconds in real time, and update a time variable accordingly?

Question

I have been doing some code on projectile motion. I initiated a timer t and used the constructor 10 for it, every action event I add 0.01 to a time variable initialised as 0.00, to simulate time going past. Using this time I calculate the height/distance etc. I cannot figure out 2 problems:

1) When printing the current time into the console it keeps going to many decimal places, run code. 2) Height will keep increasing even though it shouldn't, I feel as if it is due to the positive and negative of acceleration.

(My panel is 1920x1080 and the panel animating takes up 80pixels - that's why it is 1000-50) (1000-radius)

Images for use: https://i.stack.imgur.com/dho5L.jpg

From responses: 1) Forgot to multiply vy by t. 2) Use DecimalFormat to truncate the value

Thanks to @G_H ; @Brick ; @Tom ; @Diginoise Code:

package projectV1;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Ellipse2D;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;


public class Animate extends JPanel implements ActionListener {

double x = 0;
double time = 0.00;
String timeText2 = "0";
double y = 1000;
double velY = 0.0;
double velX = 0.0;
public static Timer t;

public Animate() { // Constructor
    super(); 
}

public void start(String acceleration, String initialVelocity, String angle){



    new Ball(acceleration, angle, initialVelocity);

    t = new Timer(10, this);
    t.start();

    }


@Override
 public void actionPerformed(ActionEvent e) {
    updateTime(); 
    move();


      }

 public void move(){ // HOlds the repaint method
        x = x + Ball.getVelX();
        Simulation.distanceText.setText(Double.toString(Ball.getDistance(time))); // Some of these are just checks for me in the gui
        Simulation.velocityText.setText(Double.toString(time));
        y = y- Ball.getVelY(time);
        Simulation.heightText.setText(Double.toString(Ball.getHeight(time)));



        repaint();
    }

 public void updateTime(){
     time= time + 0.01;

     System.out.println(Double.toString(time));  // Using this as a test to check the time output.
     timeText2 = Double.toString(time);
     Simulation.timeText.setText(timeText2);
     }


public void paintComponent(Graphics gg){
    super.paintComponent(gg);
    gg.drawRect(0, 0, 1920, 1000);
    Graphics2D g = (Graphics2D) gg;

     Ellipse2D.Double shape = new Ellipse2D.Double(x, y-50.0, 50, 50);
        g.fill(shape);

    // g.fillOval(x, y-50, 50, 50);

     }
}

Code from '' Ball Class ''

public Ball(String acceleration, String angle, String initialVelocity ) {

    Ball.acceleration = acceleration;
    Ball.initialVelocity = initialVelocity;
    Ball.angle = angle;
}


public static double getVelX(){ //  This stays constant throughout the program
    double velX = 0.0;
    int u = Integer.parseInt(initialVelocity);
    double ang = Double.parseDouble(angle);
    velX = u*(Math.cos(ang));
    return velX;

}

public static double getVelY(double time ){

    double velY = 0.0 ;
    double ang, acc = 0;
    int u = 0;
    ang = Double.parseDouble(angle);
    acc = Double.parseDouble(acceleration);
    u = Integer.parseInt(initialVelocity);

    velY = u*(Math.sin(ang))- (acc*time);
    return velY;
}

public static double getDistance(double time){
    double distance = 0;
    double ang = 0;
    int u = 0;
    ang = Double.parseDouble(angle);

    u = Integer.parseInt(initialVelocity);
    distance = (u*time)*(Math.cos(ang));

    return distance;


}
public static double predictedTime(){
    double ptime = 0;
    double ang = 0;
    ang = Double.parseDouble(angle);
    int u = Integer.parseInt(initialVelocity);
    ptime = ((u*u)*(Math.sin(ang)*Math.sin(ang))/(2*9.81));

    return ptime;
}

public static double getHeight(double time ){
    double height;
    double ang, acc;
    int u = 0;
    ang = Double.parseDouble(angle);
    acc = Double.parseDouble(acceleration);
    u = Integer.parseInt(initialVelocity);
    height = u*(Math.sin(ang))-(0.5*acc*(time*time));

    return height;


}

Answer 1

1) When printing the current time into the console it keeps going to many decimal places, run code.

This is due to the binary representation of fractional numbers. Suppose that we try to represent 1/3 in decimal, you get 0.333333... The expansion is infinite, there's no finite representation. 0.01 (1/100) has a finite representation in decimal, but not in binary. So if you write double d = 0.01;, know that the internal binary representation is an approximation, not the exact value. You'll want to truncate the output to some value with a certain number of decimal places. Check out class DecimalFormat for outputting the values in a specific format.

2) Height will keep increasing even though it shouldn't, I feel as if it is due to the positive and negative of acceleration.

Check this part: velY = u*(Math.sin(ang))- (acc*time);

u is a contant. So is sin(ang). acc*time is only going to increase. So it makes sense that velY is going to change linearly. You've posted this:

It states Sy = Vy*t - 0.5*g*t², then that Vy = u*sin(ang). There's a time component missing in the expansion of the equation. It should be this: velY = (u*(Math.sin(ang))- (acc*time)) * time; or (same thing theoretically) velY = u*(Math.sin(ang))*time - (acc*time*time); Note that acc must be 1/2 g. If you supply g as acceleration you'll need to multiply acc*time*time with 0.5 as per the formula for vertical displacement.

Final note, you make the various fields into static fields of class Ball, then assign those in the constructor. It's best not to do that. Make fields acceleration, initialVelocity and angle instance fields of Ball (they can be final because they won't change) and make the methods non-static. This would allow you to simulate multiple balls, for example, since their values remain local to them, rather than shared class state.