Need to draw projectile motion of a thrown ball

Question

So I\'m writing a code that allows a user to throw an imaginary object at an initial angle and speed to see if they can come close to hitting a ball (that is positioned based on

Answer 1

For drawing a curve you need to calculate horizontal (x) and vertical (y) position, as a function of time, start point, start speed and angle. In other words, calculate the horizontal distance, and vertical distance. The equations are well known and widely available.

You use these equations to repeatedly calculate x,y and then repaint.
See the following demonstration. Not the comments :

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Point2D;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

public class BalisticCurve extends JFrame {

    private static final double G = 9.8; //positive because Y axis is positive going down
    private int animationSpeed = 5; //millis. The smaller the faster
    private static int size = 900, ballDiameter = 10;
    private double startX, startY, ballX, ballY;
    private double xSpeed, ySpeed, lastPointX, lastPointY;
    private double time, deltaTime = 0.01 ; //in seconds
    private List<Point2D> curvePoints= new ArrayList<>();
    private Timer timer;

    BalisticCurve(){

        super("Balistic Curve");
        DrawBoard board  = new DrawBoard();
        add(board, BorderLayout.CENTER);

        ballX= lastPointX = startX = 50;
        ballY = lastPointY = startY = size - 100;
        getUserInput();

        pack();
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);

        timer = new Timer(animationSpeed, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent event) {

                board.moveBall();
                board.repaint();
                if(! inBounds()) {
                    timer.stop();
                }
            }
        });
        timer.start();
    }

    private void getUserInput() {

        double angle = 45;//todo replace with user input + verification
        double speed = 100;
        xSpeed = speed * Math.cos(angle * (Math.PI / 180));
        ySpeed = speed * Math.sin(angle * (Math.PI / 180));
    }

    private boolean inBounds() {

        //ignore if ball exceeds height
        if((ballX < 0) || (ballX > (getWidth()))
                || ( ballY  > (getHeight() - ballDiameter) ) ) {
            return false;
        }

        return true;
    }

    class DrawBoard extends JPanel {

        public DrawBoard() {
            setPreferredSize(new Dimension(size, size));
        }

        @Override
        public void paint(Graphics g) {
            super.paint(g);

            Graphics2D g2d = (Graphics2D) g;
            g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                    RenderingHints.VALUE_ANTIALIAS_ON);
            g2d.setColor(Color.RED);
            g2d.fillOval((int)ballX,(int)ballY,ballDiameter,ballDiameter);

            if((Math.abs(lastPointX - ballX)>=1) && (Math.abs(lastPointY - ballY)>=1) ) {
                curvePoints.add(new Point2D.Double(ballX, ballY));
                lastPointX = ballX; lastPointY = ballY;
            }

            drawCurve(g2d);
        }

        private void drawCurve(Graphics2D g2d) {

            g2d.setColor(Color.BLUE);
            for(int i=0; i < (curvePoints.size()-1); i++) {

                Point2D from = curvePoints.get(i);
                Point2D to = curvePoints.get(i+1);
                g2d.drawLine((int)from.getX(),(int)from.getY(), (int)to.getX(), (int)to.getY());
            }
        }

        private void moveBall() {

            ballX = startX + (xSpeed * time);
            ballY = startY - ((ySpeed *time)-(0.5 *G * Math.pow(time, 2))) ;
            time += deltaTime;
        }
    }

    public static void main(String[] args) {

        new BalisticCurve();
    }
}

Don't hesitate to ask what is not clear enough.