SolarSystem.java

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.stage.Stage;
import javafx.scene.layout.StackPane;
import javafx.scene.canvas.*;
import javafx.scene.paint.Color;
import javafx.animation.*;
import javafx.util.Duration;
import javafx.geometry.Point2D;
import javafx.scene.input.KeyCode;

import java.util.*;

public class SolarSystem extends Application{
    private static final int WIDTH = 1000;
    private static final int HEIGHT = 800;
    private static final double SCALE = 200 / 1.50e11;
    private static final double TIMESTEP = 3600*24; // 1 Day
    private static final double G = 6.6743e-11;

    private static class Planet{
        private double x, y;
        private double xVelocity, yVelocity;
        private double mass, radius;
        private Color color;
        private List<Point2D> orbit = new ArrayList<>();
        private boolean signChanged = false, renderOrbit = true;

        public Planet(Color color, double radius, double mass, double x, double y, double xVelocity, double yVelocity){
            this.color = color;
            this.radius = radius;
            this.mass = mass;
            this.x = x;
            this.y = y;
            this.xVelocity = xVelocity;
            this.yVelocity = yVelocity;
        }

        private Point2D getAttractionForce(Planet planet){
            double distanceX = planet.x-this.x;
            double distanceY = planet.y-this.y;
            double distance = Math.sqrt(Math.pow(distanceX, 2)+Math.pow(distanceY, 2));

            double force = SolarSystem.G*this.mass*planet.mass/Math.pow(distance, 2);
            double angle = Math.atan2(distanceY, distanceX);
            double forceX = force * Math.cos(angle);
            double forceY = force * Math.sin(angle);

            /*
            Distance moon-sun: 1.504E11
            Sun force for moon: 4.313523549126089E20
            Distance moon-earth: 4.0E8
            Earth force for moon: 1.831019119125E20
            */

            /*if (this == MOON && planet == SUN){
                System.out.println("Distance moon-sun: "+distance);
                System.out.println("Sun force for moon: "+force);
            } else if (this == MOON && planet == EARTH){
                System.out.println("Distance moon-earth: "+distance);
                System.out.println("Earth force for moon: "+force);
            }*/

            return new Point2D(forceX, forceY);
        }

        public void updatePosition(List<Planet> planets){
            double totalForceX = 0;
            double totalForceY = 0;

            for (Planet planet : planets){
                if (planet == this) continue;
                Point2D force = getAttractionForce(planet);
                totalForceX += force.getX();
                totalForceY += force.getY();
            }

            double temp = this.xVelocity;
            this.xVelocity += totalForceX/this.mass*TIMESTEP;
            this.yVelocity += totalForceY/this.mass*TIMESTEP;

            if (temp*this.xVelocity < 0){ // The sign changed
                this.signChanged = !this.signChanged;
                if (!this.signChanged){
                    this.renderOrbit = false;
                }
            }

            this.x += this.xVelocity*TIMESTEP;
            this.y += this.yVelocity*TIMESTEP;
        }

        public void render(GraphicsContext gc){
            final double x = this.x*SolarSystem.SCALE+WIDTH/2;
            final double y = this.y*SolarSystem.SCALE+HEIGHT/2;

            if (this.renderOrbit){
                this.orbit.add(new Point2D(x, y));
            }

            gc.setFill(this.color);
            final double radius = this.radius; // TODO
            gc.fillOval(x-radius, y-radius, radius*2, radius*2);
        }

        public void renderOrbit(GraphicsContext gc){
            gc.setStroke(Color.WHITE);
            gc.setLineWidth(2);
            for (int i = 0; i < this.orbit.size()-1; i++){
                gc.strokeLine(this.orbit.get(i).getX(), this.orbit.get(i).getY(), this.orbit.get(i+1).getX(), this.orbit.get(i+1).getY());
            }
        }
    }

    private List<Planet> planets = new ArrayList<>();
    private Map<KeyCode, Boolean> keys = new HashMap<>();
    private static int WATCHING_PLANET = 0;

    private static Planet MOON, EARTH, SUN;

    @Override
    public void start(Stage stage){
        Canvas canvas = new Canvas(WIDTH, HEIGHT);
        GraphicsContext gc = canvas.getGraphicsContext2D();
        StackPane pane = new StackPane(canvas);

        canvas.setFocusTraversable(true);
        canvas.setOnKeyPressed(e -> this.keys.put(e.getCode(), true));
        canvas.setOnKeyReleased(e -> this.keys.put(e.getCode(), false));

        reset();

        Timeline loop = new Timeline(new KeyFrame(Duration.millis(1000.0/40), e -> {
            for (Planet planet : planets){
                planet.updatePosition(planets);
            }
            update(gc);
        }));
        loop.setCycleCount(Animation.INDEFINITE);
        loop.play();

        Scene scene = new Scene(pane, WIDTH, HEIGHT);
        stage.setScene(scene);

        stage.setTitle("Solar system");
        stage.setResizable(false);
        stage.show();
    }

    private void reset(){
        planets.clear();
        Planet sun = new Planet(Color.YELLOW, 16, 1.989e30, 0, 0, 0, 0);
        Planet earth = new Planet(Color.CYAN, 8, 5.972e24, -1.50e11, 0, 0, 2.97e4);
        Planet moon = new Planet(Color.GRAY, 4, 7.35e22, -(1.50e11+4e8), 0, 0, 1022);
        Planet mercury = new Planet(Color.ORANGE, 6, 3.30e23, -6.982e10, 0, 0, 4.736e4);
        Planet mars = new Planet(Color.RED, 5, 6.41e23, -2.49e11, 0, 0, 2.407e4);
        Planet venus = new Planet(Color.LIME, 4, 4.86e24, -1.08e11, 0, 0, 3.502e4);
        planets.add(sun);
        planets.add(earth);
        planets.add(moon);
        //planets.add(mercury);
        //planets.add(mars);
        //planets.add(venus);

        MOON = moon;
        EARTH = earth;
        SUN = sun;
    }

    private void update(GraphicsContext gc){
        gc.clearRect(0, 0, WIDTH, HEIGHT);
        gc.setFill(Color.BLACK);
        gc.fillRect(0, 0, WIDTH, HEIGHT);

        if (this.keys.getOrDefault(KeyCode.ESCAPE, false)){
            System.exit(0);
        } else if (this.keys.getOrDefault(KeyCode.R, false)){
            reset();
            this.keys.put(KeyCode.R, false);
        }

        // DEBUG
        if (this.keys.getOrDefault(KeyCode.DIGIT0, false)){
            WATCHING_PLANET = 0;
            this.keys.put(KeyCode.DIGIT0, false);
        } else if (this.keys.getOrDefault(KeyCode.DIGIT1, false)){
            WATCHING_PLANET = 1;
            this.keys.put(KeyCode.DIGIT1, false);
        } else if (this.keys.getOrDefault(KeyCode.DIGIT2, false)){
            WATCHING_PLANET = 2;
            this.keys.put(KeyCode.DIGIT2, false);
        } else if (this.keys.getOrDefault(KeyCode.DIGIT3, false)){
            WATCHING_PLANET = 3;
            this.keys.put(KeyCode.DIGIT3, false);
        } else if (this.keys.getOrDefault(KeyCode.DIGIT4, false)){
            WATCHING_PLANET = 4;
            this.keys.put(KeyCode.DIGIT4, false);
        } else if (this.keys.getOrDefault(KeyCode.DIGIT5, false)){
            WATCHING_PLANET = 5;
            this.keys.put(KeyCode.DIGIT5, false);
        }

        for (Planet planet : planets){
            planet.renderOrbit(gc);
            planet.render(gc);
        }

        gc.setFill(Color.WHITE);
        gc.fillText(String.format("x: %.3f y: %.3f\nxVel: %.3f\nyVel: %.3f\nTarget: %d", planets.get(WATCHING_PLANET).x, planets.get(WATCHING_PLANET).y, planets.get(WATCHING_PLANET).xVelocity, planets.get(WATCHING_PLANET).yVelocity, WATCHING_PLANET), 50, 50);
    }

    public static void main(String[] args){
        launch(args);
    }
}