Main.java

import javax.swing.*;
import java.awt.*;
import java.util.*;
import java.awt.event.*;
import javax.swing.event.MouseInputAdapter;
@SuppressWarnings("serial")
public class Main extends JPanel implements ActionListener {
    public static final double XSPEED = .0000001;
    public static final double YSPEED = .0000001;
    int startX, startY, endX, endY, adding=0;
    boolean dragging=false,
    still=false, //determines whether the planet moves via gravity or not.
    vShow=false,
    aShow=false,
    pause=false; 
    double nextMass=100000;
    ArrayList<Orbital> planets = new ArrayList<Orbital>();
    public Main() {
        //initialization of listeners
        MooseMover mouse=new MooseMover();
        this.addMouseListener(mouse);
        this.addMouseMotionListener(mouse);
        KeyListener listener = new MyKeyListener();
        addKeyListener(listener);
        setFocusable(true);
    }
    public class MyKeyListener implements KeyListener {
        @Override
        public void keyTyped(KeyEvent e) {

        }

        @Override
        public void keyPressed(KeyEvent e) {
        	//System.out.println(e.getKeyCode());
            if(e.getKeyCode()==32/*space key*/){
                still=!still;
            }
            if(e.getKeyCode()==10/*enter*/){
                nextMass=Integer.parseInt(JOptionPane.showInputDialog("What is the next Mass? (Standard is 100000)"));
            }
            if(e.getKeyCode()==65/*a*/){
            	aShow=!aShow;
            }
            if(e.getKeyCode()==86/*v*/){
                pause=!pause;
            }
        }

        @Override
        public void keyReleased(KeyEvent e) {

        }
    }
    public class MooseMover extends MouseInputAdapter{
        public void mousePressed(MouseEvent e){
            startX=e.getX();
            startY=e.getY();
            endX=e.getX();
            endY=e.getY();
        }
        //calculates initial velocity via drag length
        public void mouseDragged(MouseEvent e){
            endX=e.getX();
            endY=e.getY();
        }

        public void mouseReleased(MouseEvent e){
            endX=e.getX();
            endY=e.getY();
            planets.add(new Orbital(nextMass,startX,startY,(endX-startX)/10,(endY-startY)/10,still));
        }
    }

    @Override
    public void paintComponent(Graphics g){
    	if(!pause) {
        g.setColor(Color.WHITE);
        g.fillRect(0,0,this.getWidth(),this.getHeight());
        g.setColor(Color.RED);
        //instructions
        g.drawString("(Press Enter key to change) Weight: "+nextMass,0,10);
        g.drawString("(Press Space key to change) Stable: "+still,0,20);
        g.drawString("xAcceleration(Blue) yAcceleration(Green)",0,30);
        g.drawString("Press a to show/hide acceleration vectors",0,40);
        g.drawString("Press v to pause/unpause",0,50);
        g.drawLine((int)startX,(int)startY,(int)endX,(int)endY);
        for(int i=0;i<planets.size();i++){
            double xAccel=0,yAccel=0;
            for(int j=0;j<planets.size();j++){
            	//calculates planet gravitational pull magnitude and direction
                double theta = Math.atan((planets.get(j).y-planets.get(i).y)/(planets.get(j).x-planets.get(i).x)),
                magnitude=6.67*Math.pow(10,-6)*planets.get(i).mass*planets.get(j).mass/Math.sqrt(Math.pow(planets.get(i).x-planets.get(j).x,2)+Math.pow(planets.get(i).y-planets.get(j).y,2));
                try{
                    if(planets.get(i).x<planets.get(j).x){
                        xAccel+=Math.cos(theta)*magnitude/planets.get(i).mass;
                    }
                }catch(Exception e){}
                try{
                    if(planets.get(i).x>planets.get(j).x){
                        xAccel-=Math.cos(theta)*magnitude/planets.get(i).mass;
                    }
                }catch(Exception e){}
                try{
                    if(planets.get(i).x>planets.get(j).x){
                        if(planets.get(i).y<planets.get(j).y){
                            yAccel-=Math.sin(theta)*magnitude/planets.get(i).mass;
                        }
                    }
                    if(planets.get(i).x<planets.get(j).x){
                        if(planets.get(i).y<planets.get(j).y){
                            yAccel+=Math.sin(theta)*magnitude/planets.get(i).mass;
                        }
                    }
                }catch(Exception e){}
                try{
                    if(planets.get(i).x<planets.get(j).x){
                        if(planets.get(i).y>planets.get(j).y){
                            yAccel+=Math.sin(theta)*magnitude/planets.get(i).mass;
                        }
                    }
                    if(planets.get(i).x>planets.get(j).x){
                        if(planets.get(i).y>planets.get(j).y){
                            yAccel-=Math.sin(theta)*magnitude/planets.get(i).mass;
                        }
                    }
                }catch(Exception e){}
            }
            //changes planet velocity
            planets.get(i).acceleration(xAccel,yAccel);
        }
        for(Orbital a:planets){
            a.move(g);
            g.fillOval((int)a.x-5,(int)a.y-5,10,10);
        }
        //x-axis acceleration vector visibility
        g.setColor(Color.BLUE);
        if(aShow)
        for(Orbital a:planets){
        	g.drawLine((int)a.x,(int)a.y,(int)(a.x+(a.vx-a.vxt)*4000),(int)a.y);
        }
        //y-axis acceleration vector visibility
        g.setColor(Color.GREEN);
        if(aShow)
        for(Orbital a:planets){
        	g.drawLine((int)a.x,(int)a.y,(int)a.x,(int)(a.y+(a.vy-a.vyt)*4000));
        }
    	}
        repaint();	
    }

    @Override
    public void actionPerformed(ActionEvent e){
        
    }
}

Orbital.java

import java.util.ArrayList;
import java.awt.*;
public class Orbital{
    double x,y,vx,vy,mass,vxt,vyt;
    boolean stationary;
    ArrayList<Double> xPoint= new ArrayList<Double>();
    ArrayList<Double> yPoint= new ArrayList<Double>();
    public Orbital(double Mass, int StartX, int StartY, int VelocityX, int VelocityY,boolean still){
        mass=Mass;
        x=StartX;
        y=StartY;
        vx=VelocityX;
        vy=VelocityY;
        vxt=vx;
        vyt=vy;
        stationary=still;
    }

    public void acceleration(double ax, double ay){
    	//changes the velocity according to the acceleration vectors
        vxt=vx;
        vyt=vy;
        vx+=ax;
        vy+=ay;
    }

    public void move(Graphics g){
    	//point movement due to velocity
        if(!stationary){
            xPoint.add(x);
            if(xPoint.size()>75){
                xPoint.remove(0);
            }
            yPoint.add(y);
            if(yPoint.size()>75){
                yPoint.remove(0);
            }
            x+=vx;
            y+=vy;
            for(int i=0; i<xPoint.size()-1;i++){
                g.drawLine((int)(double)xPoint.get(i),(int)(double)yPoint.get(i),(int)(double)xPoint.get(i+1),(int)(double)yPoint.get(i+1));
            }
        }
    }
}

ProgramStart.java

import javax.swing.*;
import java.awt.*;
public class ProgramStart{
    public static void main(String[] args){
        JFrame Movie = new JFrame();
        Container MovieFrame = Movie.getContentPane();
        Main One = new Main();
        Movie.setTitle("FUN");
        Movie.setSize(676,718);
        Movie.setLocation(0,0);
        One.setBackground(Color.CYAN);
        MovieFrame.add(One);
        Movie.setVisible(true);
    }
}