Mass to health ratio

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
import java.io.*;
import java.util.*;
/**
 * Write a description of class MyWorld here.
 * 
 * @author Matthew Benson
 * @version 1.0
 */
public class MyWorld extends World
{
    private SpaceBackground Image1, Image2;                             //images merged together to create illusion of scrolling background
    GreenfootSound backgroundMusic = new GreenfootSound("GameScreenMusic.wav");             //background music assigned to a variable
    private int delay;                                          //time between how fast asteroids spawn
    private boolean gameOver;                                           //tests whether game is over or not
    private int wait = 240;                                         //4 second delay (240/60) for explosion gif to be seen
    private double gameTimer1 = 0;                              //survived time of spaceship1
    private double gameTimer2 = 0;                              //survived time of spaceship2
    /**
     * Initialization and creation of objects
     * 
     */
    public MyWorld()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(1200, 800, 1, false); 

        Image1 = new SpaceBackground();
        addObject(Image1, getWidth()/2, getHeight()/2);
        Image2 = new SpaceBackground();
        addObject(Image2, getWidth() + getWidth()/2, getHeight()/2);

        if (StartScreen.mode == 1) { // if the mode variable is 1 (meaning one player mode), only spawn 1P ship
            Spaceship ship = new Spaceship();
            addObject(ship, 100, getHeight()/4);
        }
        if (StartScreen.mode == 2) { // if the mode variable is 2 (meaning 2 player mode), spawn both ships.
            Spaceship ship = new Spaceship();
            addObject(ship, 100, getHeight()/4);
            Spaceship2 ship2 = new Spaceship2();
            addObject(ship2, 100, (getHeight() / 2) + (getHeight() / 4));
        }
    }

    /**
     * Spawns asteroids at right edge of screen every half a second.
     * Y-location of asteroids are random.
     */
    public void spawn() {
        delay = (delay + 1)%30;         //spawns asteroids every 0.5 seconds
        if (delay == 0) addObject(new Asteroid((Greenfoot.getRandomNumber(19)-20), 2), 0, 0);
    }

    /**
     * Stop scrolling screen once game is over.
     * The game is over once both spaceships explode.
     */
    public void gameOver() {
        Image1.setLocation((this.getWidth() / 2) + 10, this.getHeight() / 2);
        Image2.setLocation((this.getWidth() / 2) + 10, this.getHeight() / 2);
    }

    /**
     * Write scores of both spaceships to an external file.
     * Store (gameTimer1/60) and (gameTimer2/60) here to store time survived (s)
     */
    public void writeToFile() {
        double score1 = gameTimer1/60;                  //convert act cycle increments to seconds
        double score2 = gameTimer2/60;                  //convert act cycle increments to seconds
        String score1s = String.valueOf(score1);        //convert double to string
        String score2s = String.valueOf(score2);        //convert double to string
        try {
            File highscores = new File("highscores.txt");       //assign file to variable
            if (!highscores.exists()) {                         //if file does not exist, create new one
                highscores.createNewFile();
            }
            FileWriter fw = new FileWriter(highscores,true);        //create new filewriter
            BufferedWriter bw = new BufferedWriter(fw);             //create new bufferedwriter, to avoid replacement when writing to a file
            bw.write(score1s);
            bw.newLine();
            bw.write(score2s);
            bw.newLine();
            bw.close();                 //close the bufferedwriter
        } 
        catch (IOException e) {         //vital part of storing, catch exceptions in case file does not exist etc.
            e.printStackTrace();
        }
    }

    /**
     * Executes following information several times per second during the course of runtime.
     * Dictates the actions of objects after game has started.
     */
    public void act() {
        if (getObjects(Spaceship.class).size() != 0 || getObjects(Spaceship2.class).size() != 0) {              //spawn asteroids if at least one spaceship is alive
            gameOver = false;
            spawn();
        }

        if (getObjects(Spaceship.class).isEmpty() && getObjects(Spaceship2.class).isEmpty()) {                  //if both spaceships are dead, end game
            gameOver = true;
            gameOver();
        }

        backgroundMusic.playLoop();    //play background music on run()

        Image1.ActivateScroll();
        Image2.ActivateScroll();

        if (gameOver) {
            if (wait>0)                 //after game is over, wait x amount of seconds based on (wait/60)
            {
                wait--;
                if(wait == 0) {
                    backgroundMusic.stop();
                    writeToFile();                          //write scores to file here to avoid repeats
                    Greenfoot.setWorld(new GameOver());
                }
            }
        }

        if (getObjects(Spaceship.class).size() != 0) {          //incrementing time survived for spaceship1
            gameTimer1++;
        }
        if (getObjects(Spaceship2.class).size() != 0) {         //incrementing time survived for spaceship2
            gameTimer2++;
        }

    }
}