Score Timer

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
import java.awt.*;
import java.text.*;

/**
 * This class should be added in to the world when needed, and will continue displaying the amount of seconds that elapsed since the addition to the world.
 * 
 * @author Nosson1459 
 * @version 12/6/2016
 */
public class Timer extends Actor
{
    private long startTime; // this is the starting time of the scenario
    private long stopTime; // this is for figuring out the diffrence in time from beginning to now
    private double elapsedTime=0.0000; // this is what timer is equal
    private static final Color transparent = new Color(0,0,0,0);
    private int a=0;
    private String prefix;
    private GreenfootImage time;
    public Timer(String prefix)
    {
        this.prefix=prefix;
        updateTime();
    }
    /**
     * Act - do whatever the Timer wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        if (a==0)
        {
            startTime=System.currentTimeMillis();
            a=1; // this "if" is so that startTime is initialized only after "Run" is pressed but still only initialized once
        }
        updateTime();
    }    
    /**
     * Update the image on screen to show seconds stopwatch
     */
    private void updateTime()
    {
        MyWorld world=(MyWorld)getWorld();
        if (a != 0)
        {
            stopTime=System.currentTimeMillis(); //gets current time in milliseconds
            elapsedTime=(stopTime-startTime)/1000.0; //converts time to seconds
            if (world.stopped==true)
            {
                startTime+=(world.startedTime-world.stoppedTime);
                world.stopped=false;
            }
        }
        time=new GreenfootImage(prefix + new DecimalFormat("0.0").format(elapsedTime), 22, Color.BLACK, transparent); // makes the timer image, the decimal format makes the timer only have one decimal place (0.0) (you could do more if you do (0.00) etc.
        setImage(time);
    }
}

public long stoppedTime;
public long startedTime;
public boolean stopped=false;

public void stopped()
{
    stopped=true;
    stoppedTime=System.currentTimeMillis();
}
public void started()
{
    if (stopped==true)
    {
        startedTime=System.currentTimeMillis();
    }
}