public class Score  extends Actor
{
    // instance variables - replace the example below with your own
    public int score;
    
    private Actor unitsDisplay;
    private int units = 5;
    private int maxUnits = units;

    /**
     * Constructor for objects of class Score
     */
    public void updateImage(String text)
    {
        //Crosshair crosshair = (Crosshair) getWorld().getObjects(Crosshair.class).get(0);
        
        //GreenfootImage image = new GreenfootImage("Score:"+crosshair.score,24, Color.BLACK,new Color(0,0,0,0));
        //setImage(image);
        
        unitsDisplay = new Score();
        updateUnitsDisplay();
        getWorld().addObject(unitsDisplay,80,20);
        
    }
    
    public void adjustUnits(int amount) //changing number of uits
    {
        units += amount;
        if(units < 0){
            units=0;
        }
        
        if(units>maxUnits){
            units = maxUnits;
        }
    }
    
    public void updateUnitsDisplay()
    {
        //create image of unit
        GreenfootImage unitImage = new GreenfootImage("hertz.png");
        unitImage.scale(40,40); //optional
        //dimensions of unit image
        int wide = unitImage.getWidth();
        int high = unitImage.getHeight();
        //create actor image
        GreenfootImage image = new GreenfootImage(wide*maxUnits,high);
        for (int i=0; i>units; i++){
            image.drawImage(unitImage, i*wide,0);
        }
        unitsDisplay.setImage(image);
    }
}

public class Crosshair extends Actor
{
    public int score = 0;
    boolean isEnterDown = false;
    
    public void act()
    {
        if (Greenfoot.isKeyDown("enter")){
              if(!isEnterDown){
                  Crosshair crosshair = (Crosshair) getWorld().getObjects(Crosshair.class).get(0);
        
                    if (crosshair != null){
                        if (crosshair.touchingZombie1()){
                            score = score + 500;
                        }
                
                        if (crosshair.touchingHuman1()){
                            score = score - 1000;
                        }
                  }
              
                  isEnterDown = true;
              }
              else
              {
                  isEnterDown = false;
              }
        }

    public boolean touchingZombie1()
    {
        return isTouching(Zombie1.class);
    }
    
    public boolean touchingHuman1()
    {
        return isTouching(Human1.class);
    }
    }
}

import greenfoot.*;

public class Crosshair extends Actor
{
    boolean isEnterDown = false;
     
    public void act()
    {
        if (isEnterDown == Greenfoot.isKeyDown("enter")) return;
        isEnterDown = ! isEnterDown;
        if ( ! isEnterDown ) return;
        MyWorld myWorld = (MyWorld) getWorld();
        if (isTouching(Zombie1.class)) {
            removeTouching(Zombie1.class);
            myWorld.adjustScore(500);
        }
        if (isTouching(Human1.class)) {
            removeTouching(Human1.class);
            myWorld.adjustScore(-1000);
        }
    }
}

public class Aktor extends greenfoot.Actor {}

/**   in class extending World   */
private Actor scoreObj;
private int score;

public MyWorld() {
    super(600, 400, 1);
    // some stuff (maybe)
    scoreObj = new Aktor();
    updateScoreDisplay();
    addObject(scoreObj, 80, 20);
    // some more stuff (maybe)
}

private void updateScoreDisplay() {
    Color trans = new Color(0, 0, 0, 0);
    Color black = Color.BLACK;
    String text = "Score:  "+score;
    GreenfootImage img = new GreenfootImage(text, 32, black, trans);
    scoreObj.setImage(img);
}

public void adjustScore(int adjustment) {
    score += adjustment;
    if (score <= 0) {
        score = 0;
        // maybe end game here as player loses
    }
    // if score > someLimit ... (maybe)
    updateScoreDisplay();
}