getWorld().removeObjects(getWorld().getObjects(Enemy.class)); 

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



/**
 * Write a description of class Board here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Board extends World
{
    private Counter actCounter;
    public Bar bar = new Bar("Health:", "" , 1000, 1000);
    
    public void act()
    {
        actCounter.setValue(actCounter.getValue() + 1);
        
        if (bar.getValue() == bar.getMinimumValue())
        {
            if (getObjects(GameOver.class).isEmpty()) showGameOver();
            return;
        }
    }
    
    public void BarSubtract(int value)
    {
        value = 0;
        bar.subtract(value); 
    }
    
    /**
     * Constructor for objects of class Board.
     * 
     */
    public Board()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(800, 800, 1); 
        GreenfootImage background = getBackground();  
        background.setColor(Color.BLACK);  
       // background.fill();  
        createStars(500);  
        
        addObject(bar, getWidth()/3, 30);
        
        addObject(new  Player(), getWidth()/2, getHeight()/2);
        actCounter = new Counter("Score: ");

        addObject(actCounter, getWidth() / 2, 30);
        Greenfoot.setSpeed(50);
    }
    
    /** 
      * Create a given number of stars in space. 
      */  
     private void createStars(int number)  
     {  
          GreenfootImage background = getBackground();  
          for(int i=0; i < number; i++)  
          {  
               int x = Greenfoot.getRandomNumber( getWidth() );  
               int y = Greenfoot.getRandomNumber( getHeight() );  
               int color = 120 - Greenfoot.getRandomNumber(100);  
               background.setColor(new Color(color,color,color));  
               background.fillOval(x, y, 2, 2);  
          }  
     }
     
     private void showGameOver()
    {
        addObject(new GameOver(), getWidth() / 2, getHeight() / 2);
    }
}

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

/**
 * Write a description of class Player here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Player extends Actor
{
    Board br = new Board();
    /**
     * Act - do whatever the Player wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        hit();
        MouseInfo info = Greenfoot.getMouseInfo();
        if(info!=null){
            int thatX=info.getX();
            int thatY=info.getY() ;
            turnTowards(thatX, thatY); 
        }
        moveOnKeyPress();
        
    }    
    
    public void hit() 
    {   
        Actor Enemy;
        Enemy = getOneIntersectingObject(Enemy.class);
        if (Enemy != null)  
        {  
            br.BarSubtract(150);
            getWorld().removeObject(Enemy);
        }  
    }
    
    public void moveRight()  
    {  
        int x = (getX() + 4);  
        int y = (getY());  
              
        setLocation(x, y);  
    }  
    public void moveLeft()  
    {  
        int x = (getX() - 4);  
        int y = (getY());  
              
        setLocation(x, y);  
    }  
    public void moveDown()  
    {  
        int x = (getX());  
        int y = (getY() + 4);  
            
        setLocation(x, y);  
    }  
    public void moveUp()  
    {  
        int x = (getX());  
        int y = (getY() - 4);  
              
        setLocation(x, y);  
    }
    
    public void moveOnKeyPress()
    {
        if(Greenfoot.isKeyDown("left")) {
            moveLeft();
        }
        if(Greenfoot.isKeyDown("right")) {
            moveRight();
        }
        if(Greenfoot.isKeyDown("up")) {
            moveUp();
        }
        if(Greenfoot.isKeyDown("down")) {
            moveDown();
        }
    }
}