Single shot

 if (Greenfoot.isKeyDown("space") && !isShoot) {
            World world = getWorld();
            world.addObject(new Speer(), getX(), getY());
isShoot = true;
        }
else 
isShoot = false;

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
/**
 * A Held swims around in the sea and eats fleas. But be careful for sharks.
 * 
 */
public class Held extends Personages
{
    public boolean isShoot;    
    /**
     * Do what a fish has to do.
     */

 

  public void act() {
checkToetsen();
checkVijhand();
}

public void checkToetsen() { 
        if (Greenfoot.isKeyDown("W")) { 
            moveUp(); 
        } 
         if (Greenfoot.isKeyDown("A")) { 
            moveLeft(); 
        } 
         if (Greenfoot.isKeyDown("S")) { 
            moveDown(); 
        } 
         if (Greenfoot.isKeyDown("D")) { 
            moveRight(); 
        } 
        
        if (Greenfoot.isKeyDown("space") && !isShoot) {
            World world = getWorld();
            world.addObject(new Speer(), getX(), getY());
        isShoot = true;
        }
        else 
        isShoot = false;      
   
}
public void checkVijhand()
{
    if(isTouching(Tijger.class))
    {
        Level1 wereld = (Level1)getWorld();
        Healthbar healthbar = wereld.haalHealthbarOp();
        healthbar.healthbarDaalt();
    }
    
}
}

// a field like
private boolean spaceDown;

// with this in act
if (spaceDown != Greenfoot.isKeyDown("space"))
{
    spaceDown = ! spaceDown;
    if (spaceDown) getWorld().addObject(new Speer(), getX(), getY());
}