Help with KeyDown (space invaders game)

Question

I'm making a space invaders type game for school using Greenfoot (new Java IDE). Whenever I press space, a bullet will fire. However, multiple bullets are being created when I press space once, and when I hold space, a long stream of bullets are being made. I want one bullet to be created when I press space. Tried using KeyPress, but apparently Greenfoot does not recognize java.awt? Thanks in advance.
public class Ship extends Vehicle {
     * Constructor for objects of class Shuttle
    public Bullet B;
    boolean canFire = false;
    public Ship() {
        GreenfootImage ship = new GreenfootImage("spaceship.gif");
        this.setImage(ship);       
    //Create a bullet
    public void fire(){
        B = new Bullet();
        getWorld().addObject(B,getX(),getY()-60);
    public boolean collision(){
        Actor enemy = getOneIntersectingObject(null);
        return (enemy!=null) ? true : false;
    public void act(){
        // move with keys
        if (this.canMove())
             move();      
        if(Greenfoot.isKeyDown("space")){               
                fire();               
        else if(collision()){
            getWorld().removeObject(this);                           
}

Answer 1

Make a Class Variable to use as a flag indicating that a key was pressed... check to see if the flag is set, and if it is not, then fire a bullet and then set the flag. Reset the flag when they keyReleased action fires.
This way only one bullet can be fired for each time a key is pressed, provided your act() is being polled, but if a keyPressed fires act(), but not a keyRelease too, then it will not work. If it does not work, then you'll need to go to a regular KeyListener.
//Your code make these changes
public class Ship extends Vehicle {
      * Constructor for objects of class Shuttle
     public Bullet B;
     boolean canFire = false;
     public Ship() {
         GreenfootImage ship = new GreenfootImage("spaceship.gif");
         this.setImage(ship);       
     //Create a bullet
     public void fire(){
         B = new Bullet();
         getWorld().addObject(B,getX(),getY()-60);
     public boolean collision(){
         Actor enemy = getOneIntersectingObject(null);
         return (enemy!=null) ? true : false;
     public void act(){
         // move with keys
         if (this.canMove())
              move();      
        if(!GreenFoot.isKeyDown("space")) canFire = true; //add this
         if(Greenfoot.isKeyDown("space") and canFire){               
                 fire();              
                 canFire = false; //add this
         else if(collision()){
             getWorld().removeObject(this);                           
}