Alien Attack

Download and Open Scenario

  1. Click on the following link to download the scenario: AlienAttack

  2. Select Open.

  3. Copy the folder AlienAttack and paste it in your myGreenfoot folder.

  4. Launch Greenfoot and select Open from the Scenario menu.

  5. Navigate to your MyGreenfoot folder, select the AlienAttack scenario, and click Open.


In this lesson we use the shooting techniques learned in the previous lesson to create a shooting game called AlienAttack. The game is played by shooting a phaser cannon at aliens that try to drop bombs on your space station (fort). The fort has a force field that can be activated a maximum of 3 times. The force field can prevent bombs from landing on the fort. If the fort is hit by a bomb 3 times the game is over.

Cannon Class

The Cannon class contains code that implements a phaser cannon. When the cannon is shot a beam of light is created and projected in the direction the cannon is facing. The following pseudocode can be used to implement the method shootCannon.

Create a Phaser object passing it a reference to the cannon
Set direction of phaser to the same direction as cannon
Add phaser to world at cannon's location
Move phaser location to end of cannon barrel
Play phaser sound

Implement the method shootCannon. The method should look like the following when completed.

There is an interesting line of code in the shootCannon method. When the method creates a Phaser object it passes the phaser a reference to the cannon by using the keyword this.

Why does the phaser to need a reference to the cannon? Well it has to do with the way the scoring for the game works. When a Phaser object hits an Alien object the score is increased by one. Which class, Cannon or Phaser, should keep track of the score? It seems logical that since the Phaser class is checking for a collision between itself and an alien that it should be the one responsible for storing the score. However, there is are two problems with this scenario. First, every time the cannon fires a new Phaser object is created. This means that each Phaser object would have it's own score variable. In order to correctly calculate a score the game would have to tally up all the score variables in each of the Phaser objects, which although possible, would be very difficult to do. Secondly, Phaser objects only exist for a short while. When they hit an alien or go off the screen they are removed from the world. This presents a whole other set of problems. Therefore the Phaser class is not the best place to keep track of the score. Since the game only has one Cannon object and this object exists for the entirety of the game it is the best place to store the score.

This brings us back to the question: Why does the phaser need a reference to the cannon? We just concluded from our discussion above that the score should be stored in the Cannon class, however there is another problem. Which class tests to see if a phaser has collided with an alien? Yes, the Phaser class. So the Phaser class needs some way of notifying the Cannon class when a collision occurs so that it can update the score. The purpose of the reference to the cannon in the Phaser class is to establish a communication channel between the phaser and the cannon.

This communication channel is established in three steps:

  1. A Cannon variable is declared in the Phaser class.

  2. A parameter of type Cannon is added to the Phaser constructor so that the Cannon variable can be assigned the reference to the Cannon object.

  3. Using the Cannon variable a call is made to the Cannon method updateScore. This is how the Phaser class communicates with the Cannon class.

Here is the 3 steps highlighted in the Phaser class.

Alien Class

In the game, aliens move across the world randomly from either left to right or right to left. They are designed to drop a bomb every 3 seconds. The code below is part of the Alien class's act method. This code creates a timer so that the dropBomb method is called every 3 seconds.

The method System.currentTimeMillis() retrieves the current time on the computer's clock in milliseconds.

Implement the Alien class's dropBomb method using the following pseudocode:

Create a Bomb object
Add the bomb to world at this alien's current location
Set rotation of bomb to 90 (90 degrees is South)
Move bomb's location to bottom of alien (use 20)

To make the game more challenging instead of having the bombs drop straight down let's have them also move horizontally as they drop. To do this modify the dropBomb method by replacing the code that creates a bomb with the following:

Bomb bomb = new Bomb(speed);

This code passes the alien's speed to the bomb so that the bomb can move horizontally at the same speed as the alien.