Space Invaders Need Help!

New to Greenfoot and need some help. This is most likely a breeze for some, but I am trying to make this a completed program and can use any help possible.

Requirements:
Movement:
1. All the alien ships exhibit the same movement pattern, moving in one of three possible directions: left, right or down.
2. The aliens move as a block. The block always stays synchronized, i.e. the gaps between each alien and its neighbors remains fixed at all times.
3. The block moves in one horizontal direction, say left, till the left most alien hits the end of the screen. At this point, the whole block moves down one step and then continues to move horizontally to the right. This behavior repeats till the aliens hit the bottom of the screen or get killed. Note that as you kill off aliens on the sides of the block, the block gains more mobility since the aliens can now move farther to the left and right.
4. The aliens move in a stop and go pattern, staying stationary for some time and then moving to their next location in an instant.

Invaders:
The alien invaders come in three varieties, green, blue and pink:
Pink requires 1 hit and awards 10 points;
Blue 2 hits-20 points
Green 3 hits- 30 points
They also fire independently with a 1% probability each time act is called.

Mothership: lose a life each time hit, however you have 3 lives. Respawn at center everytime hit.

Bullets: activated by up arrow on keyboard, and gun requires small cooldown before next bullet. Holding up will provide steady stream of bullets.

Ending: after all invaders are destroyed, or you lose all 3 lives, game over is displayed.

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Tags: game demo with-source