startPositionX += DELTA_X;
this.alien2 = new Alien(new AlienType(2), startPositionX, POSITION_Y);
startPositionX += DELTA_X;
this.alien3 = new Alien(new AlienType(3), startPositionX, POSITION_Y);
startPositionX += DELTA_X;
this.alien4 = new AlienMotherShip();
this.alien4.moveTo(startPositionX, POSITION_Y);
startPositionX += DELTA_X;
this.alienShot = new AlienShot(startPositionX, POSITION_Y);
startPositionX += DELTA_X;
this.wall = new Wall(startPositionX);