1

🚀

Setup & the Player Ship

New project, then a ship that strafes along the bottom

Active

🎯

Goal for this step

Set up the project and get your ship sliding along the bottom of space.

1New folder SpaceShooter, same Main.java + GamePanel.java template (600×640 window, near-black background).
2Ship variables: shipX centred, width 40, sitting at y = 580.
3Arrow keys strafe left/right, clamped to the screen, you have done this twice now, so it should feel easy!
4Draw the ship as a triangle using fillPolygon, three points: nose and two wing tips.
5Sprinkle some stars: draw 40 small white dots at random-but-fixed positions.

GamePanel.java

    int shipX = 280, shipW = 40;
    boolean leftHeld, rightHeld;

    void update() {
        if (leftHeld)  shipX -= 6;
        if (rightHeld) shipX += 6;
        shipX = Math.max(0, Math.min(600 - shipW, shipX));
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(new Color(77, 217, 224));
        int[] xs = { shipX + shipW/2, shipX, shipX + shipW };
        int[] ys = { 580, 620, 620 };
        g.fillPolygon(xs, ys, 3);
    }

✏️

Fill in the Blanks

+15 XP

fillPolygon draws a shape from arrays of points, our ship triangle uses points. The ship is clamped with Math.max and Math. just like the Breakout paddle.

🧠

Knowledge Check

+15 XP

The xs array starts with shipX + shipW/2. What point of the ship is that?

AThe left wing tip

BThe nose, horizontally centred on the ship

CThe engine

2

🔫

Firing Lasers

An ArrayList of bullets that fly up the screen

Locked

🎯

Goal for this step

Press Space to fire lasers that travel up and disappear off screen.

1Bullets come and go constantly, so an array will not do, use ArrayList<Rectangle> bullets.
2On Space keyPressed: add a new Rectangle at the ship nose (width 4, height 12).
3In update(), move every bullet up 9 pixels per frame.
4Remove bullets that leave the top with removeIf, never let dead objects pile up, or the game slowly chokes!
5Draw them as bright yellow slivers.

GamePanel.java

    java.util.List<Rectangle> bullets = new java.util.ArrayList<>();

    public void keyPressed(KeyEvent e) {
        // ... arrows ...
        if (e.getKeyCode() == KeyEvent.VK_SPACE)
            bullets.add(new Rectangle(shipX + shipW/2 - 2, 578, 4, 12));
    }

    void update() {
        // ... ship movement ...
        for (Rectangle b : bullets) b.y -= 9;
        bullets.removeIf(b -> b.y + b.height < 0);
    }

    // paintComponent:
    //   g.setColor(new Color(250, 204, 21));
    //   for (Rectangle b : bullets) g.fillRect(b.x, b.y, b.width, b.height);

💡

removeIf with a lambda is the tidy modern way to delete from a list while looping, no crashes, no leftover objects eating memory.

⌨️

Code Challenge

+20 XP

Make the lasers fly and clean themselves up:

GamePanel.java

for (Rectangle b : bullets) b.y  9;
bullets.(b -> b.y + b.height < );

💡 Hint: Up the screen means y gets smaller. The cleanup method takes a condition; off-screen means fully above y = 0.

🧠

Knowledge Check

+15 XP

Why must off-screen bullets be removed from the list?

AThey could hit aliens from behind

BOtherwise the list grows forever and the game gradually slows down

CJava limits lists to 100 items

3

👾

The Alien Wave

Spawn a marching grid of invaders

Locked

🎯

Goal for this step

Fill the sky with aliens that march side-to-side and step down like Space Invaders.

1Aliens also live in an ArrayList<Rectangle>, spawn a 6×4 grid of 36×26 aliens in the constructor.
2One shared direction: int alienDir = 1;, every frame each alien moves alienDir * 2 horizontally.
3If ANY alien touches a side wall, flip alienDir and move the whole wave DOWN 14 pixels.
4Do the wall check first with a flag, then apply movement, otherwise half the wave turns and half does not!
5Draw them in classic green.

GamePanel.java

    java.util.List<Rectangle> aliens = new java.util.ArrayList<>();
    int alienDir = 1;

    // constructor:
    for (int r = 0; r < 4; r++)
        for (int c = 0; c < 6; c++)
            aliens.add(new Rectangle(60 + c*80, 60 + r*52, 36, 26));

    void update() {
        boolean hitWall = false;
        for (Rectangle a : aliens)
            if (a.x + alienDir*2 < 0 || a.x + a.width + alienDir*2 > 600)
                hitWall = true;
        if (hitWall) {
            alienDir = -alienDir;
            for (Rectangle a : aliens) a.y += 14;
        }
        for (Rectangle a : aliens) a.x += alienDir * 2;
    }

✏️

Fill in the Blanks

+15 XP

The whole wave shares one direction variable called . When any alien touches a wall we flip it and every alien steps by 14 pixels.

🧠

Knowledge Check

+15 XP

Why check the wall with a flag BEFORE moving the aliens?

AIt is faster

BSo the entire wave turns together on the same frame instead of splitting apart

CJava evaluates loops backwards

4

💥

Blasting Aliens

Bullet-alien collision with double removal and score

Locked

🎯

Goal for this step

Lasers destroy aliens on contact, with score, and both objects removed cleanly.

1Loop bullets with an Iterator, and for each bullet loop the aliens.
2On intersect: remove the alien, remove the bullet, score += 20, and stop checking that bullet (it is gone!).
3Removing from a list you are looping is the classic beginner crash, the Iterator pattern below is the safe way.
4Draw the score top-left, same as always.

GamePanel.java (update)

    int score = 0;

    var bi = bullets.iterator();
    while (bi.hasNext()) {
        Rectangle b = bi.next();
        var ai = aliens.iterator();
        while (ai.hasNext()) {
            Rectangle a = ai.next();
            if (b.intersects(a)) {
                ai.remove();      // alien destroyed
                bi.remove();      // bullet used up
                score += 20;
                break;            // this bullet is gone
            }
        }
    }

⚠️

If you see ConcurrentModificationException, you removed from a list inside a for-each loop. Always use the iterator’s own remove() like above.

⌨️

Code Challenge

+20 XP

Complete the safe double-removal when a laser hits an alien:

GamePanel.java

if (b.intersects(a)) {
    ai.();
    bi.();
    score += 20;
    ;
}

💡 Hint: Both iterators have a removal method, and we must stop checking a bullet that no longer exists.

🧠

Knowledge Check

+15 XP

What does the break after a hit prevent?

AThe alien respawning

BOne bullet destroying several aliens in the same frame after it was already removed

CThe score going negative

5

📈

Waves & Rising Difficulty

New waves spawn faster and march quicker

Locked

🎯

Goal for this step

When a wave is cleared, spawn the next one, faster and worth more.

1Add int wave = 1; and move the grid-spawning code into a method spawnWave().
2In update(): if aliens.isEmpty(), wave++, spawnWave().
3Make speed scale with the wave: aliens move alienDir * (1 + wave) pixels.
4Score per kill can scale too: score += 20 * wave;.
5Show Wave: 3 next to the score, players love seeing how far they got.

GamePanel.java

    int wave = 1;

    void spawnWave() {
        for (int r = 0; r < 4; r++)
            for (int c = 0; c < 6; c++)
                aliens.add(new Rectangle(60 + c*80, 60 + r*52, 36, 26));
    }

    void update() {
        // ... everything so far ...
        if (aliens.isEmpty()) {
            wave++;
            spawnWave();
        }
        for (Rectangle a : aliens) a.x += alienDir * (1 + wave);
    }

💡

Pulling repeated code into a method like spawnWave() is called refactoring, the moment you need something twice, make it a method.

✏️

Fill in the Blanks

+15 XP

When the list is empty we increase and call (). Alien speed scales as alienDir * (1 + ).

🧠

Knowledge Check

+15 XP

Why put the grid-spawn code in its own method?

AMethods run faster than loose code

BWe need it at the start AND after every cleared wave, write once, call twice

CpaintComponent cannot contain loops

6

☠️

Game Over & Polish

Invasion line, game-over screen and final balance

Locked

🎯

Goal for this step

Lose when aliens reach your ship line, show a proper end screen, and tune the fun.

1Add boolean gameOver = false;. If any alien’s bottom passes y = 560, the invasion succeeded: gameOver = true.
2Also end the game if an alien rectangle intersects the ship’s bounding box.
3When gameOver: freeze update() with an early return and draw GAME OVER + score + wave reached.
4Balance pass: bullet speed vs alien speed vs fire rate. Try limiting shots: only fire if bullets.size() < 3, suddenly aiming matters!
5Optional flourish: give aliens a chance to drop bombs, a second ArrayList moving down. You already know every technique needed. That is the point. 🚀

GamePanel.java

    boolean gameOver = false;

    void update() {
        if (gameOver) return;
        // ... everything ...
        for (Rectangle a : aliens)
            if (a.y + a.height > 560) gameOver = true;
    }

    // paintComponent:
    if (gameOver) {
        g.setColor(Color.WHITE);
        g.setFont(new Font("Arial", Font.BOLD, 44));
        g.drawString("GAME OVER", 160, 300);
        g.setFont(new Font("Arial", Font.BOLD, 20));
        g.drawString("Score: " + score + "   Wave: " + wave, 200, 340);
    }

⌨️

Code Challenge

+20 XP

Limit the player to 3 lasers on screen at once:

GamePanel.java

if (e.getKeyCode() == KeyEvent.VK_SPACE
        && bullets.() < )
    bullets.add(new Rectangle(shipX + shipW/2 - 2, 578, 4, 12));

💡 Hint: Lists report how many items they hold with a method; we allow firing only under the limit.

🧠

Knowledge Check

+15 XP

Limiting bullets to 3 on screen makes the game…

AEasier, because bullets are stronger

BMore skilful, you must aim instead of holding down fire

CSlower to render

Defend Earth!Space Shooter

Defend Earth!
Space Shooter