Space invader game

[Tushar Goel]

I am trying to make a space invader game for self practice using TDD approach. I have written some points and did some implementation. Please suggest:

Goal: We should protect invasion from the invaders by shooting them

Some To do list:

1) Should kill the invaders using laser for protecting invasion
2) Should move laser sideways to protect it from the invaders attack
3) Should get new laser if available when destroyed by invaders
4) Should be game over when all the lasers destroyed by invaders
5) Should add 10 points into the total score whenever any invader killed


I worked on points 1 and 3. So my code is:

package test.example.game.space;

import static org.junit.Assert.*;
import main.example.game.space.Game;
import main.example.game.space.Invader;
import main.example.game.space.Laser;

import org.junit.Before;
import org.junit.Test;

public class GameTest {

	private Game game;
	
	@Before
	public void setUp() {
		game = new Game();
	}
	
	@Test
	public void isInvaderKilled_should_return_true_if_invader_killed() {
		Laser laser = game.getLaser();
		Invader invader = new Invader(20, 20, 20);
		add(invader);
		
		laser.shoot(game);
		
		assertTrue(game.isInvaderKilled(invader));
	}

	private void add(Invader invader) {
		game.add(invader);
	}

	@Test
	public void isInvaderKilled_should_return_false_if_invader_not_killed() {
		Laser laser = game.getLaser();
		Invader invader = new Invader(30, 20, 20);
		add(invader);
		
		laser.shoot(game);
		
		assertFalse(game.isInvaderKilled(invader));
	}
	
	@Test
	public void score_should_return_10_when_1_invader_killed() {
		Laser laser = game.getLaser();
		Invader invader = new Invader(20, 20, 20);
		add(invader);
		
		laser.shoot(game);
		
		assertEquals(10, game.score());
	}
	
	@Test
	public void score_should_return_20_when_2_invader_killed() {
		Laser laser = game.getLaser();
		Invader invader1 = new Invader(20, 20, 20);
		Invader invader2 = new Invader(20, 30, 20);
		add(invader1);
		add(invader2);
		
		laser.shoot(game);
		laser.shoot(game);
		
		assertEquals(20, game.score());
	}
	
	@Test
	public void score_should_return_0_when_no_invader_killed() {
		Laser laser = game.getLaser();
		Invader invader1 = new Invader(40, 20, 20);
		Invader invader2 = new Invader(50, 30, 20);
		add(invader1);
		add(invader2);
		
		laser.shoot(game);
		laser.shoot(game);
		
		assertEquals(0, game.score());
	}
	
	@Test
	public void isLaserAvailable_should_return_true_when_available() {
		Laser laser = game.getLaser();
		Invader invader = new Invader(40, 20, 20);
		
		invader.destroyLaser(game);
		
		assertTrue(game.isLaserAvailable());
	}
	
	@Test
	public void isLaserAvailable_should_return_false_when_not_available() {
		Invader invader = new Invader(40, 20, 20);
		Laser laser = game.getLaser();
		invader.destroyLaser(game);
		laser = game.getLaser();
		invader.destroyLaser(game);
		laser = game.getLaser();
		invader.destroyLaser(game);
		
		assertFalse(game.isLaserAvailable());
	}
	
	@Test
	public void isMissileHitInvader_should_return_true_if_hit() {
		Invader invader = new Invader(20, 20, 20);
		Laser laser = game.getLaser();
		
		laser.shoot(game);
		
		assertTrue(game.isShotHitInvader(laser, invader));
	}
	
}

package main.example.game.space;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Game {

	private List<Invader> invaders;
	private List<Invader> killedInvaders;
	private List<Laser>   lasers;
	
	private static int numberOfLasers = 3;
	private int laserIndex = 0;
	private int score;
	
	private static final int LASER_INITIAL_X_POSITION = 20;
	private static final int LASER_INITIAL_Y_POSITION = 20;
	
	private static final int POINT_TO_KILL_INVADER = 10;

	public Game() {
		invaders = new ArrayList<>();
		killedInvaders  = new ArrayList<>();
		lasers = new ArrayList<>();
		numberOfLasers = 3;
		
		for (int i = 0; i < numberOfLasers; i++) {
			lasers.add(new Laser(LASER_INITIAL_X_POSITION, LASER_INITIAL_Y_POSITION));
		}
	}
	
	public Laser getLaser() {
		Laser laser = lasers.get(laserIndex++);
		laser.addGameInstance(this);
		return laser;
	}

	/**
	 * Check if particular invader is killed or not.
	 * 
	 * @param invader
	 * @return
	 */
	public boolean isInvaderKilled(Invader invader) {
		return killedInvaders.contains(invader);
	}

	/**
	 * Score earn after killing invaders.
	 * @return
	 */
	public int score() {
		return score;
	}

	/**
	 * Add invader to the invader list.
	 * @param invader invader to be added
	 */
	public void add(Invader invader) {
		invaders.add(invader);
	}

	/**
	 * Killed the invader if laser shot hit the invader.
	 * @param laser
	 */
	public void killInvader(Laser laser) {
		for (Invader inv : invaders) {
			if (isShotHitInvader(laser, inv)) {
				updateInvaderList(inv);
				addKilledInvader(inv);
				updateScore();
				break;
			}
		}
	}

	public boolean isShotHitInvader(Laser laser, Invader inv) {
		return inv.getXpos() == laser.getXpos();
	}

	/**
	 * Remove the killed invader from the list.
	 * @param inv  killed invader
	 */
	private void updateInvaderList(Invader inv) {
		int index = invaders.indexOf(inv);
		invaders.remove(index);
	}

	private void addKilledInvader(Invader inv) {
		killedInvaders.add(inv);
	}

	private void updateScore() {
		score += POINT_TO_KILL_INVADER;
	}

	public boolean isLaserAvailable() {
		return laserIndex < 3;
	}

	public void destoryLaser() {
		for (Invader inv : invaders) {
			if (isShotHitLaser(inv)) {
				updateLaserList();
				break;
			}
		}
	}

	/**
	 * Destroyed laser has been removed.
	 */
	private void updateLaserList() {
		Laser laser = lasers.get(laserIndex - 1);
		laser = null;
	}

	/**
	 * Laser X coordinates and Invader X coordinates should be matched in
	 * case of invader missile hits laser.
	 * @param inv
	 * @return
	 */
	private boolean isShotHitLaser(Invader inv) {
		return inv.getXpos() == lasers.get(laserIndex - 1).getXpos();
	}

}

[Tushar Goel]

Correction in GameTest class. Method name "isShotHitInvader_should_return_true_if_hit" instead "isMissileHitInvader_should_return_true_if_hit". Now other classes are:

package main.example.game.space;

import java.util.concurrent.atomic.AtomicInteger;

public class Invader implements DestroyLaser {

	private int xpos;
	private int ypos;
	private final int width;
	private final int invaderNumber;
	
	private static AtomicInteger number = new AtomicInteger();
	
	public Invader(int xpos, int ypos, int width) {
	   this.xpos = xpos;
	   this.ypos = ypos;
	   this.width = width;
	   this.invaderNumber = number.incrementAndGet();
	}

   public int getXpos() {
	return xpos;
   }

   public int getYpos() {
	return ypos;
   }

   public int getInvaderNumber() {
	return invaderNumber;
  }

	/* (non-Javadoc)
	 * @see java.lang.Object#hashCode()
	 */
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + invaderNumber;
		return result;
	}

	/* (non-Javadoc)
	 * @see java.lang.Object#equals(java.lang.Object)
	 */
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Invader other = (Invader) obj;
		if (invaderNumber != other.invaderNumber)
			return false;
		return true;
	}

	@Override
	public void destroyLaser(Game game) {
		game.destoryLaser();
	}
	
}

package main.example.game.space;

public class Laser implements DestroyInvader {

	private int xpos;
	private int ypos;
	private int width;
	private Game game;
	
	public Laser(int i, int j) {
		this.xpos = i;
		this.ypos = j;
		width = 20;
	}
	
	public int getXpos() {
		return xpos;
	}
	
	public int getYpos() {
		return ypos;
	}
	
	public void addGameInstance(Game game) {
		this.game = game;
	}

	@Override
	public void shoot(Game game) {
		game.killInvader(this);
	}
}

package main.example.game.space;

public interface DestroyLaser {
   void destroyLaser(Game game);
}

package main.example.game.space;

public interface DestroyInvader {
   void shoot(Game game);
}

[Junilu Lacar]

Some of your code doesn't make sense to me. Why would a laser shoot a game? That's what I read when I see

laser.shoot(game);

[Tushar Goel]

Thanks Junilu, i thought laser doesn't know whether the shot has been hit to invaders or not. It just shoot. Its game responsibility to check if it hits the invaders or not.

What else do you think is wrong?

[Junilu Lacar]

Tushar,

This has the potential to turn into another very long thread, which honestly is a direction I'm not inclined to go right now. Let me just say that it seems like you have gone into the deep end of your detailed requirements without having a clear high level, big picture yet. Things like this need to be refined in small, incremental steps. By starting with a statement like "I want to write a Space Invaders Game in Java" then suddenly going to "Should kill the invaders using laser for protecting invasion" is skipping many of those intermediate refinements. It's like saying "I want to plan a trip to Hawaii" then you suddenly say "I need a bucket to put pretty shells in." A casual observer will likely say, "WTH? How did you get from wanting to plan a trip to Hawaii to collecting shells in a bucket all of a sudden?!"

Perhaps you should choose simpler problems first while you try to build up your TDD skills. A full-blown game simulation seems like overreach right now. You may be familiar with the phrase "Your eyes are bigger than your stomach" or something like that?

[Tushar Goel]

Thanks Junilu, you are right. Right now, i may not have desired skills to create complex programme. I will continue on this when i get desired skills.

I need to focus on small things now. Any suggestion?

[Junilu Lacar]

Don't worry about it, all that will come with practice. There are lots of homework in the Beginner forum that you can try your hand at. It's also interesting to compare what you come up with when you TDD some of these problems vs the solutions that the OPs post. If you do manage to come up with a solution for a homework problem though, remember not to give too much away to the OP so they can still do it themselves.

[Junilu Lacar]

Tushar, check out this thread to get some ideas: https://coderanch.com/t/628047/patterns/Parking-Lot-Design

[Tushar Goel]

Yeah thanks.. It gives me some idea about to clearly representing the todo list and other's things.