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Space Invaders (Part 15/34)

Games, Level 0, Lesson 6

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if i write only :

enemyFleet.fire(this);

in the onTurn method, the penultimate task accept. if i write :

if(enemyFleet.fire(this) != null) {

            enemyBullets.add(enemyFleet.fire(this));
            enemyFleet.fire(this);
        }

the last task accept. but when i write them together the penultimate task went wrong.

We'll store all the enemy projectiles in an enemyBullets field in the SpaceInvadersGame class. We'll add code to draw and move them in appropriate methods.
To remove projectiles that are no longer relevant, we'll create and implement a removeDeadBullets() method in the SpaceInvadersGame class.
To remove the projectiles, you can either use an iterator, or create a copy of the enemyBullets list and run through it in a loop.
If we determine that a projectile needs to be removed, we'll call enemyBullets.remove(bullet).
Projectiles are irrelevant if they have flown off the screen or if they have hit their target (isAlive == false).
Let's add a check() helper function. At this stage, it will call the removeDeadBullets() method. Later, it will perform various checks at each step of the game.
We'll call the fire() method on the fleet with the same frequency, potentially causing an enemy ship to fire.
If a shot is fired, the method returns a projectile, which must be added to the list.

Requirements:

The SpaceInvadersGame class must have a private List<Bullet> enemyBullets field.
In the createGame() method, the enemyBullets field must be initialized with a new ArrayList<Bullet> object before the call to the drawScene() method.
In the drawScene() method, the draw(Game) method must be called on each object in the enemyBullets list after the call to the drawField() method. Pass "this" to the method.
The moveSpaceObjects() method must call the move() method on each object in the enemyBullets list.
The SpaceInvadersGame class must have a private void removeDeadBullets() method.
In the removeDeadBullets() method, you need to remove all "dead" projectiles and projectiles that have flown off the screen (y coordinate is greater than or equal to HEIGHT - 1) from the enemyBullets list.
The SpaceInvadersGame class must have a private void check() method.
In the check() method, you need to call the removeDeadBullets() method.
In the SpaceInvadersGame class's onTurn(int) method, you need to call the check() method before the call to the drawScene() method.
In the onTurn(int) method, you need to call the fire(Game) method with this as an argument on the enemyFleet object before the call to the drawScene() method.
If the value returned by calling the fire(Game) method on the enemyFleet object is not null, you need to add the return value to the enemyBullets list. Add it before the call to the drawScene() method.

package com.codegym.games.spaceinvaders; public class ShapeMatrix { public static final int[][] ENEMY = new int[][]{ {7, 0, 7}, {7, 7, 7}, {0, 7, 0} }; public static final int[][] BULLET = new int[][]{ {2}, {2} }; }

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Bohdan

Level 8 , Milan, Italy

8 September 2020, 13:44

Гарна собака Tiym))

Bohdan

Level 8 , Milan, Italy

8 September 2020, 13:42

i just did it before you wrote message Tiym Divertitto))

Artem Divertitto Senior Android Developer at United Tech

8 September 2020, 13:40

solution

1. The penultimate requirement failed because you call method fire more than twice in this code

if(enemyFleet.fire(this) != null) {

            enemyBullets.add(enemyFleet.fire(this));
            enemyFleet.fire(this);
        }

2. You should put result of the method fire() inside variable, and if it not null, put at the list enemyBullets

Guadalupe Gagnon

Level 37 , Tampa, United States

8 September 2020, 13:10

solution

Each time you call .fire() a bullet is created. You should be storing this bullet temporarily then checking if it is null. If it is not null then add it to the list. This bit of code will fire 1 shot and if that first is not null, it will fire two additional shots. The second shot will be saved to the list, null or otherwise, and the third does nothing since the returned object is not used:

if(enemyFleet.fire(this) != null) { // first shot checked for null
   enemyBullets.add(enemyFleet.fire(this)); // second shot saved, could be null
   enemyFleet.fire(this); // this line does nothing
}