LibGDX making a rectangle move on its own

Question

    public class gameMain implements Screen {
    final main game;



    SpriteBatch batch;
    Texture img;
    private Texture alexTexture;
    private Rectangle alex;

    private Texture cTex;
    private Texture dropper;
    private Texture ender;
    private Texture partsImg;
    private Texture toy;

    private OrthographicCamera camera;
    private Array<Rectangle> part;       // ******
    private long lastDropTime;
    private int beltSpeed = 100;         // ******

    //Score Keeper
    private int score;
    private String scoreName;



    //basically a create method
    public gameMain(final main gam){
        this.game = gam;

        // load images into memory
        dropper = new Texture("android/assets/dropper.png");
        ender = new Texture("android/assets/endOfBelt.png");
        partsImg = new Texture("android/assets/unmadeToyParts.png");
        toy = new Texture("android/assets/toymade.png");
        cTex = new Texture("android/assets/conveyerBeltBackground.png");
        alexTexture = new Texture(Gdx.files.internal("android/assets/alexAlpha2.png"));

        // set the game window resolution
        camera = new OrthographicCamera();
        camera.setToOrtho(false, 1080,720);

        // Alex's position / hitbox
        alex = new Rectangle();
        alex.x = 250;
        alex.y = 150;
        alex.width = 126;
        alex.height = 75;

        // part stuff
        part = new Array<Rectangle>();
        spawnPart();

        // score handler
        score = 0;
        scoreName = "Toys Made: ";
    }

    private void spawnPart(){             // *******
        Rectangle parts = new Rectangle();
        parts.x = 0;
        parts.y = 210;
        parts.width = 52;
        parts.height = 60;
        part.add(parts);
        lastDropTime = TimeUtils.nanoTime();
    }                                          //********

    @Override
    public void render(float delta) {
        Gdx.gl.glClearColor(0, 0, 0, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        camera.update();

        // load alex sprite
        game.batch.setProjectionMatrix(camera.combined);
        // begin drawing to screen
        game.batch.begin();
        batch.draw(cTex, -5, 200); // draw conveyer background
        batch.draw(dropper, 0, 210); // draw the dropper
        batch.draw(ender, 600, 200); // draw the ender
        game.batch.draw(alexTexture, alex.x, alex.y); // draw alex
        for(Rectangle parts: part){
            game.batch.draw(partsImg, parts.x, parts.y); // draw part
        }
        game.font.draw(game.batch, scoreName, 25, 100); // draw scoreboard
        game.batch.end(); // end drawing to screen

       //******************************************************************************
//HERE IS WHERE I AM TRYING TO PUT IT

part.x += beltspeed;
if (part.rectangle overlaps ender.rectangle){
despawn part;
}

//******************************************************************************



    }

    @Override
    public void show() {

    }

    @Override
    public void hide() {

    }

    @Override
    public void pause() {

    }

    @Override
    public void resume() {

    }

    @Override
    public void resize(int width, int height) {

    }

    @Override
    public void dispose() {

    }
}

Hello I am very new to LibGDX, and after doing some research I decided to make a game where you are a factory worker named Alex and you build toys. I will gladly take any feedback you have to make my code more orginized or work better. I highlighted everything I believe to be relevant to my question with some asterisks.

But my question is: How do you make a 'rectangle' such as my 'part' move on its own. I added a beltspeed and a spawn zone, and once the hitbox for it goes over the 'ender' hitbox I would like to despawn it.

Also I haven't gotten this far yet, but when the 'part' hitbox passes over 'Alex' hitbox I would like a little minigame to appear where you have to press up, down, left, right (randomly) to build the toy. If you have any suggestions on how I should handle that, I would be very appreciative.

"C:\Program Files\Java\jdk1.7.0_79\jre\bin\java" -Didea.launcher.port=7532 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA Community Edition 2016.1.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_79\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_79\jre\lib\rt.jar;C:\Users\RAFiredog\Desktop\Intellij\AlexTheTemp\out\production\desktop;C:\Users\RAFiredog\Desktop\Intellij\AlexTheTemp\out\production\core;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx\1.9.2\e641cb91bec06bc64d9ddf7a7d7062caceec73cd\gdx-1.9.2.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx-box2d\1.9.2\b78eaa90aaaf7830e6dffff587ea6e859c2787b2\gdx-box2d-1.9.2.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx-freetype\1.9.2\3609253d14edb1b3ca5aacff4e06989edde75be4\gdx-freetype-1.9.2.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx-backend-lwjgl\1.9.2\d710a8704ed584ec2ba9ca52e3a6a5885dd759cc\gdx-backend-lwjgl-1.9.2.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx-platform\1.9.2\dde4cf7e9ce61c24042f512203aed87657496639\gdx-platform-1.9.2-natives-desktop.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx-box2d-platform\1.9.2\497cbfbf81e7e8b1e8433b19291c5af4bbc2ec66\gdx-box2d-platform-1.9.2-natives-desktop.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx-tools\1.9.2\37d8e3009ae0febdf93717b21980c67c8cadac07\gdx-tools-1.9.2.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx-freetype-platform\1.9.2\1e1afb0b6c9e8aeb7d495cb8f55c353d3da58cf0\gdx-freetype-platform-1.9.2-natives-desktop.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\org.lwjgl.lwjgl\lwjgl\2.9.2\a9d80fe5935c7a9149f6584d9777cfd471f65489\lwjgl-2.9.2.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\org.lwjgl.lwjgl\lwjgl_util\2.9.2\4b9e37300a87799856e0bd15ed81663cdb6b0947\lwjgl_util-2.9.2.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.jlayer\jlayer\1.0.1-gdx\7cca83cec5c1b2f011362f4d85aabd71a73b049d\jlayer-1.0.1-gdx.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\org.jcraft\jorbis\0.0.17\8872d22b293e8f5d7d56ff92be966e6dc28ebdc6\jorbis-0.0.17.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\com.badlogicgames.gdx\gdx-backend-headless\1.9.2\813e6020de85cd831f02cb5e9060fd7ea8cb208e\gdx-backend-headless-1.9.2.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\org.lwjgl.lwjgl\lwjgl-platform\2.9.2\510c7d317f5e9e700b9cfaac5fd38bdebf0702e0\lwjgl-platform-2.9.2-natives-windows.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\org.lwjgl.lwjgl\lwjgl-platform\2.9.2\d276cdf61fe2b516c7b7f4aa1b8dea91dbdc8d56\lwjgl-platform-2.9.2-natives-linux.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\org.lwjgl.lwjgl\lwjgl-platform\2.9.2\d55b46b40b40249d627a83a7f7f22649709d70c3\lwjgl-platform-2.9.2-natives-osx.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\net.java.jinput\jinput\2.0.5\39c7796b469a600f72380316f6b1f11db6c2c7c4\jinput-2.0.5.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\net.java.jutils\jutils\1.0.0\e12fe1fda814bd348c1579329c86943d2cd3c6a6\jutils-1.0.0.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\net.java.jinput\jinput-platform\2.0.5\7ff832a6eb9ab6a767f1ade2b548092d0fa64795\jinput-platform-2.0.5-natives-linux.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\net.java.jinput\jinput-platform\2.0.5\385ee093e01f587f30ee1c8a2ee7d408fd732e16\jinput-platform-2.0.5-natives-windows.jar;C:\Users\RAFiredog\.gradle\caches\modules-2\files-2.1\net.java.jinput\jinput-platform\2.0.5\53f9c919f34d2ca9de8c51fc4e1e8282029a9232\jinput-platform-2.0.5-natives-osx.jar;C:\Program Files (x86)\JetBrains\IntelliJ IDEA Community Edition 2016.1.1\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.rafiredog.game.desktop.DesktopLauncher
Exception in thread "LWJGL Application" java.lang.NullPointerException
    at com.rafiredog.game.gameMain.render(gameMain.java:97)
    at com.badlogic.gdx.Game.render(Game.java:46)
    at com.rafiredog.game.main.render(main.java:45)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:223)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:124)

Process finished with exit code 0

Lastly, I am now getting this error. I moved some code around near the 'batch.draw'. However that might not be the cause of the error? I am a little lost right now. But I will gladly screenshot what I have of the game now once the errors are resolved.

Thank you for any insight you have on fixing up this game.

Answer 1

Piggy backing onto Tenfour04's answer and attempting to answer your question in the comments. I'd suggest looking at this link: https://github.com/libgdx/libgdx/wiki/The-life-cycle

Answer 2

You need to do game updates and then rendering on each cycle of the game loop. The game update part of your code can iterate through your objects to do stuff to them.

For example, your render method would look like this. You need to explicitly use an Iterator for your for loop because you otherwise could not remove items from the list while iterating.

@Override
public void render(float delta) {

    //Update game
    Iterator<Rectangle> iterator = parts.iterator();
    while (iterator.hasNext()){
        Rectangle part = iterator.next();
        part.x += beltSpeed * delta; //distance = speed * time
        if (part.rectangle.overlaps(ender.rectangle)){
            iterator.remove(); //removes rectangle from the list
        }
    }
    //This is also where you would update movement for anything else,
    //such as a character jumping, or counting down a timer and spawning
    //something when it reaches zero, for example.

    //Draw everything
    Gdx.gl.glClearColor(0, 0, 0, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
    camera.update();
    game.batch.setProjectionMatrix(camera.combined);
    game.batch.begin();
    //...
    game.batch.end();
}

If you spawn and "despawn" a lot of objects, you will probably need to use pooling to avoid stutters from memory deallocation. You can do this by replacing new Rectangle() with Pools.obtain(Rectangle.class) and adding the line Pools.free(part) right after iterator.remove().

I also noticed you have counter-intuitive usage of the words "part" and "parts" by swapping plural and singular. Make sure that doesn't trip you up.

As for your NullPointerException, read here.