Java - array of objects not storing values (scope?)

Question

I'm trying to get an array of objects to render into a gamescreen. I've looked around and found some similar questions asked by others, but I can't seem to apply the answers they've gotten to my program.

The problem seems to occur in the nested for-loops. In trying to solve this issue, I've gotten Java NullPointerExceptions at the first line within those for loops.

package com.frfanizz.agility;

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Input.Keys;
import com.badlogic.gdx.Screen;
import com.badlogic.gdx.graphics.GL30;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;

public class GameScreen implements Screen {

AgilityGame game;
OrthographicCamera camera;

SpriteBatch batch;
Hero hero;
Spark[] sparkArray;
int totalSparks;

public GameScreen(AgilityGame game) {
    this.game = game;
    camera = new OrthographicCamera();
    camera.setToOrtho(true, 1920, 1080);

    batch = new SpriteBatch();

    hero = new Hero(60,540);

    //Variables to set spark array
    int screenX = 1400;
    int screenY = 1200;
    int numOfRow = 11;
    int numOfCol = 8;
    float spacingRow = screenY/(numOfRow + 1);
    float spacingCol = screenX/(numOfCol + 1);
    int totalSparks = numOfRow*numOfCol;
    //Spark array
    Spark[] sparkArray = new Spark[totalSparks];
    //setting bounds for sparks
    int index = 0;
    for (int i=0;i<numOfCol;i++) {
        for (int j=0; j<numOfRow;j++) {
            //sparkArray[index] = new Spark();
            sparkArray[index].bounds.x = (float) (60 + spacingCol + ((i)*spacingCol));
            sparkArray[index].bounds.y = (float) (spacingRow + ((j-0.5)*spacingRow));
            index++;
        }
    }

}

@Override
public void render(float delta) {

    Gdx.gl.glClearColor(1F, 1F, 1F, 1F);
    Gdx.gl.glClear(GL30.GL_COLOR_BUFFER_BIT);
    camera.update();
    generalUpdate();

    batch.setProjectionMatrix(camera.combined);

    batch.begin();

        //Rendering code
        batch.draw(Assets.spriteBackground, 0, 0);
        batch.draw(hero.image,hero.bounds.x,hero.bounds.y);

        for (int i=0; i<totalSparks; i++) {
            batch.draw(sparkArray[i].image,sparkArray[i].bounds.x,sparkArray[i].bounds.y);
        }

    batch.end();

}

//Other gamescreen methods

}

The classes Hero and Spark are as follows:

package com.frfanizz.agility;

import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.math.Rectangle;

public class Hero {

public Sprite image;
public Rectangle bounds;

public Hero(int spawnX, int spawnY) {
    image = Assets.spriteHero;
    image.flip(false, true);
    bounds = new Rectangle(spawnX - 16, spawnY - 16, 32, 32);
}
}

and:

package com.frfanizz.agility;

import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.math.Rectangle;

public class Spark {

public Sprite image;
public Rectangle bounds;

public Spark () {
    image = Assets.spriteSpark;
    image.flip(false, true);
    bounds = new Rectangle();
    bounds.height = 32;
    bounds.width = 32;
}
}

Within the for loops, I've printed the values of sparkArray, so I think I have an issue with values rather than references from the other questions I've read the answers to. (Here are the questions I (unsuccessfully) referenced: Java. Array of objects , Java NullPointerException with objects in array)

Thanks in advance!

Answer 1

Uncomment the line

//sparkArray[index] = new Spark();

Your array of Spark objects contains null values until you put instances of Spark into it.

Consider using a two-dimensional array of Spark, if it's appropriate to what you're doing.

int numOfRow = 11;
int numOfCol = 8;
Spark[][] sparkArray = new Spark[numOfRow][numOfCol];
//setting bounds for sparks
for (int row=0; row<numOfRow; row++) {
    for (int col=0; col<numOfCol; col++) {
        sparkArray[row][col] = new Spark();
        sparkArray[row][col].bounds.x = 1.23; // example of usage
    }
}