Java arrays - Keep getting NullPointerExcpetion

Question

So I'm trying to make a game where I spawn 20 enemies. The enemies are supposed to be loaded into an array of enemies, and then the game is meant to draw them out in a for loop, the only issue is that I keep getting a NullPointerException when I try to run the for loop.

The line in the code below:

enemy[i] = new Enemy(enemyX, enemyY, enemySize, vx, vy);

is the one that's throwing out the exception. I'm listing the full source below as well so you can see what I'm trying to do, I'm just stuck and I need some help on this, is there any other way to create the enemies and place them on the screen?

Main Class:

//define package
package auctus;

//imports
import org.newdawn.slick.AppGameContainer;
import org.newdawn.slick.BasicGame;
import org.newdawn.slick.Color;
import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Graphics;
import org.newdawn.slick.SlickException;

//create class
public class Auctus extends BasicGame{
    //Window dimensions and title
    public static final int WIDTH = 800;
    public static final int HEIGHT = 600;
    public static final String TITLE = "Auctus 0.1";

    //Defining the player variables, obviously only one player to create so no need for
    //automation in this area
    public float playerX, playerY, playerSize;
    //Creating the enemy array
    public Enemy enemy[];
    //declaring a global variable for the number of enemies
    public int numberOfEnemies;

    //slick2D API stuff
    public Auctus(String title) {       
            super(TITLE);       
    }


    //initialisation method
    public void init(GameContainer gc) throws SlickException {          
        //setting the number of enemies
        numberOfEnemies = 20; 

        //setting the player variables dynamically each time the game is started
        playerX = (float)Math.random() * WIDTH;
        playerY = (float)Math.random() * HEIGHT;
        playerSize = (float)Math.random() * 50;

        //creating the enemies
        for(int i = 0; i < numberOfEnemies; i++){           
            float enemyX = (float)Math.random() * WIDTH;
            float enemyY = (float)Math.random() * HEIGHT;
            float enemySize = (float)Math.random() * 50;
            float vx = (float)Math.random() * 2;
            float vy = (float)Math.random() * 2;

            //this is the line that's giving me trouble
            enemy[i] = new Enemy(enemyX, enemyY, enemySize, vx, vy);        
        }       
    }

    //from here it doesn't matter, this is error free both syntactically and logically
    public void render(GameContainer gc, Graphics g) throws SlickException    {     
            drawPlayer(g, playerX, playerY, playerSize);
            for(int i = 0; i < numberOfEnemies; i++){       
                enemy[i].drawEnemies(g);        
            }               
        }

    public void update(GameContainer gc, int delta) throws SlickException {     
        updatePlayer(gc, delta, playerX, playerY, playerSize);
        for(int i = 0;i < numberOfEnemies; i++){            
            enemy[i].updateEnemies(delta);      
        }       
    }

    public void drawPlayer(Graphics g, float playerX, float playerY, float playerSize){                 
    }

    public void updatePlayer(GameContainer gc, int delta, float playerX, float playerY, float playerSize){
    }

    public void updateEnemies(GameContainer gc, int delta, float enemyX, float enemyY, float enemySize, float vx, float vy){        
        for(int i = 0; i < numberOfEnemies; i++){           
            enemyX += vx;
            enemyY += vy;
        }       
    }

    public static void main(String[] args){     
        try{    
            AppGameContainer app = new AppGameContainer(new Auctus(TITLE));
            app.start();            
        } catch(SlickException e){          
            e.printStackTrace();            
    }
  }     
} 

Enemy class:

package auctus;

//imports
import org.newdawn.slick.Color;
import org.newdawn.slick.Graphics;

//class declaration
public class Enemy {
    //declaring variables
    public float enemyX, enemyY, enemySize, vx, vy;

    public Enemy(float x, float y, float size, float dx, float dy){     
        enemyX = x;
        enemyY = y;
        enemySize = size;    
        vx = dx;
        vy = dy;        
    }

    public float enemyX(){      
        return enemyY;      
    }

    public float enemyY(){
        return enemyY;      
    }

    public float enemySize(){       
        return enemySize;       
    }

    public float vx(){      
        return vx;      
    }

    public float vy(){      
        return vy;      
    }

    public void setX(float x){      
        enemyX = x;     
    }

    public void setY(float y){      
        enemyY = y;     
    }

    public void setSize(float size){    
        enemySize = size;       
    }

    public void setVX(float dx){        
        vx = dx;        
    }

    public void setVY(float dy){        
        vy = dy;        
    }

    public void drawEnemies(Graphics g){        
        g.setColor(Color.red);
        g.fillRect(enemyX, enemyY, enemySize, enemySize);       
    }

    public void updateEnemies(int delta){       
        if(enemyX < 800 || enemyX > 0){         
            enemyX += vx * delta;           
        }

        if(enemyX > 800 || enemyX < 0){         
            vx = -vx;           
        }

        if(enemyY < 600 || enemyY > 0){         
            enemyY += vy * delta;           
        }

        if(enemyY > 600 || enemyY < 0){         
            vy = - vy;          
        }       
    }   
}

The enemy class is pretty straight forward. I can't see why it would make that into a NullPointerException, but maybe someone else can spot it.

What am I doing wrong?

Answer 1

You never instantiate your array enemy.

At some point, before you use enemy, you'll need something like: enemy = new Enemy[numEnemies];. You can do it in the declaration:

// use a constant for the number of enemies:
public static final int NUM_ENEMIES = 20; 
Enemy enemy[] = new Enemy[NUM_ENEMIES];

Otherwise, since enemy is null - because you haven't instantiated it - you'll get a NullPointerException when you try to index it.