Question

I’m having a problem where I receive this error:

Exception in thread “main” java.lang.NullPointerException

at com.noxia.Main.startCombat(Main.java:101)

at com.noxia.Area1.createArea1Enemy(Area1.java:43)

at com.noxia.Main.main(Main.java:30)

I know that I need to initialize the variables because they are null, but I can’t seem to figure out what I need to put where. I’ve minimized the code to show just the relevant parts as there are many other variables and methods left out, but this seems to pertain to the issue. Any help would be greatly appreciated =)

public class Main {
    Player p;
    Enemy e;
    Area1 a1;        

    public static void main(String[] args) {
        Main main = new Main();
        main.a1 = new Area1();            
        main.p = new Player(100);
        //the line directly below this is line 30 where the error occurs
        main.a1.createArea1Enemy(10);
    }

    public void startCombat()
    {
        //the line directly below this is line 101 where the error occurs       
        while (p.getCurrentLife() > 0 & a1.e.getLife() > 0)
        {
            p.playerAttack();
            if (p.getCurrentLife() > 0 & a1.e.getLife() > 0)
            {
                e.enemyAttack();
            }
        }   
    }


public class Player extends Main {
    private int currentLife;

    public int getCurrentLife()
    {
        return currentLife;
    }
    public void setCurrentLife(int cl)
    {
        currentLife = cl;
    }

    public Player(int cl)
    {
        currentLife = cl;
    }


public class Enemy extends Main {
    private int life;

    public int getLife()
    {
        return life;
    }
    public void setLife(int lf)
    {
        life = lf;
    }

    public Enemy (inf lf)
    {
        life = lf;
    }


public class Area1 extends Main {

    public void createArea1Enemy(int enemyCounter)
    {
        while (enemyCounter > 0)
        {
            String[] enemyList = {"Enemy1", "Enemy2"} //code for Enemy2 left out below
            int enemyListLength = enemyList.length; 
            int randomEnemy = (int) (Math.random() * enemyListLength);

            if (enemyList[randomEnemy].equals("Enemy1"))
            {
                Enemy enemy1 = new Enemy("Enemy1", 100);
                //the line directly below this is line 43 where the error occurs
                startCombat();
            }
        enemyCounter--;
        }
    }
}

Answer 1

The simple answer is that you have to set e to an enemy before calling startCombat.

But a better way to do this would be to remove e, and pass the enemy object to startCombat using a method parameter. The e field is conceptually wrong. To understand the wrongness, try to come up with a coherent explanation of what it means in terms of the state of a Main object.

---

Clearly this is beginner code … and there are a number of bad things about what you have written:

• The fields of a class should be for object state, not for passing parameter values to methods.
• You should avoid writing code that accesses the innards of a class … like your main method does.
• Best practice is to make fields private, and defined getter and / or setter methods (as required) for external classes to access / modify them.
• You need to learn how to write constructors with parameters.
• You need to design your code properly. Everything extending Main means that there is going to be no rational model of what the objects “mean”. And there’s the problem that each instance of Enemy and Area1 will have their own copies of the p, e, and a1 fields, and a whole bunch of inappropriate methods.