non-static variable arr cannot be referenced from a static context

Question

I've read the Oracle documents regarding scope and controlling access yet it just isn't sticking, so I'm assuming that my issue comes from my failure to understand... Anyways Here's my code. I'm trying to access the unique Player objects created in an array, and change their unique variables like their balances, using methods from the Player class. Any solutions and ESPECIALLY explanations are welcome!

public class Player
{
private int currentBal;
private String myName;
private int rollOne;
private int rollTwo;
private int rollTotal;
private int doublesCount;
private int currentPosition;
private int currentDoubles;
private int move;
private int moveMult;
private int newBal;
private boolean rollAgain;
private boolean inJail;
public Player(String userName, int changeInMoney)
{
    myName = userName;
    currentBal -= changeBalance(changeInMoney);
}

public int changeBalance(int changeInMoney){newBal -= changeInMoney; return newBal;}

public int viewBalance(){return currentBal;}

Here is my PlayerArray class.

public class PlayerArray
{
Scanner scan = new Scanner(System.in);
private int numbHuman;
private Player[] arr;
private String[] userName;
private int startingMoney;
public PlayerArray()
{
    Scanner scan = new Scanner(System.in);
    System.out.println("There will be 4 players, how many do you wish to be human? 0><4");
    numbHuman = scan.nextInt();
    while (numbHuman < 1 || numbHuman > 4)
    {
        System.out.println("Invalid entry, try again.");
        numbHuman = scan.nextInt();
    }       
    arr = new Player[numbHuman];
    userName = new String[numbHuman];
    startingMoney = 1500;
    for(int i = 0; i < arr.length; i++)
    {
        System.out.println("Player " + (i + 1) + ", Please enter your first name:");
        userName[i] = scan.next();
        arr[i] = new Player(userName[i],startingMoney);
    }
}

public Player[] getPlayerArray()
{   
    int charge = 500;
    arr[0].changeBalance(charge);
    System.out.println(arr[0].viewBalance()); //look here as example
    return arr;
}
}

this is my player class, minus some methods I can't use till later. Bellow is my main method to call it,

import java.util.Scanner;
import java.util.Random;
public class Launcher
{
private Planet myTest;
private PlanetInfo myPlanetInfo;
private static Player[] arr;
public static void main(String[] args)
{
    Launcher testLauncher = new Launcher();
    PlayerArray myArray = new PlayerArray();
    Pay myCharge = new Pay(); // continue work on charges 
    myArray.getPlayerArray();
    //STILL TRYING TO GET BELLOW LINE TO WORK LAST NIGHT!!!
    int testBal =  arr[0].viewBalance(); //ERROR HERE
    System.out.println("player 1's balance: " + testBal);
}
}

Error "Java.lang.NullPointerException: null"

Answer 1

Your main method is a static method. It actually exists before any object is created from your class, and thus cannot access instance variables and methods directly. You cannot access non static methods or variables from the same class unless you create an object for the class, i.e Launcher launcher = new Launcher();.

In this case, your player array arr is not static. You either need to make this static or create a Launcher object and access the variable from there. In the latter case, you will need to make the arr array public.

The first option requires you to change your player array declaration to private static Player arr;.

The second requires you to change the access of the arr array to public and access it like so: launcher.arr.

Regarding your second error, you need to either do this: arr = myArray.getPlayerArray(); or just access the array directly like this: myArray.getPlayerArray()[0] (for the first item in that array).

Answer 2

Make your variables static, since main is a static method, you can't reference non-static methods.

so make your variables on top something like:

private static Player[] arr;

Answer 3

Variable 'Player[] arr' is instance variable, i.e. it should belongs to particular instance. Static method 'main' can't directly access an instance variable from the class it's in, because it doesn't have an explicit reference to any particular instance of the class (simply said at this point, you don't have an instance to whom your Player[] arr belongs).

Answer 4

Since your Launcher has no instance none of its non static members exist. Static method main cannot access such - in this case your arr does not exist. You may want to make it static to make it work.

Read static and instance members