Exception in thread "main" java.lang.NullPointerException when trying to use a method in another class?

Question

I am new to Java and OOP. I'm having trouble understanding this error that I'm getting:

Exception in thread "main" java.lang.NullPointerException
    at main.MyBlock.displayBlock(ProgFunAssignment1.java:66)
    at main.ProgFunAssignment1.main(ProgFunAssignment1.java:38)

It also happens with the clearBlock() method as well. I'm assuming it might be to do with me not understanding how to use the methods I've defined in the myBlock class properly in main? or maybe that I haven't initialized block properly?

I've looked through some other answers about the same error but can't really seem to find anything that makes enough sense for me to get started on. I've pasted my code below, please help me! Explanations on what this error actually means would be great too, I'd like to actually understand what I've done wrong.

Please ask me for any extra information you might need, sometimes I'm not great at including everything!

package main;
import java.util.Scanner;
import java.lang.*;

public class ProgFunAssignment1 {

    public static void main(String[] args) {
         Scanner console = new Scanner(System.in);
         int maxRows;
         int maxColumns;

         // initial value selection (TASK D)     
         do { System.out.println("Please enter the desired row and column lengths of your block");
         maxRows = console.nextInt();
         maxColumns = console.nextInt();
         if ((maxRows < 3 || maxRows > 10) || (maxColumns < 3 || maxColumns > 10)) {
             System.out.println("Values invalid, please enter values between 3 and 10");}
         } while ((maxRows < 3 || maxRows > 10) || (maxColumns < 3 || maxColumns > 10));

         MyBlock block = new MyBlock(maxRows, maxColumns);


         //Menu creation (TASK E)
         int choice;
         int colPosition;
         int rowPosition;

        do { System.out.println("Please select one of the following options \n 1. Add a House \n 2. Display Block \n 3. Clear Block \n 4. Quit");
         choice = console.nextInt();

         if (choice == 1) {
             System.out.println("Please enter position coordinates of the house:");
             rowPosition = console.nextInt();
             colPosition = console.nextInt();
             // If coordinates not valid, go back to menu
         }
         else if (choice == 2) {
             block.displayBlock();
         }
         else if (choice == 3) {
             block.clearBlock();
         }
         else if (choice == 4) {
             System.exit(0);
         }
         else {
             System.out.println("Choice was not valid, please try again.");
             }} while (choice < 0 || choice > 4);    
    }
}

class MyBlock {
    private int[][] block;
    boolean vacant;


    public MyBlock(int maxRows, int maxColumns) {

        int block[][] = new int[maxRows][maxColumns];
        vacant = true;

    }

    public void displayBlock() {
        // content of displayBlock() method here
        for (int[] x : block)
        {
           for (int y : x)
           {
                System.out.print(y + " ");
           }
           System.out.println();
        }

        }

    public void clearBlock() {
        // content of clearBlock() method here
        vacant = true;
        for (int i = 0; i < block.length; i++) {
            for (int j = 0; j< block[i].length; j ++)
                block[i][j] = 0;
        }
    }

    public boolean buildHouse(int rowPos, int colPos, int rows, int columns) {
        // content of buildHouse() method here

        // useful code
        if (true) 
            return true;
        else 
            return false;
}
}

// useful code if (true) return true; else return false; -> this is far from useful code. — Stultuske, Apr 16 '20 at 08:52
`int block[][] = new int[maxRows][maxColumns];` this doesn't do what you think it does. So the `for` loop in your method goes boom. — Federico klez Culloca, Apr 16 '20 at 08:53
@FedericoklezCulloca darn, I didn't think it did. Could you point me in the right direction in terms of being able to initialize an array using those variables? — Jaimee-lee Lincoln, Apr 16 '20 at 08:55
@FedericoklezCulloca It took me a good moment to realize what is wrong with that... Jaimee-lee, you have two variables called `block`, one on the class level and other in the constructor. They aren't the same. — Amongalen, Apr 16 '20 at 08:58

score 1 · Accepted Answer · answered Apr 16 '20 at 08:57

1

In the MyBlock constructor, change:

int block[][] = new int[maxRows][maxColumns];

to:

block = new int[maxRows][maxColumns];

You are simply hiding the instance attribute with a local one.

answered Apr 16 '20 at 08:57

Giancarlo Romeo

663
1
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Exception in thread "main" java.lang.NullPointerException when trying to use a method in another class?

1 Answers1