Java: Methods not Functioning

Question

I seem to be having trouble with the three user created methods and calling them correctly in the main method. I don't have any syntax errors that I know of, so that leaves logical ones, the parameters look fine too.

The program is suppose to have an output similar to the last output below. It's a program that takes a 2D array and checks if it's a magic square. Each method has a responsibility regarding the vetting/checking process to determine if it's a magic square or not.

I'd really appreciate some help getting this working, I'm stumped.

Code:

import java.util.Arrays;

public class Magic10
{
    // A method to check whether rows equal the target sum
    private static boolean rowsEqTargetSum(int[][] a, int targetSum, int n, int row, int col, int sum)
    {
    // Calculate the sum of each row ... if magic, then equal to targetSum      
        for(row=0; row<n; row++)
        {
            System.out.print("row "+row+": ");
            for(col=0; col<n; col++)
            {
                int value = a[row][col];
                sum += value;
                if (col > 0)
                    System.out.print(" + "); // print plus before all except 1st
                System.out.print(value);
            }
            System.out.println(" = "+sum);
            if(sum != targetSum)
            {
                System.out.println("Row sum incorrect : Not a magic Square!");
            }
        }
        return rowsEqTargetSum(null, 0, sum, sum, sum, sum);
    }
    // A method to check whether diagonals equal the target sum 
    private static boolean diagonalEqTargetSum(int[][] a, int targetSum, int n, int row, int col, int sum)
    {
        System.out.print("diagonal: ");
        for(int pos=0; pos<n; pos++)
        {

            row = n-1 - pos;
            col = pos;
            int value = a[row][col];
            sum += value;
            if (pos > 0)
                System.out.print(" + "); // print plus before all except 1st
            System.out.print(value);
        }
        System.out.println(" = "+sum);
        if(sum != targetSum)
        {
            System.out.println("Diagonal is incorrect : Not a magic Square!");
        }
        return diagonalEqTargetSum(null, 0, 0, sum, sum, sum);
    }
    // A method to check whether all numbers are used exactly once in the 2D array
    private static boolean allNumbersRepresented(int[][] a, int n, int col, int row)
    {
    // Lastly, we check that every number from 1 to n is represented
        final int nSquare=n*n;
        boolean[] flag= new boolean[n*n];

        for(row=0; row<n; row++)
        {
            for(col=0; col<n; col++)
            {
                int num = a[row][col];
                if (n < 1 || num > nSquare)
                {
                    System.out.println("Number out of range : Not a magic Square!");
                }
                if (flag[num-1]) 
                {
                    System.out.println("Duplicate number : Not a magic Square!");
                }
                flag[num-1] = true;
            }
        }
        return allNumbersRepresented(null, 0, 0, 0);
    }

    public static void main(String []args)
    {
        int[][] a ={{4,9,2},
                    {3,5,7},
                    {8,1,6}};
        final int n=a.length;
        final int targetSum=n*(n*n+1)/2;

        System.out.println(" The following two dimensional array is Magic!");   
        for (int i = 0;i< a.length;i++)
        {
            System.out.println(Arrays.toString(a[i]));
        }

        // Calls rowsEqTargetSum Method
        if (!rowsEqTargetSum(a, targetSum, targetSum, targetSum, targetSum, targetSum))
        {
            return;
        }
        // Calls diagonalEqTargetSum Method
        if (!diagonalEqTargetSum(a, targetSum, targetSum, targetSum, targetSum, targetSum))
        {
                 return;
        }
        // Calls allNumbersRepresented Method
        if (!allNumbersRepresented(a, targetSum, targetSum, targetSum))
        {
                return;
        }
    }
}

Current Output (Something is wrong with calling the methods?)

 The following two dimensional array is Magic!
[4, 9, 2]
[3, 5, 7]
[8, 1, 6]
row 0: 4 + 9 + 2Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
    at Magic10.rowsEqTargetSum(Magic10.java:14)
    at Magic10.main(Magic10.java:91)

Output looking somewhat like this:

row 0: 4 + 9 + 2 = 15
row 1: 3 + 5 + 7 = 15
row 2: 8 + 1 + 6 = 15
diagonal: 8 + 5 + 2 = 15
 The following two dimensional array is Magic !
[4, 9, 2]
[3, 5, 7]
[8, 1, 6]

Answer 1

Your method is not the problem. This is a case of G.I.G.O (Garbage in, Garbage out). You have called your method using incorrect arguments. In this case the third argument of the rowsEqTargetSum method should be n and instead you pass n*(n*n+1)/2 (targetSum). This value will always be above n for any n greater than 1 and therefore lead to an index out of bounds exception based on your code.

This same issue happens again within the method in your tail call of rowsEqTargetSum(null, 0, sum, sum, sum, sum) which is an infinite recursion (your next bug) since there is no alternative (completion) option in the method. This I believe will lead to your next error of a stack overflow IF your code is repaired to call methods with proper arguments.

Answer 2

You need to be able to understand what an Exception is and how to resolve it. Your code execution in its output says java.lang.ArrayIndexOutOfBoundsException and at what line you are getting it.

First step to correct your logic is to avoid java.lang.ArrayIndexOutOfBoundsException

You can control the iteration as below

for(row=0; row<a.length; row++)
    {
        int[] rowArr = a[row];
        System.out.print("row "+row+": ");
        for(col=0; col<rowArr.length; col++)
        {
            int value = a[row][col];
            sum += value;
            if (col > 0)
                System.out.print(" + "); // print plus before all except 1st
            System.out.print(value);
        }
        System.out.println(" = "+sum);
        if(sum != targetSum)
        {
            System.out.println("Row sum incorrect : Not a magic Square!");
        }
    }

Step 1 -> int[] rowArr = a[row];

Step 2- >

int[] rowArr = a[row];
System.out.print("row "+row+": ");
for(col=0; col<rowArr.length; col++)

This leads to java.lang.NullPointerException when you make call return rowsEqTargetSum(null, 0, sum, sum, sum, sum);