Java：方法不起作用

Question

我似乎在使用三个用户创建的方法并在主方法中正确调用它们时遇到了问题。我没有任何我知道的语法错误，所以留下了合乎逻辑的错误，参数看起来也很好。

假设该程序的输出类似于下面的最后一个输出。这是一个接受二维数组并检查它是否为magic square 的程序。每种方法都负责审查/检查过程以确定它是否是幻方。

我真的很感激能帮我解决这个问题，我很难过。

代码：

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;
        }
    }
}

当前输出（调用方法有问题？）

 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)

输出看起来有点像这样：

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

您需要能够理解异常是什么以及如何解决它。您的代码执行在其输出中显示 java.lang.ArrayIndexOutOfBoundsException 以及您在哪一行得到它。

纠正逻辑的第一步是避免 java.lang.ArrayIndexOutOfBoundsException

您可以如下控制迭代

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!");
        }
    }

第 1 步 -> int[] rowArr = a[row];

第二步->

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

当你调用 return rowsEqTargetSum(null, 0, sum, sum, sum, sum); 时会导致 java.lang.NullPointerException；

Answer 2

你的方法不是问题。这是 G.I.G.O（Garbage in, Garbage out）的一个例子。您使用不正确的参数调用了您的方法。在这种情况下，rowsEqTargetSum 方法的第三个参数应该是 n，而是传递 n*(n*n+1)/2 (targetSum)。对于任何大于 1 的 n，此值将始终高于 n，因此会根据您的代码导致索引越界异常。

同样的问题在您的 rowsEqTargetSum(null, 0, sum, sum, sum, sum) 的尾部调用中再次发生，这是一个无限递归（您的下一个错误），因为在其中没有替代（完成）选项方法。如果您的代码被修复以调用具有正确参数的方法，我相信这将导致您的下一个堆栈溢出错误。