java编程题，不知道该怎么做啊

Java程序：

import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int row = 3;
		int col = 4;
		double[][] n = new double[row][col];
		int i, j;
		
		System.out.printf("Enter a %d-by-%d matrix row by row:\n", row, col);
		for(i=0; i			for(j=0; j				n[i][j] = scan.nextDouble();
			}
		}
		
		for(i=0; i			System.out.printf("Sum of the elements at column %d is %f\n", i, sumColumn(n, i));
		}
	}
	
	public static double sumColumn(double[][] n, int columnIndex) {
		double sum = 0;
		
		for(int i=0; i			sum += n[i][columnIndex];
		}
		
		return sum;
	}
}

运行测试：

Enter a 3-by-4 matrix row by row:

1.5 2 3 4

5.5 6 7 8

9.5 1 3 1

Sum of the elements at column 0 is 16.500000

Sum of the elements at column 1 is 9.000000

Sum of the elements at column 2 is 13.000000

Sum of the elements at column 3 is 13.000000