如何在没有nullpointerexception错误的情况下显示锯齿状数组 [英] How do I display my jagged array without nullpointerexception error

问题描述

每次运行我的代码时，都会收到此错误：



线程main中的异常java.lang.NullPointerException



我尝试过：

 import java.util.Scanner; 
 public class JaggedArray 
 {
 
 public static void main（String [] args）
 {
 
 int [] [] jArr; 
 jArr = new int [25] []; 
 System.out.println（请输入学生人数：）; 
 Scanner sc = new Scanner（System.in）; 
 int students = sc.nextInt（）; 
 for（int i = 0; i< students; ++ i）
 {
 System.out.println（学生多少年级+（i + 1）+有：）; 
 int gradeNum = sc.nextInt（）; 
 jArr [i] = new int [gradeNum]; 
 for（int j = 0; j< gradeNum; ++ j）
 {
 System.out.println（Please enter grade+（j + 1）+： ）; 
 jArr [i] [j] = sc.nextInt（）; 
} 
} 
 System.out.println（你的数组是：）; 
 
 //在我的方法中显示一个错误，调用
 displayArray（jArr）; 
 
 
 public static void displayArray（int [] [] jArr）
 {
 
 for（int i = 0; i< jArr.length） ; i ++）
 {
 //它显示了这个嵌套for循环的错误
 for（int j = 0; j< jArr [i] .length; j ++）
 {
 System.out.print（jArr [i] [j] +）; 
} 
 System.out.println（）; 
} 
} 
} 

解决方案

Quote：

jArr = new int [25] [];

以上行是罪魁祸首。向下移动数组定义：

  //   ...  
扫描仪sc = 新扫描仪（System.in）; 
  int  students = sc.nextInt（）; 
 jArr =  new   int  [students] [];  //  <  - 这是正确的地方（因为你知道'学生'） 
  for （ int  i =  0 ; i < students; ++ i）
  //   ..  
 

Every time I run my code, i get this error:

Exception in thread "main" java.lang.NullPointerException

What I have tried:

import java.util.Scanner;
public class JaggedArray 
{

	public static void main(String[] args)
	{
		
		int[][] jArr; 
		jArr = new int[25][];
		System.out.println("Please enter number of students:");
		Scanner sc = new Scanner(System.in);
		int students = sc.nextInt();
		for (int i = 0; i < students; ++i)
		{
			System.out.println("How many grades does student " + (i+1) + " have:");
			int gradeNum = sc.nextInt();
			jArr[i] = new int[gradeNum];
			for (int j = 0; j < gradeNum; ++j)
			{
				System.out.println("Please enter grade " + (j+1) + ":");
				jArr[i][j] = sc.nextInt();
			}
		}
		System.out.println("Your array is: ");

                //It shows an error in my method calling
		displayArray(jArr);
		
	
	public static void displayArray(int[][] jArr)
	{

		for(int i = 0; i < jArr.length; i++)
		{
                        //It shows an error on this nested for loop
			for(int j = 0; j < jArr[i].length; j++)
			{
				System.out.print(jArr[i][j] + " ");
			}
			System.out.println();
		}
	}
}

解决方案

Quote:

jArr = new int[25][];

The above line is the culprit. Move down a bit the array definition:

//...
Scanner sc = new Scanner(System.in);
int students = sc.nextInt();
jArr = new int[students][]; // <- this is the right place (because you know 'students')
for (int i = 0; i < students; ++i)
//..

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