我正在尝试反转二维数组，并不断收到null异常 [英] I am trying to reverse a two dimensional array and keep getting a null exception

问题描述

这里是我的方法，它想反转数组.(此外，请记住，此方法可以接收锯齿状数组)

Here is my method that is suppose to reverse the array. (Also, keep in mind that this method can receive a jagged array)

    public static int[][] reverse(int[][]a){

int[][] rev = new int[a.length][];

int row = a.length;

for(int x=0;x<a.length;x++){
   int col = a[x].length-1;
   for(int y=0; y< a[x].length;y++){

       rev[row-1][col-1]= a[x][y];
       col--;

       }
   row--;
   }

   return rev;


}// reverse method

我不断得到

Exception in thread "main" java.lang.NullPointerException
at Home.reverse(Home.java:259)
at Home.main(Home.java:56)

Java结果:1

这是我调用该方法的主要对象

Here is my main that calls that method

int[][] two = {{1,2,3,4},{0,1,2},{9},{1,3,5,7}};
System.out.println("The size of array os " + ar.length);
System.out.println("The columns of array is " + ar[0].length); 

int[][] rev;
//int[] b = concatenate(ar);
//int[] c = concatenate(two);
//int[][] rev = reverse(ar);

//for(int x=0;x<c.length;x++){System.out.println("worked" + c[x]);}

rev = reverse(two);

//for(int x = 0;x<rev.length;x++){
//  for(int y = 0;y<rev[x].length;y++){
//    System.out.print(rev[x][y]);
//}
//}
for(int x=0;x<rev.length;x++){
for(int y=0;y<rev[x].length;y++){
    System.out.print(rev[x][y]);
}
}

}// main

所以我的问题是，我哪里出了错?我追踪了我的反向方法，看起来它应该完成它的工作，但显然不是.

So my question is really, where did I go wrong? I traced my reverse method and it looks like it should be doing its job but apparently not.

推荐答案

说这句话

int[][] rev = new int[a.length][];

您已经创建了一个长度为 a.length 个数组的数组，但是内部数组都是 null .因此，您在此行上收到 NullPointerException :

You have created an array of length a.length of arrays, but the internal arrays are all null. So you are getting a NullPointerException on this line:

rev[row-1][col-1]= a[x][y];

您需要在第一个for循环内创建内部数组.这将满足您的锯齿状阵列"要求.

You need to create your internal arrays inside the first for loop. This will satisfy your "jagged array" requirement.

for(int x=0;x<a.length;x++){
   int col = a[x].length-1;
   rev[row-1] = new int[a[x].length];  // Add this to create differing lengths.
   for(int y=0; y< a[x].length;y++){

这篇关于我正在尝试反转二维数组，并不断收到null异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！