I am trying to reverse a two dimensional array and

Question

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

I keep getting

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

Java Result: 1

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

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.

Answer 1

When you say this line

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

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];

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++){

Answer 2

Your array rev has not been initialized in its second coordinate. After your declaration int[][] rev = new int[a.length][], all your rev[i] are null objects (arrays are objects, even for primitive types). To avoid that, initialize with int[][] rev = new int[a.length][a[0].length] or with rev[i] = new int[a[i].length] or so, if the array is jagged.