Mock Exam Array question

[David Attard]

I found this question on Dan Chrisholm's mock exam:
http://www.danchisholm.net/apr18/topic/section1/arrays1.html
Questions 17

class A {}
class B {
  public static void main(String[] arg) {
    A[]     a1 = new A[1];         // 1
    A[][]   a2 = new A[2][1];      // 2
    A[][][] a3 = new A[3][3][3];   // 3
    System.out.print(a3[2][2][2]); // 4
    a1[0] = new A();               // 5
    a2[0] = a2[1] = a1;            // 6
    a3[0] = a3[1] = a3[2] = a2;    // 7
    System.out.print(a3[2][2][2]); // 8
  }
}

What is the result of attempting to compile and run the above program?
I didn't understand why it gives a runtime-exception, and the answer posted, can anyone put it in simpler terms please

[Corey McGlone]

Okay, let me try to make this easy to understand - I'll do my best.
a3 references a 3 dimensional array. Another way to look at a a 3-D array is as a single dimensional array in which each element of the array references a 2-D array. Think of it like this (forgive the poor ASCII art):

        +---------+---------+---------+
a3 ---> |    |    |    |    |    |    |
        +----|----+----|----+----|----+
             |         |         |
      +------+         |         |
      |             +--+         ++
      |             |             |
      V             V             V
+---+---+---+ +---+---+---+ +---+---+---+
|   |   |   | |   |   |   | |   |   |   |
+---+---+---+ +---+---+---+ +---+---+---+
|   |   |   | |   |   |   | |   |   |   |
+---+---+---+ +---+---+---+ +---+---+---+
|   |   |   | |   |   |   | |   |   |   |
+---+---+---+ +---+---+---+ +---+---+---+

However, in this question, in line 7, you're assigning each of the elements of a3 to a reference to a 2-D array which has dimensions 2x1. Therefore, your picture looks something like this now:

                                    a3[2]
        +---------+---------+---------+
a3 ---> |    |    |    |    |    |    |
        +----|----+----|----+----|----+
             |         |         |
       +-----+         |         |
       |             +-+         |
       |             |           |
       V             V  a3[2][0] V  a3[2][1]
   +---+---+     +---+---+    +---+---+
   | | | | |     | | | | |    | | | | |
   +-|-+-|-+     +-|-+-|-+    +-|-+-|-+
     |   |         |   |        |   |
     V   V         V   V        V   V
    +-+ +-+       +-+ +-+      +-+ +-+
    | | | |       | | | |      | | | |
    +-+ +-+       +-+ +-+      +-+ +-+
                       a3[2][0][0] a3[2][1][0]

Notice that now each element of a3 references another array which has dimensions 2x1. I've also written in the index values for each of the elements indexed by a3[2]. As you can see, the array element a3[2][2][2] no longer exists! That's why you end up with the run-time exception.
I hope that helps,
Corey

[David Attard]

Excellent Thanks a lot!