Uninitialized array access : one vs multi dimension

[Ek Chotechawanwong]

Hi,
I have a basic language question about array access. There seems to be difference in how java compiler handle uninitilized member between one and multi dimensional array. (I use Sun - j2sdk 1.4.2_03 on Win2k).
In the one dimension array,

public class ArrayTest {
        public static void main(String[] args) {
                int[] a ;
                System.out.println("a[0] = " + a[0]);
        }
}

In this case I cannot compile the program.

ArrayTest.java:4: variable a might not have been initialized
System.out.println("a[0] = " + a[0]);
^
1 error

However, in case of 2 dimensional array, I can compile but I got runtime exception (NullpointerException).

public class ArrayTest {
        public static void main(String[] args) {
                int [][] a = new int[2][];
                System.out.println("a[1][5] = " + a[1][5]);
        }
}

In this case I can run the program but got NullPointerException.

Exception in thread "main" java.lang.NullPointerException
at ArrayTest.main(ArrayTest.java:4)

Question
Could you please explain why the behaviour is different?
Thanks so much in advance.

[Dirk Schreckmann]

ekc,
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[Jeanne Boyarsky]

In the second one, you did initialize the array (a).

  int [][] a = new int[2][];

It is a valid case to have an array that contains all null elements. For example, you may want to populate it later. So the compiler doesn't know for sure whether the array has been initialized.
In the first example, the array was not initialized at all. This is a case that the compiler can identify.

[Ek Chotechawanwong]

Hi Jeanne,
I think

int[][] a = new int[2][];

just allocates the first dimension aka. array of 2 int[] elements however it doesn't explicitly construct the second dimension of each - it doesn't say how many int elements will be in each int[] array.
Therefore, it's the same as the first case to me.
If it's a kind of java compiler limitation/design issue, I would be OK with the difference. But I am afraid, I am missing something on language concept.
In other words, I see that javac can check whether one dimensional array is initilized or not. Is there any reason why java compiler doesn't check the subsequent dimension of the multi-dimensional array?
Thanks,
Ek