String array without initial size gives null pointer exception

Question

I was just checking some OCJP questions and came across this difference during String array initialization and exceptions.

Case 1

try {
    String[][] b = new String[10][10]; // 1
    System.out.println(b[0][0]);       // 2
} catch (Exception e) {
    System.out.println("Exception during array 'b' initialization");
        e.printStackTrace();
}

Case 2

try {
    String[][] a = new String[10][]; // 3
    System.out.println(a[0][0]);     // 4
} catch (Exception e) {
    System.out.println("Exception during array 'a' initialization");
    e.printStackTrace();
}

Line 2 does not throw any exception whereas line 4 throws a null pointer exception. Line 2 does output the value as null though.

Does java have a difference in default values for initialization when the size of an array is specified and when its not?

Answer 1

This sets the type of a, as array of arrays :

String[][] a

When you write

String[][] a = new String[10][];

you initialize the external array, but you don't create the internal arrays, so that a[0] is null.

When you write

String[][] b = new String[10][10];

the runtime creates also the internal arrays. It's described here in the specification :

At run time, evaluation of an array creation expression behaves as follows:

If there are no dimension expressions, then there must be an array initializer.

A newly allocated array will be initialized with the values provided by the array initializer as described in §10.6.

The value of the array initializer becomes the value of the array creation expression.

Otherwise, there is no array initializer, and:

First, the dimension expressions are evaluated, left-to-right. If any of the expression evaluations completes abruptly, the expressions to the right of it are not evaluated.

Next, the values of the dimension expressions are checked. If the value of any DimExpr expression is less than zero, then a NegativeArraySizeException is thrown.

Next, space is allocated for the new array. If there is insufficient space to allocate the array, evaluation of the array creation expression completes abruptly by throwing an OutOfMemoryError.

Then, if a single DimExpr appears, a one-dimensional array is created of the specified length, and each component of the array is initialized to its default value (§4.12.5).

Otherwise, if n DimExpr expressions appear, then array creation effectively executes a set of nested loops of depth n-1 to create the implied arrays of arrays.

A multidimensional array need not have arrays of the same length at each level.