EXPRESSIONS

Run time Evaluation of Array Creation Expressions

15.9.1

Then, if a single

DimExpr

 appears, a single dimensional array is created of

the specified length, and each component of the array is initialized to its standard

default value ( 4.5.4).

If an array creation expression contains

N DimExpr

 expressions, then it effec 

tively executes a set of nested loops of depth

N

  1  to create the implied arrays of

arrays. For example, the declaration:

float[][] matrix = new float[3][3];

is equivalent in behavior to:

float[][] matrix = new float[3][];

for (int d = 0; d < matrix.length; d++)

matrix[d] = new float[3];

and:

Age[][][][][] Aquarius = new Age[6][10][8][12][];

is equivalent to:

Age[][][][][] Aquarius = new Age[6][][][][];

for (int d1 = 0; d1 < Aquarius.length; d1++) {

Aquarius[d1] = new Age[8][][][];

for (int d2 = 0; d2 < Aquarius[d1].length; d2++) {

Aquarius[d1][d2] = new Age[10][][];

for (int d3 = 0; d3 < Aquarius[d1][d2].length; d3++) {

Aquarius[d1][d2][d3] = new Age[12][];

}

}

}

with

d, d1

,

d2

 and

d3

 replaced by names that are not already locally declared.

Thus, a single

new

 expression actually creates one array of length 6, 6 arrays of

length 10, 6

10 = 60  arrays of length 8, and 6

10

8 = 480  arrays of length

12. This example leaves the fifth dimension, which would be arrays containing the

actual array elements (references to

Age

 objects), initialized only to null refer 

ences. These arrays can be filled in later by other code, such as:

Age[] Hair = { new Age("quartz"), new Age("topaz") };

Aquarius[1][9][6][9] = Hair;

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

level; thus, a triangular matrix may be created by:

float triang[][] = new float[100][];

for (int i = 0; i < triang.length; i++)

triang[i] = new float[i+1];

There is, however, no way to get this effect with a single creation expression.

317