Double array initialization in Java

Question

I was reading a book on Java and came across an example in which an array of type double was initialized in a way that I haven't seen before. What type of initialization is it and where else can it be used?

double m[][]={     {0*0,1*0,2*0,3*0},     {0*1,1*1,2*1,3*1},     {0*2,1*2,2*2,3*2},     {0*3,1*3,2*3,3*3} }; 

Answer 1

This is array initializer syntax, and it can only be used on the right-hand-side when declaring a variable of array type. Example:

int[] x = {1,2,3,4}; String[] y = {"a","b","c"}; 

If you're not on the RHS of a variable declaration, use an array constructor instead:

int[] x; x = new int[]{1,2,3,4}; String[] y; y = new String[]{"a","b","c"}; 

These declarations have the exact same effect: a new array is allocated and constructed with the specified contents.

In your case, it might actually be clearer (less repetitive, but a bit less concise) to specify the table programmatically:

double[][] m = new double[4][4];  for(int i=0; i<4; i++) {     for(int j=0; j<4; j++) {         m[i][j] = i*j;     } } 

Answer 2

You can initialize an array by writing actual values it holds in curly braces on the right hand side like:

String[] strArr = { "one", "two", "three"}; int[] numArr = { 1, 2, 3}; 

In the same manner two-dimensional array or array-of-arrays holds an array as a value, so:

String strArrayOfArrays = { {"a", "b", "c"}, {"one", "two", "three"} }; 

Your example shows exactly that

double m[][] = {     {0*0,1*0,2*0,3*0},     {0*1,1*1,2*1,3*1},     {0*2,1*2,2*2,3*2},     {0*3,1*3,2*3,3*3} }; 

But also the multiplication of number will also be performed and its the same as:

double m[][] = { {0, 0, 0, 0}, {0, 1, 2, 3}, {0, 2, 4, 6}, {0, 3, 6, 9} };