[java] Double array initialization in Java

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}
};

It is called an array initializer and can be explained in the Java specification 10.6.

This can be used to initialize any array, but it can only be used for initialization (not assignment to an existing array). One of the unique things about it is that the dimensions of the array can be determined from the initializer. Other methods of creating an array require you to manually insert the number. In many cases, this helps minimize trivial errors which occur when a programmer modifies the initializer and fails to update the dimensions.

Basically, the initializer allocates a correctly sized array, then goes from left to right evaluating each element in the list. The specification also states that if the element type is an array (such as it is for your case... we have an array of double[]), that each element may, itself be an initializer list, which is why you see one outer set of braces, and each line has inner braces.

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;
    }
}

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} };

double m[][] declares an array of arrays, so called multidimensional array.

m[0] points to an array in the size of four, containing 0*0,1*0,2*0,3*0. Simple math shows the values are actually 0,0,0,0.

Second line is also array in the size of four, containing 0,1,2,3.

And so on...

I guess this mutiple format in you book was to show that 0*0 is row 0 column 0, 0*1 is row 0 column 1, and so on.

If you can accept Double Objects than this post is helpful: Initialization of an ArrayList in one line

List<Double> y = Arrays.asList(null, 1.0, 2.0);
Double x = y.get(1);