Why is this Java array considered two dimensional?

Question

Consider this code:

class arraytest {

    public static void main(String args[]) {

        int[] a = null , b[] = null;

        b = a;
        System.out.println( b );

    }

}

The line

b = a;

is flagged by the compiler saying:

Incompatible types, found int[], required int [][]

Why is b considered two dimensional? I realize the "shortcut" declaration

int[] a = null , b[] = null;

is to blame, but why does it make the array two dimensional when only one set of brackets have been written? I find this syntax unclear and obfuscating.

RichieHindle · Accepted Answer · 2009-07-18 21:34:34Z

19

Take this example:

int a, b;

Then both a and b are ints, right? So now take this:

int[] a, b;

The both a and b are int arrays. So by adding another set of brackets:

int[] a, b[];

you have added another set of brackets to b.

Either of these would be fine:

int[] a = null , b = null;
int a[] = null , b[] = null;

or as you say, simply putting them on separate lines would work too (and be much easier to read).

answered Jul 18, 2009 at 21:34

RichieHindle

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Comments

Matthieu · Accepted Answer · 2009-07-18 21:37:43Z

5

int[] a = null , b[] =null;

.... it's equal to :

int[] a = null;
int[][]b = null;

You have to do :

int[] a = null , b =null;

answered Jul 18, 2009 at 21:37

Matthieu

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Comments

user92983 · Accepted Answer · 2009-07-18 22:12:20Z

3

In addition to the other answers, Java does not have multi-dimensional arrays. Java has arrays of any type. This "any type" may itself be an array. Multi-dimensional arrays are distinctly different, although subtly.

answered Jul 18, 2009 at 22:12

user92983

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user8681 · Accepted Answer · 2009-07-18 22:24:02Z

2

If you take out the a declaration, this is what you have:

int[] b[] = null;

which is, unfortunately, a legal alternative to

int[][] b = null;

answered Jul 18, 2009 at 22:24

user8681

Comments

Jay R. · Accepted Answer · 2009-07-19 04:09:03Z

1

I was lead to believe that the syntax of allowing the [] after the b was there solely for the C-language crowd who would feel more at home with it. Unfortunately, it has the side effect of allowing confusing declarations like

 int[] b[];

which ends up the same as

 int[][] b;

answered Jul 19, 2009 at 4:09

Jay R.

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