Array declaration and initialization in Java. Arrays behave differently, when the position of their subscript indices is changed in their declaration

Question

The following is the obvious and usual array declaration and initialization in Java.

int r[], s[];       //<-------
r=new int[10];
s=new int[10];

A very similar case behaves differently, when the position of [] is changed in the declaration statement like as shown below.

int []p, q[];       //<-------
p=new int[10];
q=new int[10][10];

Please look at the declaration. The position of [] has been changed from r[] to []p. In this case, the array q behaves like an array of arrays of type int (which is completely different from the previous case).

The question: Why is q, in this declaration int []p, q[]; treated as a two dimensional array?

Additional information:

The following syntax looks wonky.

int []a[];

This however, complies fine and just behaves like int a[][]; or int [][]a;.

Hence, the following cases are all valid.

int [][]e[][][];
int [][][][][]f[][][][];

Try this one public int numbers()[] { return new int[5]; } :D — Peter Lawrey
– Peter Lawrey, Commented May 25, 2013 at 18:34
Personally, I prefer to declare one array variable per line, and initialize the variable in the same line, in order to avoid this kind of confusion. — Code-Apprentice
– Code-Apprentice, Commented Jul 3, 2013 at 18:35

gsamaras · Accepted Answer · 2016-05-14 01:46:39Z

43

Look at JLS on Arrays:

The [] may appear as part of the type at the beginning of the declaration, or as part of the declarator for a particular variable, or both.

and

Brackets are allowed in declarators as a nod to the tradition of C and C++. The general rules for variable declaration, however, permit brackets to appear on both the type and in declarators, so that the local variable declaration:
float[][] f[][], g[][][], h[];  // Yechh!
is equivalent to the series of declarations:
float[][][][] f;
float[][][][][] g;
float[][][] h;

So for example:

int []p, q[];

is just

int[] p, q[]

which is in fact

int p[]; int q[][]

The rest are all similar.

edited May 14, 2016 at 1:46

gsamaras

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answered May 25, 2013 at 18:33

zw324

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2 Comments

Amir Pashazadeh Over a year ago

It is strange that q becomes a two dimension array.

DVK Over a year ago

@AmirPashazadeh - It's not strange at all if you pretend that the variables are surrounded by parenthesis, ala Perl's syntax my $x, $y; being same as my($x, $y);. If you do this theoretical grouping, you get int[] p, q[] as int[] (p, q[]) => commutatively becoming int[] (p), int[] (q[]) => int []p, int []q[] => int p[], int q[][]

Lion · Accepted Answer · 2013-05-26 19:15:25Z

10

The sane way of declaring a variable is

type name

So if type is int[], we should write

int[] array

Never write

int array[]

it is gibberish (though it's legal)

edited May 26, 2013 at 19:15

Lion

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answered May 26, 2013 at 0:36

ZhongYu

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