Arrays with trailing commas inside an array initializer in Java

Question

Array initializers can be used to initialize arrays at compile-time. An initializer with trailing commas as shown below compiles fine.

int a[][] = {{1,2,} ,{3,4,} , {5,6,},}; //Trailing commas cause no compiler error

for(int i=0;i<a.length;i++)
{
    for(int j=0;j<2;j++)
    {
        System.out.print(a[i][j]+"\t");
    }
    System.out.println();
}

Output :

1        2        
3        4        
5        6

Also legal with one dimension arrays as obvious with the above discussion.

int[] b = {1, 2, 3, 4, 5, 6,}; //A trailing comma causes no compiler error

for(int i=0;i<b.length;i++)
{
    System.out.print(b[i]+"\t");
}

Output :

1        2        3        4        5        6

Even the following is a legal syntax and compiles fine.

int c[][] = {{,} ,{,} , {,},};

The compiler should expect a constant value (or another initializer) after and before a comma ,. How is this compiled? Does the compiler simply ignore such commas or something else happens in such a scenario?

A trailing comma is ignored. It's allowed by the language since some find it convenient. — Daniel Fischer
– Daniel Fischer, Commented Jul 23, 2012 at 23:18
@Daniel Well it keeps diffs more succinct when adding things to arrays, allows easy reordering of array expressions and enables a simpler grammar for the language all in one go. Quite some advantages for no downsides I can see. — Voo
– Voo, Commented Jul 24, 2012 at 0:00
@Voo How does it make the grammar simpler? If the trailing comma were mandatory, that would make it simpler, but with an optional trailing comma, I don't see it. Nor can I see the greater succinctness of diffs. Reordering, yes. — Daniel Fischer
– Daniel Fischer, Commented Jul 24, 2012 at 0:14
@Daniel Right you are about the grammar, I forgot that they're still optional. About diffs: Assume that for larger arrays every item gets its own line (I see that relatively often and while it's not always sensible, there are lots of situations where it's useful - also leaves place for comments without resorting to /* */ inlining). Adding a new line then only changes a single line in the diff, without it you'd have to change two. — Voo
– Voo, Commented Jul 24, 2012 at 0:22
@Voo I see. But if you lay out your arrays Haskell style, with leading commas, it wouldn't change two lines - except if you add an entry at the first position, but that's much rarer than adding at the end. — Daniel Fischer
– Daniel Fischer, Commented Jul 24, 2012 at 0:28

Mark Byers · Accepted Answer · 2012-07-23 23:19:15Z

38

The trailing comma is ignored. From the Java specification:

A trailing comma may appear after the last expression in an array initializer and is ignored.

answered Jul 23, 2012 at 23:19

Mark Byers

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1 Comment

user719662 Over a year ago

as a side note: the above statement is present even in JLS 1.0 titanium.cs.berkeley.edu/doc/java-langspec-1.0 (reaching back to Java 1.0), so we can safely say "it was always there".

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