How does the Java array argument declaration synta

Question

I have been writing java for a while, and today I encountered the following declaration:

public static void main(String... args) {

}

Note the "dot dot dot" in the array declaration, rather than the usual bracket []. Clearly it works. In fact I wrote a small test and verified it works. So, I pulled the java grammar to see where this syntax of argument declaration is, but did not find anything.

So to the experts out there, how does this work? Is it part of the grammar? Also, while I can declare function like this, I can't declare an array within a function's body like this.

Anyway, do you know of any place that has this documented. It is curiosity, and perhaps not worth of any time invested in it, but I was stumped.

Answer 1

It is varargs

In simple term its an Array of Member like

public setMembers(Member[] members);

When to use:

Generally while designing API it is good to use when number of argument is not fixed.

Example from standard API of this is String.format(String format,Object... args)

Also See

• var-arg-of-object-arrays-vs-object-array-trying-to-understand-a-scjp-self-test

Answer 2

Variable arguments. Can have 0 or more String arguments.

The function can access the parameters as an array of Strings.

Answer 3

Check out the Java Language Specification, Third Edition, Chapter 8 (Classes). Buried in there is this nugget:

If the last formal parameter is a variable arity parameter of type T, it is considered to define a formal parameter of type T[]. The method is then a variable arity method. Otherwise, it is a fixed arity method. Invocations of a variable arity method may contain more actual argument expressions than formal parameters. All the actual argument expressions that do not correspond to the formal parameters preceding the variable arity parameter will be evaluated and the results stored into an array that will be passed to the method invocation (§15.12.4.2).

Basically, the last parameter of any method call can have T.... If it has that, it is converted to T[].

So basically, what you have is a fancy way of reproducing the more traditional

String[] args

Answer 4

It means you can pass zero or more Member objects to the setMembers() method. In the setMembers method, members will have array semantics.

Answer 5

I believe this was implemented in Java 1.5. The syntax allows you to call a method with a comma separated list of arguments instead of an array.

public static void main(String... args);
main("this", "is", "multiple", "strings");

is the same as:

public static void main(String[] args);
main(new String[] {"this", "is", "multiple", "strings"});

http://today.java.net/article/2004/04/13/java-tech-using-variable-arguments http://download.oracle.com/javase/1.5.0/docs/guide/language/varargs.html

Answer 6

It's the so-called varargs syntax. In the method body, you can read the members parameter as it were an array - actually, it /is/ 'just' an array.

However, the magic bit is in calling the method. Before the varargs syntax was introduced, you'd call the method a bit like so:

setMembers(new Members[] {member1, member2, member3});

With the new varargs syntax however, you don't need to explicitly create the array anymore, and you can pass:

setMembers(member1, member2, member3);

This does mean however that a varargs argument has to be the last argument in a method. Something like this is therefore not permitted:

void setMembers(Member ... members, String memberType);

Summarized: It's a bit of syntactic sugar, really. I'm no expert on the inner workings of the Java compiler, but I'm pretty sure that methods calling a method that accept a varargs parameter are rebuilt into methods that build an array of the given type.