Use of ellipsis(...) in Java?

Question

I was looking through some code and saw the following notation. I'm somewhat unsure what the three dots mean and what you call them.

void doAction(Object...o);

Thanks.

Answer 1

It means that this method can receive more than one Object as a parameter. To better understating check the following example from here:

The ellipsis (...) identifies a variable number of arguments, and is demonstrated in the following summation method.

static int sum (int ... numbers)
{
   int total = 0;
   for (int i = 0; i < numbers.length; i++)
        total += numbers [i];
   return total;
}

Call the summation method with as many comma-delimited integer arguments as you desire -- within the JVM's limits. Some examples: sum (10, 20) and sum (18, 20, 305, 4).

This is very useful since it permits your method to became more abstract. Check also this nice example from SO, were the user takes advantage of the ... notation to make a method to concatenate string arrays in Java.

Another example from Variable argument method in Java 5

public static void test(int some, String... args) {
        System.out.print("\n" + some);
        for(String arg: args) {
            System.out.print(", " + arg);
        }
    }

As mention in the comment section:

Also note that if the function passes other parameters of different types than varargs parameter, the vararg parameter should be the last parameter in the function declaration public void test (Typev ... v , Type1 a, Type2 b) or public void test(Type1 a, Typev ... v recipientJids, Type2 b) - is illegal. ONLY public void test(Type1 a, Type2 b, Typev ... v)

Answer 2

It's called VarArgs http://www.javadb.com/using-varargs-in-java. In this case, it means you can put multiple instances of Object as a parameter to doAction() as many as you wants :

doAction(new Object(), new Object(), new Object());