8.4. Varags

Consider the code below.

public static void main(String[] args) {
  double d1 = 10.0;
  double d2 = 20.0;
  double d3 = 30.0;
  double d4 = 40.0;

  System.out.println("d1 = " + d1);
  System.out.println("d2 = " + d2);
  System.out.println("d3 = " + d3);
  System.out.println("d4 = " + d4);

  System.out.println("Average of d1 and d2 is " + average(d1, d2));
  System.out.println("Average of d1, d2, and d3 is " + average(d1, d2, d3));
  System.out.println("Average of d1, d2, d3, and d4 is " + average(d1, d2, d3, d4));
}

It compiles and prints out the following.

d1 = 10.0
d2 = 20.0
d3 = 30.0
d4 = 40.0
Average of d1 and d2 is 15.0
Average of d1, d2, and d3 is 20.0
Average of d1, d2, d3, and d4 is 25.0

Why do you think we can call average() with 1, 2, 3 and 4 numbers?

average(d1, d2);
average(d1, d2, d3);
average(d1, d2, d3, d4);

What if I told we could also call it with 5, 6, 7, 8, 9, and even 10 numbers?

average(d1, d2, d3, d4, d5, d6, d7, d8, d9, d10);

Variable-length arguments list

We can make all those calls because the method was defined using varargs!

public static double average(double... values) {
  double sum = 0;

  for (double value : values) {
      sum += value;
  }

  return sum / values.length;
}

The three periods (...) after the final parameter's type indicate that the final argument may be passed as

as a sequence of arguments

System.out.println("Average of d1, d2, d3, and d4 is " + average(d1, d2, d3, d4));

or an array

double[] doubles = {d1, d2, d3, d4};
System.out.println("Average of d1, d2, d3, and d4 is " + average(doubles));

Varargs can be used only in the final argument position.

In the method body, we treat the variable-length argument as an arrays.