How can I do a Math.random() to generate a random number between a and b

Question:

How can I do a Math.random() to generate a random number between a and b

Code:

public class Example {
  public static void main(String[] args) {
    double a=10;
    double b=20;
    double range = b - a;
    double r = Math.random() * range + a;
    System.out.println(r);
  }
}

Output:

$ java Example 
17.09178855580154