Sentinel Controlled Loop

In a sentinel controlled loop the change part is often getting data from the user. It would be awkward to do this inside a for. So the change part is omitted from the for and put where it is more convenient. Here is an example. The program keeps asking the user for a value for x. Then it prints out the square root of x. The user tells the program to end the loop by entering a negative number.

import java.io.*;

class evalSqrt
{
  public static void main (String[] args ) throws IOException
  {
    BufferedReader userin =  new BufferedReader(new InputStreamReader(System.in) );
    String xChars;
    double x;

    System.out.println("Enter a value for x or -1 to exit:")  ;
    xChars = userin.readLine()  ;
    x      = ( Double.valueOf( xChars ) ).doubleValue();

    for (    ; x >= 0.0 ;   )  
    {
      System.out.println( "Square root of " + x + " is " + Math.sqrt( x ) ); 

      System.out.println("Enter a value for x or -1 to exit:")  ;
      xChars = userin.readLine()  ;
      x      = ( Double.valueOf( xChars ) ).doubleValue();
    }
  }
}

It probably would be preferable to use a while in this case.