We often get students struggling to read and validate console input from the user. The following example gives a simple example of an approach that can be used.

It reads the input from the user and checks if it is a valid double. If its not it prompts the user again. Could easily be modified for different validation that you may require.

  Scanner input = new Scanner( System.in );
  Double value = null;
  while (value==null) {
	  System.out.print("Please enter number: ");
	  String s = input.nextLine();
	  if (s.length()==0) {
		  System.out.println("You did not enter a value; Try again");
	  } else {
		  try {
			  value = new Double(s);
		  } catch (Exception ex) {
			  System.out.println("You did not enter a valid value; Try again");
		  }
	  }
  }
  double d = value.doubleValue();