Thread: Just got started with Java - looking for feedback!

Hi

About to start a maths degree and what with the computer age opening up at the business-side I figure having programming experience will help on a CV. So, yesterday morning I started reading Java: A Beginner's Guide (The one you can preview from Oracle's website). Today I wrote my first proper program: this iterative code to print out the digital root of a number (you keep summing its digits until you get a 1-digit number). Here is my code:

package test;
 
public class DigitalRootTest {
	public static void main(String args[])
	throws java.io.IOException {
		System.out.println("Type in a whole number and press ENTER.");
		int sum = 0, num = (int) System.in.read();
		boolean failedinput = false;
		while(num != 13) { //I put this here to break the loop of skimming characters off the end of the buffer since a carriage return is 13 in Unicode
			if(num < 48) { //The digits 0-9 span the range 48-57 in Unicode
				System.out.println("Invalid input.  Please enter a whole number.");
				failedinput = true;
				break;
			} else if(num > 57) { //The digits 0-9 span the range 48-57 in Unicode
				System.out.println("Invalid input.  Please enter a whole number.");
				failedinput = true;
				break;
			} else {
				num -= 48; //Converting the digits from Unicode integers to their actual values
				sum += num; //Totalling up the digits as they're skimmed from the buffer
				num = (int) System.in.read(); // getting a new character
			}
		}
		System.out.println(sum);
		if(failedinput == false) {
			while(sum > 9) {
				int i, lastdigit, worksum = sum, lim = 1 + (int) Math.log10(sum);
/* i is a counting variable
    worksum is just a copy of sum that I can operate on safely without messing up the end result
    lastdigit is a variable just to hold the last digit of the current worksum as I work on it
    lim is a limit to be imposed on the for loop below, which is just the number of digits of the sum */
				sum = 0; //reset the sum
				for(i = 1; i <= lim; i++) {
					lastdigit = worksum % 10; //get the last digit of the worksum
					sum += lastdigit;
					worksum -= lastdigit;
					worksum = (int) worksum/10; //Note
				}
				System.out.println(sum);
			}
		}
	}
}

Note: I realised here I can omit the line where I take the last digit off the worksum since casting to an integer would lop this off anyway.

Can I get some feedback on this code? Can anyone spot a case in which it would fail (I haven't found one yet through putting in some large-ish numbers)? Also, how can I extend the code to send it back to the start after completing an operation? Would sticking it in an infinite for loop be okay or is that frowned upon?

That seems like a goofy way to get input. Why don't you use a Scanner instead?

Putting this inside a loop (such as a do-while loop) is reasonable. Just make sure you give the user a way to exit the loop, that way it's not an infinite loop anymore.

This is the first result for googling "java scanner tutorial": Scanning (The Java™ Tutorials > Essential Classes > Basic I/O)

Google is your best friend for these types of questions. Googling and reading tutorials and API documentation is a huge part of programming.

This is the first result for googling "java importing" and it contains the answer to your question: Using Package Members (The Java™ Tutorials > Learning the Java Language > Packages)