Main.java

public class Main {

    public static void main(String[] args) {
		   Scanner key = new Scanner(System.in);
       String num = promptForInput(key);

       while(true) {
         System.out.println("The decimal value " + num + " is " + binary(num) + " in binary.");
         num = promptForInput(key);
       }
    }       

  public static String promptForInput(Scanner key) {
    System.out.println();
    System.out.println("Enter an integer value (negative value to quit): ");
    String str = key.nextLine();
    if(Pattern.matches("[a-zA-Z]+", str)) {
      System.out.println("Error - value must contain only digits");
      //retry input prompt
      return promptForInput(key);
    }
    Integer num = Integer.parseInt(str);
    if(num < 0) {
      key.close();
      System.exit(0);
    }
    return num.toString();
  }

	public static String binary(String in) {
		int i = Integer.parseInt(in);
		String bin = "";
    Integer currentValue = i;
    while (currentValue != 0) {
      bin = (currentValue % 2) + bin;
      currentValue = currentValue / 2;
    }
    return bin;
	}
}