gistfile1.java

import java.util.*;

class Untitled {
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		Integer number = scanner.nextInt();
		kaprekar(number);
	}
	
	public static boolean kaprekar(Integer numberInteger) {
		String numberString = numberInteger.toString();
		
		int[] digits = new int[4];
		for (int i = 0; i < 4; i++) {
			digits[i] = Integer.parseInt("" + numberString.charAt(i));
		}
		
		Arrays.sort(digits);
		
		String smallestString = "" + digits[0] + digits[1] + digits[2] + digits[3],
		       biggestString  = "" + digits[3] + digits[2] + digits[1] + digits[0];
		int smallest = Integer.parseInt(smallestString),
		    biggest  = Integer.parseInt(biggestString);
		
		int difference = biggest - smallest;
		System.out.println("Differenz: " + difference);
		
		if (difference == 6174) {
			return true;
		} else {
			return kaprekar(difference);
		}
	}
}