private static ArrayList<String> possibilities = new ArrayList<>();

	public static void main(String[] args) {
		String s = "abcd";
		loop(s, null);

		for(String string : possibilities)
			System.out.println(string);
	}

	private static void loop(String charSet, String potential) {
		for(int i = 0; i < charSet.length(); i++) {
			StringBuilder builder = new StringBuilder();

			if(potential != null)
				builder.append(potential);

			builder.append(charSet.charAt(i));

			String newPotential = builder.toString();

			String remaining = getRemainingChars(charSet, newPotential);

			if(remaining.length() > 0)
				loop(remaining, newPotential);

			if(charSet.length() == 1)
				possibilities.add(newPotential);
		}
	}

	private static String getRemainingChars(String charSet, String usedChars) {
		StringBuilder remainingBuilder = new StringBuilder();

		for(char c : charSet.toCharArray()) {
			if(usedChars.indexOf(c) == -1) {
				remainingBuilder.append(c);
			}
		}

		return remainingBuilder.toString();
	}