Below is a simple solution printing all the permutations of a string. I wrote it in about 1 minute in order to test it out as an interview questions for co-op students. The solution can obviously be improved.

public class MyMain {

	static int permcount = 0;

	public static void main(String[] args) {
		String teststring = "abcd";
		String curString = "";

		MyMain.printPermutations(teststring, 0, curString);

	}

	static void printPermutations(String teststring, int curChar,
			String curString) {
		if (curString.length() == teststring.length()) {
			permcount += 1;
			System.out.println(permcount + ".) " + curString);
		}

		for (int i = 0; i < teststring.length(); i += 1) {
			String c = teststring.substring(i, i + 1);
			if (curString.indexOf(c) >= 0) {
				continue;
			}
			MyMain.printPermutations(teststring, curChar + 1, curString + c);
		}
	}
}