Generate 3-Letter Words

/* Write a program to generate and print all 3-letter words consisting of given set
 * of characters. For example if we have the characters 'a' and 'b',
 * all possible words will be "aaa", "aab", "aba", "abb", "baa", "bab", "bba" and "bbb".
 * The input characters are given as string at the first line of the input.
 * Input characters are unique (there are no repeating characters). */

import java.util.Locale;
import java.util.Scanner;

public class _02_Generate3_LetterWords {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Locale.setDefault(Locale.ROOT);
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter your letters in 1 String, without spaces:");
        String givenLetters = scan.next();

        boolean isValidLetters = checkAllGivenCharactersIsValidLetters(givenLetters);
        if (isValidLetters) {
            int i1, i2, i3;
            System.out.println("All 3-letter words consisting of given characters are:");
            for (i1 = 0; i1 < givenLetters.length(); i1++) {
                for (i2 = 0; i2 < givenLetters.length(); i2++) {
                    for (i3 = 0; i3 < givenLetters.length(); i3++) {
                        printTheThreeLettersWord(givenLetters, i1, i2, i3);
                    }
                }
            }
        } else {
            System.out.println("Error! - Invalid Input!!!");
        }
    }

    private static boolean checkAllGivenCharactersIsValidLetters(String chars) {
        for (int i = 0; i < chars.length(); i++) {
            char symbol = chars.charAt(i);

            // Checking characters is alphabetical letters:
            if (!Character.isLetter(symbol)) {
                return false;
            }

            // Check current letter for repeating:
            if (i > 0) {
                for (int j = i - 1; j >= 0; j--) {
                    if (chars.charAt(j) == symbol) {
                        return false;
                    }
                }
            }
        }

        return true;
    }

    private static void printTheThreeLettersWord(String givenLetters, int i1,
            int i2, int i3) {
        StringBuilder outputWord = new StringBuilder();
        outputWord.append(givenLetters.charAt(i1));
        outputWord.append(givenLetters.charAt(i2));
        outputWord.append(givenLetters.charAt(i3));

        System.out.print(outputWord + " ");
    }
}