public String generate() {
        StringBuilder sb = new StringBuilder();

        sb.append(firstCharacterAlphabet.charAt(rng.nextInt(firstCharacterAlphabet.length())));

        for (int i = 1; i < length; i++) {
            sb.append(alphabet.charAt(rng.nextInt(alphabet.length())));
        }

        return sb.toString();
    }
@Test
    public void firstLetterAlphabet() {
        PasswordGenerator generator = new PasswordGenerator(10, "a", "b");
        String password = generator.generate();
        assertThat(password.substring(0, 1), is("a"));
        assertThat(password.substring(1, 10), is("bbbbbbbbb"));
    }