RandomPasswordGenerator.java

public class RandomPasswordGenerator {

    private RandomPasswordGenerator() {
    }

    private static final char[] LOWERCASES = createCharArray('a', 'z');
    private static final char[] UPPERCASES = createCharArray('A', 'Z');
    private static final char[] DIGITS = createCharArray('0', '9');
    private static final SecureRandom RANDOM = new SecureRandom();

    public static final String getRandomPassword(int length) {
        if (length < 0) {
            throw new IllegalArgumentException("Negative length: " + length);
        }
        char[] pa = new char[length];

        for (int i = 0; i < length; i++) {
            int d = RANDOM.nextInt(3);
            if (d == 0) {
                pa[i] = LOWERCASES[RANDOM.nextInt(26)];
            } else if (d == 1) {
                pa[i] = UPPERCASES[RANDOM.nextInt(26)];
            } else if (d == 2) {
                pa[i] = DIGITS[RANDOM.nextInt(10)];
            }
        }
        return new String(pa);
    }

    private static char[] createCharArray(int start, int end) {
        char[] ar = new char[end - start + 1];
        for (int i = 0; i < end - start + 1; i++) {
            ar[i] = (char) (start + i);
        }
        return ar;
    }