PasswordValidator.java

import java.util.ArrayList;
import java.util.List;

public class PasswordValidator {

    /**
     * Validates a password against the following rules and returns a list of
     * unmet requirements. An empty list means the password is valid.
     *
     * Rules (checked in this order):
     *   1. At least 8 characters long
     *   2. At least one uppercase letter (A-Z)
     *   3. At least one lowercase letter (a-z)
     *   4. At least one digit (0-9)
     *   5. At least one special character from: !@#$%^&*()-_=+[]{}|;':",.<>?/`~
     *   6. No whitespace characters allowed
     *
     * @param password the password to validate (may be null)
     * @return list of violation messages; empty if all rules pass
     */
    public static List<String> validate(String password) {
        List<String> errors = new ArrayList<>();

        if (password == null) {
            errors.add("Password must not be null");
            return errors;
        }

        if (password.length() < 8) {
            errors.add("Must be at least 8 characters long");
        }

        if (!password.matches(".*[A-Z].*")) {
            errors.add("Must contain at least one uppercase letter");
        }

        if (!password.matches(".*[a-z].*")) {
            errors.add("Must contain at least one lowercase letter");
        }

        if (!password.matches(".*[0-9].*")) {
            errors.add("Must contain at least one digit");
        }

        if (!password.matches(".*[!@#$%^&*()\\-_=+\\[\\]{}|;':\",.<>?/~].*")) {
            errors.add("Must contain at least one special character");
        }

        if (password.matches(".*\\s+.*")) {
            errors.add("Must not contain whitespace");
        }

        return errors;
    }

    // -------------------------------------------------------------------------
    // Test runner
    // -------------------------------------------------------------------------

    public static void main(String[] args) {
        int passed = 0, failed = 0;

        Object[][] tests = {
            // { input, expected violations (String[]) }

            // valid password
            { "Secure@99",        new String[] {} },
            // valid - special chars variety
            { "Hello#World1",     new String[] {} },
            // valid - minimum length exactly 8
            { "Abcde1@x",        new String[] {} },

            // too short
            { "Ab1@",            new String[] { "Must be at least 8 characters long" } },
            // missing uppercase
            { "secure@99",       new String[] { "Must contain at least one uppercase letter" } },
            // missing lowercase
            { "SECURE@99",       new String[] { "Must contain at least one lowercase letter" } },
            // missing digit
            { "Secure@xx",       new String[] { "Must contain at least one digit" } },
            // missing special char
            { "Secure999",       new String[] { "Must contain at least one special character" } },
            // contains whitespace
            { "Secure @99",      new String[] { "Must not contain whitespace" } },
            // multiple violations: too short, no uppercase, no digit, no special
            { "abc",             new String[] {
                                    "Must be at least 8 characters long",
                                    "Must contain at least one uppercase letter",
                                    "Must contain at least one digit",
                                    "Must contain at least one special character"
                                 } },
            // null
            { null,              new String[] { "Password must not be null" } },
            // empty string
            { "",                new String[] {
                                    "Must be at least 8 characters long",
                                    "Must contain at least one uppercase letter",
                                    "Must contain at least one lowercase letter",
                                    "Must contain at least one digit",
                                    "Must contain at least one special character"
                                 } },
        };

        for (Object[] test : tests) {
            String input          = (String)   test[0];
            String[] expectedArr  = (String[]) test[1];

            List<String> actual   = validate(input);
            List<String> expected = List.of(expectedArr);

            boolean ok = actual.size() == expected.size()
                      && actual.containsAll(expected)
                      && expected.containsAll(actual);

            String display = input == null ? "null" : "\"" + input + "\"";
            System.out.printf("  %-20s -> %s%n",
                display,
                ok ? "PASS" : "FAIL\n    expected: " + expected + "\n    actual:   " + actual);

            if (ok) passed++; else failed++;
        }

        System.out.printf("%nResults: %d passed, %d failed%n", passed, failed);
    }
}