import java.util.Objects;
import java.util.Locale;

public final class NullPointerDebuggingLab {
    private NullPointerDebuggingLab() {}

    static final class Address {
        private final String city;

        Address(String city) {
            this.city = city;
        }

        String getCity() {
            return city;
        }
    }

    static final class Student {
        private final Address address;

        Student(Address address) {
            this.address = address;
        }

        Address getAddress() {
            return address;
        }
    }

    static String cityLabel(Student student) {
        Objects.requireNonNull(student, "student must not be null");

        Address address = student.getAddress();
        if (address == null) {
            return "City unavailable";
        }

        String city = address.getCity();
        return city == null || city.trim().isEmpty()
            ? "City unavailable"
            : city.toUpperCase(Locale.ROOT);
    }

    private static void expect(String expected, String actual) {
        if (!expected.equals(actual)) {
            throw new AssertionError(
                "Expected '" + expected + "' but received '" + actual + "'"
            );
        }
    }

    public static void main(String[] args) {
        expect("JAIPUR", cityLabel(new Student(new Address("Jaipur"))));
        expect("City unavailable", cityLabel(new Student(null)));
        expect("City unavailable", cityLabel(new Student(new Address(null))));
        expect("City unavailable", cityLabel(new Student(new Address(""))));
        expect("City unavailable", cityLabel(new Student(new Address("  "))));

        try {
            cityLabel(null);
            throw new AssertionError("Expected a NullPointerException");
        } catch (NullPointerException exception) {
            expect("student must not be null", exception.getMessage());
        }

        System.out.println("All null-contract checks passed.");
    }
}