Bathroom Protocol Design

🧠 Problem Statement

A bathroom is being designed for shared use by both males and females in an office. The following constraints must be met:

• ❌ Men and women cannot use the bathroom at the same time.
• 🔢 A maximum of 3 employees can use the bathroom simultaneously.
• 🔒 The system must avoid deadlocks.
• ✅ Starvation is not considered in this version.

❗ Constraints

1. Gender Separation: Males and females cannot use the bathroom simultaneously
2. Capacity Limit: Maximum of 3 people can use the bathroom at once
3. Thread Safety: Must handle concurrent access from multiple threads safely
4. Deadlock Prevention: System must not deadlock under any circumstances
5. Fair Access: Should provide reasonable access to both genders
6. Resource Management: Must properly track occupancy and gender

📥 Input / Output

Input:

• Multiple threads representing employees of different genders
• Requests to enter and exit the bathroom
• Gender information for each employee

Output:

• Controlled access to bathroom based on gender and capacity
• Real-time occupancy tracking
• Proper synchronization without deadlocks

💡 Intuition About Solving This Problem

The key insight is that we need to maintain two pieces of state: the current gender using the bathroom and the current occupancy count. We use a single lock to protect both pieces of state and ensure atomic operations. The condition variable allows threads to wait efficiently when the bathroom is unavailable.

🚀 Steps to Take to Solve

1. Design the state variables: Track current gender and occupancy count
2. Choose synchronization mechanism: Use ReentrantLock with Condition for efficiency
3. Implement entry logic: Check gender compatibility and capacity before allowing entry
4. Handle gender switching: Reset gender when bathroom becomes empty
5. Implement exit logic: Decrement occupancy and signal waiting threads
6. Add safety checks: Ensure occupancy never goes negative

⚠️ Edge Cases to Consider

1. Gender switching: Handle when bathroom becomes empty and gender can change
2. Capacity overflow: Prevent more than 3 people from entering
3. Concurrent exits: Handle multiple people exiting simultaneously
4. Thread interruption: Handle InterruptedException properly
5. Negative occupancy: Prevent occupancy count from going below zero
6. Gender conflict: Handle when someone tries to enter with wrong gender

💻 Code (with comments)

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class BathroomAccessControl {
    private static final int MAX_CAPACITY = 3;

    public enum Gender { MALE, FEMALE }

    private int occupancy = 0;           // Current number of people in bathroom
    private Gender currentGender = null; // Gender currently using bathroom

    private final ReentrantLock lock = new ReentrantLock();
    private final Condition bathroomAvailable = lock.newCondition();

    public void enterBathroom(Gender gender) throws InterruptedException {
        lock.lock();
        try {
            // Wait until bathroom is available for this gender
            while (isBathroomUnavailable(gender)) {
                bathroomAvailable.await();
            }

            // If bathroom is empty, set the gender
            if (occupancy == 0) {
                currentGender = gender;
            }

            occupancy++; // Increment occupancy count
            System.out.println(gender + " entered. Current occupancy: " + occupancy);
        } finally {
            lock.unlock();
        }
    }

    public void exitBathroom() {
        lock.lock();
        try {
            if (occupancy > 0) {
                occupancy--; // Decrement occupancy count
                System.out.println("Person exited. Current occupancy: " + occupancy);

                // If bathroom becomes empty, reset gender
                if (occupancy == 0) {
                    currentGender = null;
                }

                // Signal waiting threads that bathroom is available
                bathroomAvailable.signalAll();
            }
        } finally {
            lock.unlock();
        }
    }

    private boolean isBathroomUnavailable(Gender gender) {
        // Bathroom is unavailable if:
        // 1. At maximum capacity, OR
        // 2. Different gender is currently using it
        return occupancy >= MAX_CAPACITY ||
               (currentGender != null && currentGender != gender);
    }

    public int getOccupancy() {
        lock.lock();
        try {
            return occupancy;
        } finally {
            lock.unlock();
        }
    }

    public Gender getCurrentGender() {
        lock.lock();
        try {
            return currentGender;
        } finally {
            lock.unlock();
        }
    }
}

🔍 Dry Run Example

Scenario: 2 males and 1 female trying to use bathroom

Initial State:

• Bathroom empty (occupancy = 0, currentGender = null)

Step 1: Male-1 wants to enter

• Check: isBathroomUnavailable(MALE) → false (empty bathroom)
• Set currentGender = MALE
• Increment occupancy to 1
• Male-1 enters bathroom

Step 2: Male-2 wants to enter

• Check: isBathroomUnavailable(MALE) → false (same gender, under capacity)
• Increment occupancy to 2
• Male-2 enters bathroom

Step 3: Female-1 wants to enter

• Check: isBathroomUnavailable(FEMALE) → true (different gender)
• Female-1 waits on bathroomAvailable condition

Step 4: Male-1 exits

• Decrement occupancy to 1
• currentGender remains MALE (bathroom not empty)
• Signal waiting threads

Step 5: Male-2 exits

• Decrement occupancy to 0
• Reset currentGender to null (bathroom now empty)
• Signal waiting threads

Step 6: Female-1 can now enter

• Check: isBathroomUnavailable(FEMALE) → false (empty bathroom)
• Set currentGender = FEMALE
• Increment occupancy to 1
• Female-1 enters bathroom

Result: Gender separation maintained, capacity limit respected, no deadlocks.

⏱️ Time Complexity / Space Complexity

Time Complexity:

• Enter bathroom: O(1) - constant time to check conditions and update state
• Exit bathroom: O(1) - constant time to update state and signal threads
• Wait time: O(1) - constant time to block/unblock threads

Space Complexity:

• State variables: O(1) - constant space for occupancy and gender
• Lock overhead: O(1) - constant space for ReentrantLock and Condition
• Thread management: O(1) - constant space regardless of number of threads

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🚻 Usage Example

public class BathroomDemo {
    public static void main(String[] args) {
        BathroomAccessControl bathroom = new BathroomAccessControl();

        for (int i = 0; i < 5; i++) {
            createEmployee(bathroom, BathroomAccessControl.Gender.MALE, "Male-" + i);
            createEmployee(bathroom, BathroomAccessControl.Gender.FEMALE, "Female-" + i);
        }
    }

    private static void createEmployee(BathroomAccessControl bathroom,
                                      BathroomAccessControl.Gender gender,
                                      String name) {
        new Thread(() -> {
            try {
                System.out.println(name + " waiting to enter bathroom");
                bathroom.enterBathroom(gender);
                System.out.println(name + " using bathroom");
                Thread.sleep((long) (Math.random() * 2000) + 1000);
                bathroom.exitBathroom();
                System.out.println(name + " left bathroom");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }).start();
    }
}

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This solution ensures:

• Mutual exclusion of genders 👥
• Bounded occupancy 🚻
• Deadlock-free locking 🔒