You’re simulating a rocket launch system.

Before the rocket can launch, it must pass several system checks (e.g., fuel, navigation, engine, weather, etc.).

Each system is checked independently by a different thread.

👉 Only after all checks are complete, the main launch system can proceed and say:

✅ "All systems go. Launching!"

Solution Approaches

Basic Thread Join Approach
1. Create threads - One thread for each system check (fuel, navigation, etc.)
2. Start all threads - All checks run simultaneously
3. Wait for completion - Main thread waits until all checks finish
4. Launch - Only proceed if all checks were successful

public class RocketLaunchBasic {

    // Task for checking individual systems
    static class SystemCheck implements Runnable {
        private final String systemName;

        public SystemCheck(String name) {
            this.systemName = name;
        }

        public void run() {
            try {
                // Simulate check time (0-2 seconds)
                System.out.println("⏳ Checking " + systemName + "...");
                Thread.sleep((long)(Math.random() * 2000));
                System.out.println(systemName + " ✅");
            } catch (InterruptedException e) {
                System.out.println(systemName + " ❌ (Interrupted)");
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        String[] systems = {"Fuel", "Navigation", "Engine", "Weather", "Comm"};

        // 1. Create threads for all systems
        Thread[] threads = new Thread[systems.length];
        for (int i = 0; i < systems.length; i++) {
            threads[i] = new Thread(new SystemCheck(systems[i]));
        }

        // 2. Start all checks in parallel
        System.out.println("🚀 Starting pre-launch checks...");
        for (Thread t : threads) {
            t.start(); // Launch parallel execution
        }

        // 3. Wait for all to complete
        for (Thread t : threads) {
            t.join(); // Block until thread finishes
        }

        // 4. Final verification
        System.out.println("\n🔍 All checks completed!");
        System.out.println("🚀 All systems go. Launching!");
    }
}

❌ Failure Test Cases

1. Thread Never Completes (Deadlock)

One system check hangs indefinitely. Lets say one of started thread never finish its run.

public void run() {
    if (systemName.equals("Fuel")) {
        while(true) { /* Infinite loop */ } 
    }
    // ... normal check ...
}

⏳ Checking Fuel...
⏳ Checking Navigation...
Navigation ✅ ...
(Program hangs forever)

To fix this, use join(timeout), which will only wait for specific amount of time

for (Thread t : threads) {
    t.join(3000); // Wait max 3 seconds
    if (t.isAlive()) {
        System.out.println("⏰ " + systemName + " timeout!");
        t.interrupt();
    }
}

2. Exception in System Check

Thread crashes during check.

public void run() {
    if (systemName.equals("Navigation")) {
        throw new RuntimeException("Sensor error");
    }
    // ... normal check ...
}

⏳ Checking Fuel...
⏳ Checking Navigation...
⏳ Checking Engine...
Exception in thread "Thread-1" java.lang.RuntimeException: Sensor error
Fuel ✅
Engine ✅

🔍 All checks completed!
🚀 All systems go. Launching! (⚠️ Despite failure!)

How we will fix this ?, will be covered in our course.

More Approaches and Failing Test Cases will be covered in our course.

Solution Approaches

Basic Thread Join Approach

❌ Failure Test Cases

🚀 Rocket Launch Countdown Sync

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