What is a Race Condition?

Copy codevar balance = 100

// Thread 1: Withdraw 80
fun withdraw(amount: Int) {
    if (balance >= amount) {     // Check
        Thread.sleep(10)          // Simulate delay
        balance -= amount         // Update
    }
}

// Thread 2: Withdraw 80
// Both threads check balance=100, both proceed
// Final balance: -60 or 20 (undefined!)

Copy codeTime →
Thread1: if(100>=80)✓ ----delay---- balance=100-80=20
Thread2:    if(100>=80)✓ ----delay---- balance=20-80=-60
                                     ^ Race condition!

Copy code// Both may read old value, one write is lost
prefs.edit().putInt("count", prefs.getInt("count", 0) + 1).apply()

Copy codeobject Singleton {
    var instance: Database? = null
    fun get(): Database {
        if (instance == null) {          // Thread 1 & 2 both see null
            instance = Database()        // Two instances created!
        }
        return instance!!
    }
}

Copy code// May crash if activity is destroyed
thread {
    val data = fetchData()
    textView.text = data  // Not on UI thread + activity may be gone
}

Copy code@Synchronized
fun withdraw(amount: Int) {
    if (balance >= amount) balance -= amount
}

Copy codeval balance = AtomicInteger(100)
balance.addAndGet(-80)

Copy codeval mutex = Mutex()
suspend fun withdraw(amount: Int) {
    mutex.withLock {
        if (balance >= amount) balance -= amount
    }
}