Explain Runnable vs Callable and thread-safe Singleton

## Concurrency & Design Patterns (Java) 1. In Java concurrency, what is the difference between `Runnable` and `Callable`? - When would you choose one over the other? - How do you retrieve results and handle exceptions for each? 2. Name one or two design patterns you have used in real projects. 3. For the Singleton pattern, explain what it means for a Singleton to be **thread-safe**. - Why can a naïve lazy Singleton be unsafe under concurrency? - Describe at least one correct way to implement a thread-safe Singleton in Java and why it works.