Java.Multithreading.What is FutureTask?

📦 What is FutureTask?

✅ FutureTask<V> is a class that implements both Runnable and Future<V>, allowing you to:

• Wrap a Callable or Runnable task,
• Run it in a thread or executor,
• And retrieve a result later using Future methods.

🔧 It’s in java.util.concurrent

public class FutureTask<V> implements RunnableFuture<V> { ... }

Where:

• RunnableFuture<V> extends both Runnable and Future<V>

So a FutureTask is:

• A task you can execute (because it’s Runnable)
• A handle to get the result later (because it’s Future)

✅ Real-World Example

Callable<String> task = () -> {
    Thread.sleep(1000);
    return "Hello from Callable";
};

FutureTask<String> futureTask = new FutureTask<>(task);

// Run with Thread
new Thread(futureTask).start();

// Get the result
String result = futureTask.get(); // blocks until done
System.out.println(result);

🟢 Output after 1 second:

Hello from Callable

🧠 Why Use FutureTask?

Feature	Description
✅ Combines Runnable + Future	Can run the task and get result
✅ Works with Executors or Threads	Flexible execution
✅ Can be cancelled	Via futureTask.cancel(true)
✅ Thread-safe result caching	Only runs once, result is stored

🔁 Common Use Cases

• Wrapping Callable for custom thread usage
• Lazy initialization with result caching
• Manually managing task lifecycle (run, cancel, get)
• Submitting to thread pools and getting a result

---

📌 Bonus: Runnable Version

You can also wrap a Runnable with a predefined result:

Runnable runnable = () -> System.out.println("Runnable executed");
FutureTask<String> futureTask = new FutureTask<>(runnable, "Task Done");

new Thread(futureTask).start();
System.out.println(futureTask.get()); // prints: Task Done

⚠️ Notes

Caveat	Detail
get() blocks	Unless task is finished or timed out
Only executes once	Even if run() is called multiple times
Canceling is optional	You can call cancel() before/during execution

🧠 Summary

Trait	FutureTask
Type	Class implementing Runnable and Future
Use	Run task + get result later
Works with	Threads, Executors, manually
Handles	Callable, or Runnable + result
Result access	Via get(), isDone(), cancel()