What is Callable and Future in Java concurrency
💡 Concept: Callable and Future
Callable and Future are used to perform tasks in threads that return results and may throw exceptions. They provide a more flexible alternative to Runnable.
📘 Quick Intro
Callable is a functional interface that returns a result and can throw checked exceptions. Future is used to retrieve the result of a Callable task.
🧠 Analogy
Think of Callable like ordering food online — you're promised a result. Future is the receipt that lets you check the status and get your food (result) when it's ready.
🔧 Technical Explanation
Callable<V>has acall()method that returns a value of type V.ExecutorService.submit(Callable)returns aFuture<V>.Future.get()blocks until the result is available or an exception occurs.Futurealso provides methods likecancel(),isDone(), andisCancelled().
🎯 Use Cases
- ✅ Performing parallel computations and aggregating results.
- ✅ Running background tasks that return values (e.g., file reading).
- ✅ Timed tasks where cancellation is required.
💻 Example: Callable and Future
import java.util.concurrent.*;
public class CallableExample {
public static void main(String[] args) throws Exception {
ExecutorService executor = Executors.newSingleThreadExecutor();
Callable task = () -> {
Thread.sleep(1000);
return "Task completed";
};
Future future = executor.submit(task);
System.out.println("Waiting for result...");
String result = future.get(); // blocks until result is ready
System.out.println("Result: " + result);
executor.shutdown();
}
}
❓ Interview Q&A
Q1: What does Callable interface do?
A: Represents a task that returns a result and may throw exceptions.
Q2: What does Future.get() do?
A: Waits for the result of a Callable task.
Q3: How is Callable different from Runnable?
A: Callable returns a result and can throw checked exceptions.
Q4: Can Future be cancelled?
A: Yes, using future.cancel().
Q5: What is isDone() in Future?
A: Checks if the task has completed.
Q6: Does Future support timeout?
A: Yes, get(timeout, unit) method exists.
Q7: Can Future be reused?
A: No, it's for one-time use per task.
Q8: Which package contains Callable and Future?
A: java.util.concurrent.
Q9: Can multiple threads use the same Callable instance?
A: Yes, if it's stateless or thread-safe.
Q10: How to execute multiple Callables?
A: Use invokeAll() with a list of Callables.
📝 MCQs
Q1. What does Callable return?
- Nothing
- Only boolean
- A result or exception
- Only exceptions
Q2. Which method executes a Callable?
- start()
- call()
- submit()
- execute()
Q3. What does Future.get() do?
- Returns immediately
- Throws exception
- Blocks until the task is complete
- Starts new thread
Q4. Which interface allows retrieving result from thread?
- Runnable
- Thread
- Future
- Executor
Q5. How do you cancel a task?
- Thread.stop()
- Executor.shutdown()
- future.cancel()
- task.abort()
Q6. Is Callable part of java.util.concurrent?
- No
- Yes
- Only in Java 11
- Only for lambdas
Q7. Can Callable throw checked exceptions?
- No
- Yes
- Only RuntimeException
- Only IOException
Q8. What does Future.isDone() check?
- Thread is alive
- Task is cancelled
- Whether the task is completed
- Task has errors
Q9. Which Executor method returns a Future?
- execute()
- invoke()
- start()
- submit()
Q10. What does invokeAll() do?
- Cancels tasks
- Executes Runnable
- Executes a list of Callables
- Creates threads
💡 Bonus Insight
Use CompletableFuture for more advanced async programming with chaining and non-blocking operations in Java 8+.
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