What is the use of Callable in Java?

Callable allows you to create threads that return a result and can throw checked exceptions.

What is a Future in Java?

Future represents the result of an asynchronous computation that can be retrieved once the task completes.

What is Callable and Future in Java Concurrency | FullStackPrep.Dev

💡 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 a call() method that returns a value of type V.
ExecutorService.submit(Callable) returns a Future<V>.
Future.get() blocks until the result is available or an exception occurs.
Future also provides methods like cancel(), isDone(), and isCancelled().

🎯 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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What is Callable and Future in Java concurrency