Callable and Future

01What are Callable and Future?

Callable is similar to Runnable, but its task can return a result and can throw a checked exception. Future represents the eventual result of a Callable task, which may still be running in the background when you first receive it.

02Why is it used?

Runnable cannot return a value, but many real tasks need to produce a result — like a calculation running in the background whose result you need later. Callable and Future together support this "run now, collect result later" pattern.

03Real-Life Example

Think of placing an online order and receiving a tracking number (the Future) immediately, without waiting for the actual delivery. Later, you can check the tracking number to see if your order (the actual result) has arrived.

java
Callable<ResultType> task = () -> {
    return result;
};

Future<ResultType> future = executor.submit(task);
ResultType result = future.get(); // waits for and retrieves the result

java
import java.util.concurrent.*;

public class CallableFutureDemo {
    public static void main(String[] args) throws Exception {
        ExecutorService executor = Executors.newSingleThreadExecutor();

        Callable<Integer> task = () -> {
            return 10 * 10;
        };

        Future<Integer> future = executor.submit(task);
        System.out.println("Result: " + future.get());

        executor.shutdown();
    }
}

Result: 100