Java Asynchronous Programming

🚀 Introduction

🔄 What is Asynchronous Programming?

Asynchronous programming allows tasks to run in the background without stopping the main thread. This enables:

• 🧵 Non-blocking execution
• ⚡ Better resource use
• 💻 Smooth user interfaces
• 📈 High scalability for concurrent tasks
• Goal → better responsiveness and parallelism

🔁 Synchronous vs Asynchronous

Feature	Synchronous	Asynchronous
Execution	Blocking, sequential	Non-blocking, concurrent
Thread Usage	Single	Multiple
UI Responsiveness	May block	Remains responsive
Complexity	Simple	Requires more control
Error Handling	try-catch	callbacks, handlers

✅ When to Use Async Code

• I/O operations (network, file, DB)
• CPU-heavy tasks (math, ML)
• UI apps (to avoid freezing)
• Backend services (handle many requests)

👉 Example (Real-life analogy):

• Synchronous: Order coffee → wait until it’s made → then do other work.

• Asynchronous: Order coffee → work on laptop → barista notifies you when coffee is ready.

☎️ Callable Interface (Why not Runnable?)

⚖ Callable vs Runnable

Feature	Callable	Runnable
Returns value	✅ Yes (V)	❌ No
Throws Exception	✅ Yes (checked exceptions allowed)	❌ No (only unchecked)
Generics	✅ Supports generics	❌ No
Introduced in	Java 5	Java 1.0

🔍 Overview

• Callable<V> is like Runnable but:

  1. Can return a result (V)

  2. Can throw checked exceptions

• Designed for asynchronous tasks that produce a result.

✨ Key Features

• ✔ Returns result (V)

• ❗ Can throw checked exceptions

• 🧠 Uses generics for type safety

Future & Callable

• Runnable cannot return results.

• Callable can return a value + throw exceptions.

• Future holds the result of asynchronous computation.

☎️ Callable Interface (Why not Runnable)

import java.util.concurrent.*;

public class CallableExample {
    public static void main(String[] args) throws Exception {
        Callable<String> task = () -> "Hello from Callable!";

        FutureTask<String> future = new FutureTask<>(task);
        new Thread(future).start();

        String result = future.get(); // Waits for result
        System.out.println(result);
    }
}

🔮 Future Interface

Future represents the result of an async task, which will be available later.

🛠 Key Methods

• get() – wait for result
• cancel() – try to stop
• isDone() – check completion
• isCancelled() – check cancel

🚦 Example

Future<Integer> f = executor.submit(() -> 42);
Integer result = f.get(); // Waits here

⏳ Timeout Example

future.get(2, TimeUnit.SECONDS); // Throws TimeoutException if late

⚠️ Limitations of Future

• ❌ get() is blocking → doesn’t feel truly async

• ❌ No built-in chaining (cannot say do this after completion)

• ❌ No easy error handling (exceptions wrapped in ExecutionException)

• ❌ Cannot combine multiple futures (e.g., wait for both Task A & B)

👉 That’s why CompletableFuture (Java 8+) was introduced to overcome these issues.

✅ CompletableFuture

📚 Overview

CompletableFuture improves on Future by adding:

• ✅ Non-blocking chaining
• 🔗 Composability
• 💥 Error handling
• 🧪 Manual completion

🔑 Key Features

Feature	Description
thenApply	Transforms result
thenAccept	Consumes result (no return)
exceptionally	Handle errors
thenCombine	Combine multiple futures
allOf, anyOf	Wait for all/any futures

🧪 Basic Example

CompletableFuture.supplyAsync(() -> "Hi")
    .thenApply(data -> data.toUpperCase())
    .thenAccept(System.out::println);

🔁 Chaining & Error Handling

Use thenCompose() to chain dependent tasks, and exceptionally() to handle errors.

🔗 Combining Multiple Futures

thenCombine(f2, (a, b) -> a + b);
CompletableFuture.allOf(f1, f2).join();

🧵 Manual Completion

CompletableFuture<String> future = new CompletableFuture<>();
future.complete("Done"); // Manually set result