Java promise tutorial

Java promise tutorial

I'd be happy to provide a comprehensive tutorial on promises in Java!

What are Promises?

In programming, a promise is a fundamental concept in asynchronous programming that allows you to handle asynchronous operations in a more readable and maintainable way. A promise represents an operation that may not have completed yet (i.e., it's pending) but will eventually produce a value or throw an exception.

Why do we need Promises?

Before promises, dealing with asynchronous operations was tricky and error-prone. We used to use callbacks, which made our code hard to read and maintain. With promises, we can write asynchronous code that looks just like synchronous code! This makes it easier to handle errors, compose asynchronous operations, and write more robust code.

Creating a Promise

In Java, you don't have built-in support for promises like some other languages (e.g., JavaScript). However, we can create a promise using the CompletableFuture class from Java 8's java.util.concurrent package. Here's an example:

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class PromiseExample {
public static void main(String[] args) throws ExecutionException, InterruptedException {
CompletableFuture promise = new CompletableFuture<>();
// Simulate some asynchronous operation that takes 2 seconds to complete
new Thread(() -> {
try {
Thread.sleep(2000);
promise.complete("Hello World!");
} catch (InterruptedException e) {
promise.cancel(true);
}
}).start();
// Get the value of the promise when it's completed
String result = promise.get();
System.out.println(result);  // Output: "Hello World!"
}
}

In this example, we create a CompletableFuture and simulate some asynchronous operation that takes 2 seconds to complete. We then use the get() method to get the value of the promise when it's completed.

Working with Promises

Promises can be used in various ways:

Here's an example that demonstrates chaining and composing promises:

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class PromiseExample {
public static void main(String[] args) throws ExecutionException, InterruptedException {
CompletableFuture promise1 = new CompletableFuture<>();
// Simulate some asynchronous operation that takes 2 seconds to complete
new Thread(() -> {
try {
Thread.sleep(2000);
promise1.complete("Hello World!");
} catch (InterruptedException e) {
promise1.cancel(true);
}
}).start();
// Get the value of the first promise when it's completed
CompletableFuture promise2 = promise1.thenApply(s -> s.toUpperCase());
CompletableFuture promise3 = promise2.thenCombine(promise1, (s1, s2) -> s1 + " " + s2);
// Get the final result when all promises are completed
String result = promise3.get();
System.out.println(result);  // Output: "HELLO WORLD! HELLO WORLD!"
}
}

In this example, we create a chain of promises where each promise depends on the previous one. We use thenApply() to uppercase the result of the first promise and thenCombine() to combine the results of two promises.

Conclusion

Promises are a powerful tool for handling asynchronous operations in Java. By using CompletableFuture and chaining/composing promises, you can write more robust and maintainable code that's easier to read and debug. I hope this tutorial has been helpful in getting you started with promises in Java!

What is the difference between promise and Future in Java?

I'm happy to help you with that question!

In Java, Promise and Future are two distinct concepts that can be a bit confusing at first glance. Both are used for asynchronous programming, but they serve different purposes.

Promise

A Promise is an object that represents the result of an asynchronous operation. It's like a container that holds a value or an exception, which will be available when the operation completes. A Promise can be in one of three states:

When a Promise is created, it's initially in the "pending" state. As the operation progresses, the state can change to either "fulfilled" or "rejected". Once the operation completes, the Promise object becomes immutable.

Future

A Future is an interface that represents the result of an asynchronous operation. It's like a handle or a reference to a Promise. A Future provides methods for checking if the operation has completed and retrieving the result (if successful) or exception (if failed).

Here are some key differences between Promise and Future:

In summary:

I hope this helps clarify the differences between Promise and Future in Java!