Q

Q is a promise library for Java. It implements promises ("deferred objects") similar to the ones found in ECMAScript, Angular.js's $q, and Kris Kowal's Q.

Usage

When you want a method to return data that does not exist yet, create a Promise for that data and return the promise instead. The Q class acts as the controller for a promise object. Only the Q instance which created a promise may resolve or reject it.

public static Promise<Integer> getInt() {
	final Q<Integer> futureInt = Q.defer();
	new Thread("some long operation") {
		public void run() {
			if (something) {
				futureInt.resolve(5);
			} else {
				futureInt.reject("this is not the integer you are looking for");
			}
		}
	}.start();
	return futureInt.promise;
}

The receiver of a promise may then block on it, or specify a callback function to be executed when data is available or the promise is rejected.

public static void main(String... args) {
	Promise<Integer> futureInt = getInt();
	//do some other operations while we wait
	int i = futureInt.get();
	System.out.println("i is " + i);
}

In the last example, a PromiseRejectionException would have been thrown at the call of futureInt.get() if the promise had been rejected. Instead of blocking synchronously, we could have specified a callback to be executed on the resolving thread:

public static void main(String... args) {
	Promise<Integer> futureInt = getInt().then(new PromiseCallback() {
		public void resolved(int i) {
			System.out.println("i is " + i);
		}
		public void rejected(String err, Throwable cause) {
			System.err.println("Could not get int because of " + err);
			if (cause != null) cause.printStackTrace();
		}
	});
	//do some other operations while we wait
}