Difference between Job and Deferred in Coroutines Kotlin

Question

I am new to coroutines, I understand launch and async but still confusing part is Deferred. What is Deferred? and difference between Job and Deferred. Clear explanation and example is more helpful. Thanks in advance.

Answer 1

So job is sort of an object that represents a coroutine's execution and is related to structured concurrency, e.g. you can cancel a job, and all the children of this job will be also cancelled.

From docs:

Job is a cancellable thing with a life-cycle that culminates in its completion.

Deferred is some kind of analog of Future in Java: in encapsulates an operation that will be finished at some point in future after it's initialization. But is also related to coroutines in Kotlin.

From documentation:

Deferred value is a non-blocking cancellable future — it is a Job that has a result.

So, Deferred is a Job that has a result:

A deferred value is a Job. A job in the coroutineContext of async builder represents the coroutine itself.

An example:

someScope.launch {
    val userJob: Deferred<User> = async(IO) { repository.getUser(id) }
    //some operations, while user is being retrieved 
    val user = userJob.await() //here coroutine will be suspended for a while, and the method `await` is available only from `Deferred` interface
    //do the job with retrieved user
}

Also, it is possible to structure this async request with an existing scope, but that is a talk of another question.