4- Callables and Futures

Jul 03, 2026 22:59 · 5:00 · English · Whisper Turbo · 2 Гучні

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Sometimes we want to return a value from a task.

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For example,

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we may call the Twitter API to get the latest tweets for a given user.

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That's when we use the callable interface.

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If you look at the signature of the submit method,

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you can see that this method is overloaded.

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So far,

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we've been passing a runnable object to this method,

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but we can also pass a callable object.

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This callable is an interface very similar to the runnable interface,

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but this interface represents a task that returns a value.

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So, if you scroll down,

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you can see this method has a single abstract method called call.

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This is similar to the run method of the runnable interface,

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but instead of returning void,

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it returns v where v is the generic type parameter

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here.

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So,

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if we return a value in the body

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of this lambda, now this lambda matches a method with no parameters that

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returns a value.

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That is our call method here.

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So this lambda expression that we have passed here represents a callable object.

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Now,

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we store the result in a variable called future.

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Let's look at the type of this variable.

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That is a future of integer.

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This future is an interface that represents the future result of an operation.

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An operation that will complete in the future.

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It may complete in 500 milliseconds or 2 seconds or 5

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seconds, sometime in the future.

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So the result of this operation is not going to be calculated immediately using

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the CPU.

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So with this future object,

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we can get the future result of an operation.

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Okay?

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Now,

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to make this demo more interesting,

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let's simulate a long running operation.

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So,

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In this package,

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the executors package,

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I'm going to add a new class called longTask.

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In this class we're going to add a public static method,

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simulate,

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and here we want to put the current thread to sleep for 3 seconds.

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So thread to sleep for 3 seconds.

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Now let's handle the interrupted exception.

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Now,

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back in our demo class,

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In this task,

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instead of printing the name of the current thread,

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I'm going to call longTask .simulate.

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So we're simulating an operation that's going to take 3 seconds.

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Let's say we're going to call the tweeter API to get the latest tweets for a user.

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Eventually this task is going to return a value,

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in this case 1.

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Now,

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when we call the submit method,

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the submit method is going to return immediately.

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So it's not going to wait for 3 seconds.

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It's going to immediately return a future object.

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With this future object,

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we can get the result of this operation.

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So this object has a bunch of useful methods.

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Get,

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we can use this to get the value of this operation.

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The get is overloaded.

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With this version,

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we can pass a timeout.

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So if that operation is going to take so long,

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perhaps we don't want to wait forever.

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We want to timeout in,

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let's say, two seconds.

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We can also call the get method without a timeout value.

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We have cancel for canceling an operation.

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we have is cancelled with this we can check to see if this operation is cancelled

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and is done which tells us if this operation is complete or not.

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So I told you that this submit method is going to return immediately.

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So here we can do some more work.

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Let's say do more work and at some point

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you really need the result of this operation.

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That is when we call the get method.

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Now this method is going to block the current thread and have it wait until the

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result of this operation is ready.

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Eventually,

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it's going to return a value.

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In this case,

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it's going to return an integer because our task returns

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an integer.

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So let's store the result over here.

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Take a look.

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The type of this variable is integer.

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Okay?

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Now here we need to handle a couple of exceptions.

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One of them is interrupted exception that you have seen before.

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The other is execution exception.

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And this happens if this task throws an exception.

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So we can cache this exception over here and do something with it.

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Now in this demo I'm going to simplify this code and combine

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these catch blocks.

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So vertical bar,

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execution,

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exception.

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Now let's get rid of the second catch block.

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So, when I run this program,

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you will see that we immediately see this message on a terminal because the submit

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method is not going to wait for the completion of this task.

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It's going to start this task on a separate thread and eventually when we

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call future .get,

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we have to wait for this operation to complete.

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Now we get the result and print it.

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Let's see what happens.

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So,

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we see do more work,

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now 1,

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3 seconds later,

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the result is ready and we print it over here.

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