10- Handling Exceptions

Jul 04, 2026 00:57 · 4:15 · English · Whisper Turbo · 2 ተናጋሪ

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You'll learn how to consume the result of an asynchronous task.

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But what if this task throws an exception?

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For example,

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let's say we try to call a remote weather service to get the current weather in

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a given city.

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Now this remote call may fail for various reasons.

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So let's see how we can handle exceptions and recover by transforming

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an exception into a value.

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So I'm going to create a completable future object

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using the supply async method in the body

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of this lambda

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First we print a message like getting the current weather and

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then we throw a new Illegal state

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exception the type of the exception doesn't matter.

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I just want to throw an exception now this returns a

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completable future So let's run our program and see what happens We

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didn't see anything because this exception

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was thrown on a different thread to get this exception We have to call the

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get method of the future interface

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So if you look at the documentation of the future interface,

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you can see that the get method returns a value of type v and

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it may throw an exception of type interrupted exception or execution

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exception.

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So if an exception is thrown on the thread that is executing our task,

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the get method is able to propagate that exception and bring it into

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our main thread.

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Let me show you.

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So here we call future

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.get and then wrap this inside a try catch

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block

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then run our program,

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now we get the exception.

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So you are trying to get the current weather,

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but we got an execution exception.

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This execution exception is caused by an illegal state

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exception.

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So the exception that we threw in our lambda is wrapped inside

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an execution exception.

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So when we call the get method,

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you may get an interrupted exception.

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This happens if the thread is sleeping,

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but we try to interrupt it.

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Or we may get an execution exception.

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This happens if something goes wrong during the execution of our asynchronous

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task.

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Now we can get the actual cause of this exception by calling e

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.get cause.

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This will return the exception that we threw here.

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Now,

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we don't want our application to crash.

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So what should we do here?

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Well,

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in this case we can recover and provide a default value

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like the last temperature that we successfully read before.

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Let me show you how to do that.

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So first I'm going to delete this line.

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Before we call the get method,

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we call another method called exceptionally.

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Now look at the signature of this method.

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It takes a function that maps a throwable to a different

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type.

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We talked about throwables earlier in the section about exceptions.

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So this throwable class is the base or parent

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of all errors and exceptions in Java.

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So here we need to pass a lambda that maps this exception to a different type.

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So we can get this exception and map it to a numeric value like

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Let's imagine this is the last temperature that we read successfully.

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Now,

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when we call the get method,

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we get the result.

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That is the last temperature.

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So let's store it over here and then print it.

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We didn't get

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an exception this time.

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So this is the benefit of using the exceptionally method.

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Now,

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one thing you need to understand is that this method will return a new completable

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future.

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So this completable future is different from the one we originally

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created.

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With this method,

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we're programming this completable future to return a default value if

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an exception is thrown.

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So when we call the get method on this new completable future,

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we'll get that default value.

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If you call the get method on the original completable future,

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we'll get an exception.

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So this is something that you're going to see again and again throughout this section.

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A lot of methods in the completable future class return a new

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completable future.

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With this we can build a recipe for our asynchronous operations.

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