In this lesson we'll focus on how to use Kotlin's exception handling mechanisms—both
We'll also introduce a few new wrinkles to round out our understanding of this particular system.
Our focus is on real-word patterns for working with and handling errors.
But! Let's warm up with another graded debugging challenge!
Before we go on, let's look at a few further wrinkles in Kotlin exception handling that we haven't touched on yet.
First, you may have wondered: what happens if you throw an exception inside a
Let's try it:
You'll notice that the second exception is thrown out of the
Of course, we can rewrap the entire
try-catch in another
Note that exceptions thrown inside a
catch block cannot be caught by the same
try-catch in which they were thrown.
So this doesn't work:
try-catch blocks can include an additional component: a
finally block is always executed, regardless of whether an exception was thrown or not.
Let's see how that works:
If you run this a few times, you'll notice that regardless of whether we complete the
try block successfully or enter the
finally block is always executed.
One of the cool things about
finally is that it is always executed.
Even if the try includes a
This feature of
finally makes it useful when a method needs to do some kind of cleanup before exiting.
We haven't run into this scenario yet, but you will sometimes.
And when you do,
finally will be there for you!
Next let's look at a few common exception handling patterns.
assert in the past, particularly when writing test cases.
It's fine for that, but there's something about Kotlin
assertions you need to know: they are not on by default!
assert is enabled in all of our playgrounds.
However, in other Kotlin environments—such as on Android—it may be hard or impossible to enable assertions.
While they can be useful during testing, the right thing to do in most cases is to throw an
check, rather than rely on
However, we can replace many of the places where we had previously used
assert with Kotlin's convenient
When checking parameters to a method, use
When designing our Kotlin classes, there are times when we want to enforce patterns of usage that span multiple methods. To make that more concrete, let's look at an example together:
Sometimes our code needs to take a series of steps to complete some action.
If any of those steps fail, the entire operation fails.
This can be a good place to use a
try-catch to avoid having to do a lot of error checking after each step.
Let's look at an example of this:
Sometimes when an error occurs we just want to log that it happened, but then let it continue to propagate. We can do this by rethrowing the error out of the catch block:
Once you start writing your own libraries and sharing code with others, it can be helpful to design your own custom exceptions.
One reason to do this is that it allows users of your code to better understand what went wrong.
And to do more accurate error handling by establishing multiple
catch blocks that deal with different kinds of exceptions.
Exception could not be easier:
Need more practice? Head over to the practice page.