Working with Exceptions

Created By: Geoffrey Challen
/ Updated: 2022-10-07

In this lesson we'll focus on how to use Java's exception handling mechanisms—both try-catch and throw. 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.

Warm Up Debugging Challenge

But! Let's warm up with another graded debugging challenge!

Re-throw and finally

Before we go on, let's look at a few further wrinkles in Java exception handling that we haven't touched on yet.


First, you may have wondered: what happens if you throw an exception inside a catch block? Let's try it:

You'll notice that the second exception is thrown out of the catch block. Of course, we can rewrap the entire try-catch in another try-catch:

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. The 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 catch, 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 return!

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!

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Exception Handling Patterns

Next let's look at a few common exception handling patterns.

Why Not assert?

Previously we've presented how to handle bad inputs using assert:

But here's the dirty truth about Java assertions: they are not on by default! If you examine the Java command line options you'll see that you need to pass a special flag to java to turn assertions on. Let's look at how that works in practice in a brief screencast:

Use a command-line example to demonstrate how assert is not always on.

assert is enabled in all of our playgrounds. However, in other Java 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 Exception rather than rely on assert.

Input Validation

However, we can replace many of the places where we had previously used assert with commonly available non-checked Java exceptions. When checking parameters to a method, IllegalArgumentException is usually what you want to use:

Note that because IllegalArgumentException is unchecked, you don't need to use throws to declare that the method throws it. And the compiler will not require that you catch it. This is good, since usually an IllegalArgumentException indicates that the inputs to a method contradict the documentation.

State Validation

When designing our Java 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:

Work through an example showing how to use IllegalStateException to catch problems with state validation spanning multiple methods.

Batching Failures

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:

Walk through how to avoid a bunch of null checks by wrapping everything in a try-catch.

Rethrowing Exceptions

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:

You can also use this technique to convert a checked exception to an unchecked exception:

Why would we do this? In some cases, even if the compiler will insist that we "handle" a checked exception, there really isn't anything reasonable to do other than crash. In that case, converting the checked exception to an unchecked RuntimeException (like IllegalStateException above) simplifies the error handling. Methods that call our method now don't themselves need to handle the exception, because it is unchecked.

Designing Your Own Exceptions

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.

Happily, extending Exception could not be easier:

Note that this creates a checked exception. Usually that's what you want, since there is usually a reasonable unchecked exception already available for you. But you can create new unchecked exceptions similarly by extending RuntimeException.

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Show how to complete the homework problem above. Feel free to cover multiple approaches!

More Practice

Need more practice? Head over to the practice page.