Next we’ll continue exploring Java exceptions.
First, we’ll look at how we can
throw our own exceptions.
Next, we’ll discuss the different types of Java exceptions and how they are used.
Aren’t errors fun?
Let’s do this.
Last time we looked at
try-catch, Java’s syntax for handling exceptions.
But where do exceptions come from?
Some exceptions are generated directly by Java.
For example, executing this code will generate a
However, Java also provides a way for us to
throw exceptions directly—the
Let’s see how that works.
Java has several built-in exceptions that you may find useful in your own code.
throw is particularly useful when there is no valid way to continue.
Maybe your method has received bad input, or encounters a situation that it is unprepared to handle.
We’ll discuss this more in future lessons.
As a initial example, many of the situations that we’ve been handling using
assert are better handled using the
Let’s look at an example:
Create a public class called
Exceptioner that provides one
exceptionable accepts a single
int as a parameter.
You should assert that the int is between 0 and 3, inclusive.
int is 0, you should return an
If it's 1, you should return a
If it's 2, you should return a
And if it's 3, you should return a
In Java the
Throwable class is the superclass of all objects that can be thrown, either using
throw or in case of an error like a null pointer exception.
Throwable has three important subclasses that are handled differently in Java: checked exceptions, unchecked exceptions, and errors.
Let’s look at each category in turn. First, let’s browse the documentation together!
Checked exceptions in Java must be handled.
The compiler will check and enforce this!
They are always generated using
throw, which is how the compiler knows where they are and when you need to handle them.
Exception class itself is checked, as are most of its descendants.
Let’s see a example of checked examples.
(Note that this example uses a playground that works a bit differently than the ones that we’ve used previously. I’ll explain why in the walkthrough.)
Unchecked exceptions don’t need to be handled. But, the reason for this is that, in many cases, they arise from programmer error. Let’s look at some examples.
The final category are Java errors. These are serious problems that you should not try to handle. According to the documentation:
An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.
However, it is worth noting that
assert generates an error.
It is also possible to generate errors through programming errors, like this one:
This generates a
You may get much more familiar with this kind of error soon…
Create a class
CapacityManager to manage restaurant dining capacity.
You should provide a public constructor that allows the capacity of the restaurant to be set as an
int when the
CapacityManager is created.
You don't need to provide either a getter or a setter for the capacity, which will not change.
Provide two methods:
Each accepts a count of guests (as an
int) that are either arriving (
enter) or departing (
leave) from the
Both should return the number of guests after the operation completes as an
If the count is negative or zero, both methods should throw an
enter should also throw a
IllegalStateException if the new party would exceed the capacity of the restaurant.
This restaurant also refuses to admit parties larger than 8, and if one arrives
enter should throw a
PartyTooLarge exception, which you can create without importing:
Note that this is a checked exception!
leave should also throw an
IllegalStateException if a party leaves that is larger than 8, or if the leave
would cause the number of diners to fall below zero, since both represent mistakes in our
leave, if an exception is thrown, the number of diners should not be changed.
When you are finished, here is how your class should work:
Do not use
assert to solve this problem!
Now that we know how to
throw exceptions, we'll utilize that ability in many of the places that we used
Need more practice? Head over to the practice page.