The exception hierarchy in Java: what are the different types of Exception in Java and when do you use them?

In our introduction to exceptions, we mentioned that Java exceptions are also objects. Since they're objects, different types of exceptions can have different Exception subclasses. As an example, we mentioned that FileNotFoundException is an extension of IOException. Why would we create exceptions as subclasses of other "master" Exception classes in this way? Firstly, creating such a hierarchy can be extremely useful when it comes to catching exceptions. If we catch IOException, then by implication, we also catch FileNotFoundException. The programmer can decide what level of granularity they need when organising their catch blocks.

In addition to any hierarchy that may be defined within a particular program or library, Java exception classes are organised into a fundamental hierarchy at the very top level. There is a basic exception class called Exception as you might expect. But in fact, the base of the hierarchy starts not with Exception but with a class called Throwable, which is then subclassed into Exception and Error. Part of this fundamental hierarchy is illustrated in Figure 1.

Figure 1: The base of the Java exception hierarchy. Types in red, and their subclasses, represent unchecked exceptions which do not need to be explicitly declared and caught (see below).

Why are there different base exception classes?

Why are there different types of base exception classes set up in this way, and which type of "exception" should you use when? The answer comes down to how your program expects to handle (or not) the condition in question. As a general guide, this is how you should generally use the different exception class types:

• Exception subclasses represent errors that a program can reasonably recover from. Except for RuntimeException and its subclasses (see below), they generally represent errors that a program will expect to occur in the normal course of duty: for example, network connection errors and filing system errors.
• Error subclasses represent "serious" errors that a program generally shouldn't expect to catch and recover from. These include conditions such as an expected class file being missing, or an OutOfMemoryError.
• RuntimeException is a further subclass of Exception. RuntimeException and its subclasses are slightly different: they represent exceptions that a program shouldn't generally expect to occur, but could potentially recover from. They represent what are likely to be programming errors rather than errors due to invalid user input or a badly configured environment.

In reality, there are exceptions to these uses. For example, there are cases where the organisation of the JDK libraries means that we have to catch an exception that we essentially know will never occur. And there are some cases where we've "got nothing to loose" by trying to catch an Error, even though we may not actually be able to in practice.

Checked vs unchecked exceptions

We mentioned above that certain exceptions are generally indicative of a programming error rather than errors that we'd expect to occur in the normal course of events. (Whereas, it is reasonable for the programmer to expect that a networking or file system error will occur from time to time.) We also mentioned that subclasses of Error are not necessarily programming errors, but still errors that we wouldn't really expect to occur in the normal course of events.

A feature built into the Java language is that Errors and RuntimeExceptions (and their subclasses– marked in red in Figure 1) are what are called unchecked exceptions:

• unchecked exceptions can be thrown "at any time";
• methods don't explicitly have to declare that they can throw an unchecked exception;
• callers don't have to handle them explicitly.

On the next page, we'll look in more detail at unchecked exceptions in Java.

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