RuntimeException & Error

Question

In the exceptions hierarchy, the descendants of RuntimeException and those of Error are runtime exceptions/errors.

The difference between the two is: Those under RuntimeException are the ones caused by poor programming/design, and those of Error are the ones that can't/shouldn't be controlled by the developer.

For coding an exception within the application, for instance, to throw an exception when something in the business logic occurs, the RuntimeException is extended.

The question is, what exactly is the difference between extending RuntimeException and extending Error-- except that extending Error is bad practice?

Who says that it is bad practice? Certainly not something you'd do every day, but I don't doubt that there are cases where it might be merited. — Hot Licks
– Hot Licks, Commented Dec 9, 2013 at 2:00
@HotLicks Joshua Bloch recommends to do so in his book Effective Java: "While the Java Language Specification does not require it, there is a strong convention that errors are reserved for use by the JVM to indicate resource deficiencies, invariant failures, or other conditions that make it impossible to continue execution. Given the almost universal acceptance of this convention, it’s best not to implement any new Error subclasses. Therefore, all of the unchecked throwables you implement should subclass RuntimeException (directly or indirectly)." — Nir Alfasi
– Nir Alfasi, Commented Jan 9, 2014 at 20:06
@alfasin - So the XYZ operating system might detect a hardware failure while retrieving the system clock, but it shouldn't call that an Error? An API that controls the valves in a chemical plant might detect a hardware failure and again should not call it an Error? — Hot Licks
– Hot Licks, Commented Jan 9, 2014 at 20:24
@HotLicks In the same chapter, Joshua says that RuntimeException should indicate "programming errors", for example, invalid pre-conditions when calling a method. The specific case you describe (HW failure) may potentially crash the JVM (in which case, an Error will be raised) but anyways, it does not sound like "programming error". I guess that the answer depends on how such a problem will effect your program/OS. It's important to remember that there's no behavioral difference between RuntimeException and Error - only semantic which is why RuntimeException would probably still fit. — Nir Alfasi
– Nir Alfasi, Commented Jan 9, 2014 at 23:54
@Hot Licks this convention, like any convention, is arbitrary. — Nir Alfasi
– Nir Alfasi, Commented Jan 10, 2014 at 5:44

Genhis · Accepted Answer · 2017-06-03 13:57:20Z

28

Both Error and RuntimeException are unchecked exceptions, meaning that it indicate a flaw with the program, and usually should not be caught. (NullPointerException, IndexOutOfBoundsException, etc.)

I think the main difference between the two is that RuntimeException indicate there is a error with the program, and an Error is something that is fatal but out of the program's control. (OutOfMemorryError, ThreadDeath, etc.)

Therefore subclassing an Error is bad practice because an error is usually not something that could be fixed by your program at runtime. In your program, should you need to throw something, use an Exception.

edited Jun 3, 2017 at 13:57

Genhis

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answered Dec 9, 2013 at 1:51

Pita

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2 Comments

Pita Over a year ago

i bolded the part that i think should answer your question

Marcus Junius Brutus Over a year ago

Something else that cannot be fixed by your program at runtime are bugs in the code. Therefore assertion exceptions can legitimately subclass Error (or better yet, AssertionError which is an Error subclass).

Sotirios Delimanolis · Accepted Answer · 2013-12-09 01:58:28Z

The Q is, what exactly is the difference between extending RuntimeException and extending Error-- except that extending Error is bad practice?

You've already mentioned the main differences. The Java Language Specification says the same thing in different terms. For Error, it states

Error is the superclass of all the exceptions from which ordinary programs are not ordinarily expected to recover.

For RuntimeException, it states

The class RuntimeException is a direct subclass of Exception. RuntimeException is the superclass of all the exceptions which may be thrown for many reasons during expression evaluation, but from which recovery may still be possible.

What you should take away from these quotes is that you will commonly see

try {
   ...
} catch (Exception e) { // catches RuntimeException
   ...
}

as a catch all case since Exception is a super type of RuntimeException. But you will almost never see (I've never seen it)

try {
   ...
} catch (Error e) {
   ...
}

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