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I would like to ask a code style question around Java Exceptions. I am using Java to call a C/C++ library using JNI. The convention in the library I am using is that most of the methods I can call will throw the same exception type for all errors. Let's call that a LibException. Now LibException is really a wrapper exception for a multitude of errors that can come up ranging from authentication problems, connection problems or more serious problems like corrupt input etc. LibException also contains an error code int as well as the error description string.
Even more confusingly LibException can also wrap one of my own exceptions if I throw it at the library in a callback! What I mean by that is that I sometimes provide a callback method, it is called by the library and I sometimes have to throw an exception in the callback. In that case the library picks up the exception, wraps it up in a LibException and throws it back at me in the original method call.
I would like each of the underlying problem to be handled differently. Authentication problems need to be shown to the user so he can retry, users should be notified of connection problems, but more serious problems may have to trigger my diagnostics report system (an automated mechanism that can send me parts of the logfile for debugging) and of course any exceptions I throw towards the library in the callback need to be rethrown as the original exception type.
Since I am calling different methods at multiple locations, I thought it would be a good idea to put some structure around the exception handling of LibExceptions. This is to avoid code repetition but most importantly to make sure that the different exception types are handled properly and that future me does not forget to, for example, notify the user that authentication failed.
I have tried a bunch of approaches but I am not entirely happy with the code structure I get so I would like some ideas on best practices from the community.
Static method that includes logic to sort the exceptions and throw a bunch of other exceptions
public static void handleException(LibException e) throws AuthenticationException, ConnectionException, SeriousException, MyException (MyException would be my exception that I throw in the callback)
+ve This works well in that it forces the handling of all the thrown exceptions.
+ve If a new exception type is added, the compiler would force me to handle the new exception
-ve Even though handleException() is a method that always throws an exception, the compiler (rightly) does not know this. This means that if we use handleException() in a method that has to return something, the compiler complains that a return type was missed. To make the compiler happy I have to throw another exception right after calling handleException() so the compiler understands that it will not be getting a return value because an exception is definitely thrown.
-ve I don't like the line that looks like Handler.handleException(libEx) since it's not entirely obvious that it throws a bunch of exceptions.
-ve Hard to customize the exception messages with context from the location the exception occurred (i.e. which URL we could not connect to).
A method that returns an enum with the different types of exceptions that I defined. Based on the enum I can then create the different exceptions to throw.
public static ExceptionTypeEnum Handler.
+ve Since I create the exceptions, I can now customize some error messages with extra context
-ve I could still forget to handle an enum (especially if I create a new category in the future)
-ve I still need a bunch of custom code to create the exceptions at every place I catch a LibException
Similar to #2 but instead of an enum return I could have multiple methods such as isAuthenticationException(libEx) or isConnectionProblem(libEx) and then accordingly throw the exceptions myself.
-ve I would definitely forget to handle all sections properly especially if a new exception type is added later.
The exception handler could return an exception to throw up. But since we throw a number of different exceptions the getException() method would have to return the Exception base class. This means that exception handlers would have to have prior knowledge of what exceptions could be thrown and also catch the Exception class making my exception handling more difficult.
Now in case I confused everyone with this question, I guess what I am trying to find an elegant solution to is similar to the problem described on section 'Checked exceptions inappropriately expose the implementation details' on page http://www.ibm.com/developerworks/java/library/j-jtp05254/index.html
Does the community have any other suggestions of a coding method to properly and elegantly handle these LibExceptions?
Should exceptions without messages ever be thrown? In what cases? For example, when subclassing Exception, should a constructor with no parameters even be given?
public class LexerException extends Exception {
public LexerException(String message) {
super(message);
}
}
or
public class LexerException extends Exception {
public LexerException() {
super();
}
public LexerException(String message) {
super(message);
}
}
Should exceptions without messages ever be thrown? In what cases?
An exception without a message will be thrown any time some code instantiates and throws an exception without a message. Anyone can write code like that.
Of course, if the exception that you are trying to throw does not allow you to instantiate it without a message ... or a with null message ... then you can't. But I've never come across an exception class that insists that the exception is non-null.
One case where exceptions typically have no message is NullPointerException when it is thrown by the JVM itself.
If you are asking should it ... in the sense of whether it is good practice to write code that throws exceptions without a message, the answer is (IMO) No. But you can make up your own mind. (I guess, if the exception name says all that there is to be said, then a message would be redundant. However, it is always useful to have extra information in the stacktrace when debugging.)
Re these comments:
Generally, you should avoid custom exceptions. Use ones that already exist. – mre
Why should I avoid custom exceptions? – Kyranstar
The point is that before you code a custom exception, you should look to see if there is an existing exception that means the same thing as your proposed new one. For example, don't write a custom IllegalLexerArgumentException exception when there is an existing IllegalArgumentException that would serve your purposes.
Why?
For the same reasons that we don't write (say) custom collection classes without good reason. It is a bad idea to write unnecessary code, 'cos that is just more code to compile and test, more space at runtime, more code for the maintainer to read, etc.
Because having lots of exception classes that mean the same thing can make exception handling messy. This effect gets worse as you combine / reuse more libraries to make large applications.
If you don't give the exception any message so you can pop-up the message into anther position when you get the error, but it's recommended to give every exception you create a message and to create an instance of this exception and to throw it.
If the documentation explicitly states there is a single case triggering the exception, then IMHO it is not require to have a message in the exception. Otherwise, it is. A good rule of thumb is: Make it easy for whoever is going to use/debug that code.
Well, honestly I've just searched our code and we use Exceptions without messages, but they are really obvious ones. Like:
UsernameCantBeEmptyException// example only
I think this depends on the use case. The NullPointerException for instance has no message. What could be helpful? No idea. On the other hand there where versions of java where the ClassCastException had no message. That's extremly annoing as the type that should be casted is hart to figure out and is available to the program.
As long as I've no idea for a use case where skipping the message seems appropiate, I'd provide no empty constructor. AFAIK a lexer figures out things like this variable has that type and thus that acion is appropriate or not. This would indicate that the exception message might even have to be presented to he end user. In such a case even localization might be mandatory. If so even some 'factory infrastructre' might be a good idea.
On custom exceptions in general: There's been a tendency to do that to often and then many figured out that in fact no custom exceptions have ever been needed. And in fact there are even hudge programs that need not a single one. On business I work on a 200kLOC program and I don't think we have one. But for sub-systems with specific needs on handling (Like catching all the LexerException to show them in a specific way, separate them from programming bugs in your own code, whatsoever) it might make much sense to have a few. I'm working on a small OSS project and there I have even more than one. But you should always keep in mind that additional exception type must make the exception handling simpler not more complicated. If you often have several catch block one after another, then your definitly on the wrong track. That's a typical symptom of unnecessary custom exceptions.
I find that Java's reflection API is exception verbose, and I often want to catch each specific exception, but just throw Exception. Is this poor practice, or do you really throw all of those exceptions on the method signature? Every method that calls that method must then deal with each of those specific exceptions in some way. Instead, I'm thinking of doing this:
public void someMethod()
throws Exception {
try {
// do a bunch of reflection...
} catch(ClassCastException classCastException) {
throw new Exception("Some specific message.", classCastException);
} catch(InvocationTargetException invocationTargetException) {
throw new Exception("Some specific message.", invocationTargetException);
}
}
Is that poor practice?
Rethrowing different kinds of exceptions that a method throws as a single unified exception can sometimes be a reasonable approach.
Using java.lang.Exception specifically for this is a bad idea, because it's too unspecific.
You might rethrow each different exception as some custom MyException or something like that. That's sometimes a reasonable approach when you're not really interested in why someMethod failed.
Short answer
Reflection is verbose by design
Exception translation is idiomatic (catching lower level exception and abstracting it to a higher level)
... but don't overuse!
Prefer custom exceptions to java.lang.Exception; it's probably too high of an abstraction
Point 1: Avoid reflection if at all possible
Here's a quote from Effective Java 2nd Edition, Item 53: Prefer interfaces to reflection:
[The power of reflection], however, comes at a price:
You lose all the benefits of compile time checking, including exception checking.
The code required to perform reflective access is clumsy and verbose. It is tedious to write and difficult to read.
Performance suffers. Reflective method invocation is much slower than normal method invocation.
[...] As a rule, objects should not be accessed reflectively in normal application at runtime. There are a few sophisticated applications that require reflection. Examples include [omitted on purpose]. If you have any doubts to whether your application falls into one of these categories, it probably doesn't.
Point 2: Exception translation can be a good thing, but don't overuse
Here's a quote from Effective Java 2nd Edition, Item 61: Throw exceptions appropriate to the abstraction.
It is disconcerting when a method throws an exception that has no apparent connection to the task that it performs. This often happens when a method propagates an exception thrown by a lower-level abstraction. [...]
To avoid this problem, higher layers should catch lower-level exceptions and, in their place, throw exceptions that can be explained in terms of higher-level abstraction. This idiom is known as exception translation.
[...] While exception translation is superior to mindless propagation of exceptions from lower layers, it should not be overused. Where possible, the best way to deal with exceptions from lower layers is to avoid them, by ensuring that lower-level methods succeed. [...]
If it is impossible to prevent exceptions from lower layers, the next best thing is to have the higher layer silently work around these exceptions, insulating the caller of the higher-level method from lower-level problems. Under these circumstances, it may be appropriate to log the exception using some appropriate logging facility. This allows an administrator to investigate the problem, while insulating client code and end user from it.
Point 3: java.lang.Exception is way too general
Looking at the documentation, one can see a long list of direct known subclasses, with a wide-ranging topics from Reflection, RMI, XML parsing, I/O, GUI, cryptography, etc.
Declaring that a method throws Exception is probably a poor API design decision, because it does not immediately tell the user anything useful about what category the exceptions may be, or under what circumstances they may be thrown. Contrast this with a hypothetical ReflectionException; this is a lot more informative, and it also allows user to catch specifically ReflectionException and not, say, an IOException that somehow slipped through.
Related questions
In Java, is using throws Exception instead of throwing mulitple specific exceptions good practice?
Catching several exceptions and rethrowing a general exception
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Need authoritative source for why you shouldn’t throw or catch java.lang.Exception
In Java, when should I create a checked exception, and when should it be a runtime exception?
When to choose checked and unchecked exceptions
The case against checked exceptions
I agree with your approach, except I think throwing Exception is not good, eventually every method declaration in your application will end with "throws Exception". For cases like this, where the exception will only occur if you made an error, it's better to make the wrapping exception an unchecked exception.
If the exception thrown by the method (contained in the InvocationTargetException) have the potential to be interesting to you, you could make it the wrapper a different subclass of unchecked exception or create a specific checked exception subclass for it, but wrap things like the ClassCastException in unchecked exceptions.
To use throws Exception is always a bad idea. In your case you have to consider the following things.
Can I recover from the error at the next imediate level? If so use all the different exceptions for clarity.
Can I recover from the error some levels up? If so use a wrapper exception because it would pollute all methods on the way up with technical details to use the explicit ones.
I can't recover from the error or it happens in a completely different place/many levels above. If wrap each exception into RuntimeException or a custom ReflectionFailedRuntimeException to avoid technical exceptions leaking throughout your application.
Alos, normally you should code it in a way that you can expect the reflection to work in most cases without Exceptions, so it would be also considerable to make it RuntimeException wrapping anyway.
Think of what you want to achieve. The caller of your method is not supposed to know you're using reflection to achieve your goal. That's none of its business. If you would just put all those exceptions in your throws clause that would break that encapsulation.
Catching these exceptions and wrapping them in your own is therefore the way to go. Then you just have to decide which exception to throw. Exception itself is a very bad idea (I'm of the opinion it should even be abstract to suggest that) The type of the exception should be related to the problem.
Throw an exception that relates to what you're trying to do and to why it failed (if it failed because a given argument is inappropriate, IllegalArgumentException would be one). Also make an informed decision on whether you would like to throw a checked or an unchecked exception (RuntimeException).
I would in some cases.
The normal way I would decide is look at what methods calling someMethod() would expect or reasonably be expected to know. So if someMethod() was doing something IO related I'd throw an IOException, if it was related to a library I was writing I might throw a custom exception..
I'd only throw the individual exceptions if the callers of someMethod() would reasonably be expected to know it used reflection for its implementation.
I guess the key question is "If someMethod() didn't use reflection, and something went wrong, what would it do?" That will normally give you the answer.
Thats still an acceptable policy if you are collecting all exceptions and throwing a custom exception of yours.
public void someMethod()
throws MyCustomException {
try {
// do a bunch of reflection...
} catch(ClassCastException classCastException) {
MyCustomException ex1 = new MyCustomException("Some specific message.", classCastException);
// Logic to add more details to exception AND/OR handling current exception.
throw ex1;
} catch(InvocationTargetException invocationTargetException) {
MyCustomException ex1 = new MyCustomException("Some specific message.", classCastException);
// Logic to add more details to exception AND/OR handling current exception.
throw ex1;
}
}
Generally catching exception should be for a reason of handling them in some way. Simply re-throwing an exception of larger scope makes no sense anywhere to me.
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Possible Duplicate:
When to choose checked and unchecked exceptions
When should I create a checked exception, and when should I make a runtime exception?
For example, suppose I created the following class:
public class Account {
private float balance;
/* ... constructor, getter, and other fields and methods */
public void transferTo(Account other, float amount) {
if (amount > balance)
throw new NotEnoughBalanceException();
/* ... */
}
}
How should I create my NotEnoughBalanceException? Should it extend Exception or RuntimeException? Or should I just use IllegalArgumentException instead?
There's a LOT of disagreement on this topic. At my last job, we ran into some real issues with Runtime exceptions being forgotten until they showed up in production (on agedwards.com), so we resolved to use checked exceptions exclusively.
At my current job, I find that there are many who are for Runtime exceptions in many or all cases.
Here's what I think: Using CheckedExceptions, I am forced at compile time to at least acknowledge the exception in the caller. With Runtime exceptions, I am not forced to by the compiler, but can write a unit test that makes me deal with it. Since I still believe that the earlier a bug is caught the cheaper it is to fix it, I prefer CheckedExceptions for this reason.
From a philosophical point of view, a method call is a contract to some degree between the caller and the called. Since the compiler enforces the types of parameters that are passed in, it seems symmetrical to let it enforce the types on the way out. That is, return values or exceptions.
My experience tells me that I get higher quality, that is, code that JUST WORKS, when I'm using checked exceptions. Checked exceptions may clutter code, but there are techniques to deal with this. I like to translate exceptions when passing a layer boundary. For example, if I'm passing up from my persistence layer, I would like to convert an SQL exception to a persistence exception, since the next layer up shouldn't care that I'm persisting to a SQL database, but will want to know if something could not be persisted. Another technique I use is to create a simple hierarchy of exceptions. This lets me write cleaner code one layer up, since I can catch the superclass, and only deal with the individual subclasses when it really matters.
In general, I think the advice by Joshua Bloch in Effective Java best summarises the answer to your question: Use checked expections for recoverable conditions and runtime exceptions for programming errors (Item 58 in 2nd edition).
So in this case, if you really want to use exceptions, it should be a checked one. (Unless the documentation of transferTo() made it very clear that the method must not be called without checking for sufficient balance first by using some other Account method - but this would seem a bit awkward.)
But also note Items 59: Avoid unnecessary use of checked exceptions and 57: Use exceptions only for exceptional conditions. As others have pointed out, this case may not warrant an exception at all. Consider returning false (or perhaps a status object with details about what happened) if there is not enough credit.
When to use checked exceptions? Honestly? In my humble opinion... never. I think it's been about 6 years since I last created a checked exception.
You can't force someone to deal with an error. Arguably it makes code worse not better. I can't tell you the number of times I've come across code like this:
try {
...
} catch (IOException e) {
// do nothing
}
Whereas I have countless times written code like this:
try {
...
} catch (IOException e) {
throw new RuntimeExceptione(e);
}
Why? Because a condition (not necessarily IOException; that's just an example) wasn't recoverable but was forced down my throat anyway and I am often forced to make the choice between doing the above and polluting my API just to propagate a checked exception all the way to the top where it's (rightlfully) fatal and will be logged.
There's a reason Spring's DAO helper classes translate the checked SQLException into the unchecked DataAccessException.
If you have things like lack of write permissions to a disk, lack of disk space or other fatal conditions you want to be making as much noise as possible and the way to do this is with... unchecked exceptions (or even Errors).
Additionally, checked exceptions break encapsulation.
This idea that checked exceptions should be used for "recoverable" errors is really pie-in-the-sky wishful thinking.
Checked exceptions in Java were an experiment... a failed experiment. We should just cut our losses, admit we made a mistake and move on. IMHO .Net got it right by only having unchecked exceptions. Then again it had the second-adopter advantage of learning from Java's mistakes.
IMHO, it shouldn't be an exception at all. An exception, in my mind, should be used when exceptional things happen, and not as flow controls.
In your case, it isn't at all an exceptional status that someone tries to transfer more money than the balance allows. I figure these things happen very often in the real world. So you should program against these situations. An exception might be that your if-statement evaluates the balance good, but when the money is actually being subtracted from the account, the balance isn't good anymore, for some strange reason.
An exception might be that, just before calling transferTo(), you checked that the line was open to the bank. But inside the transferTo(), the code notices that the line isn't open any more, although, by all logic, it should be. THAT is an exception. If the line can't be opened, that's not an exception, that's a plausible situation.
IMHO recap: Exceptions == weird black magic.
being-constructive-edit:
So, not to be all too contradictive, the method itself might very well throw an exception. But the use of the method should be controlled: You first check the balance (outside of the transferTo() method), and if the balance is good, only then call transferTo(). If transferTo() notices that the balance, for some odd reason, isn't good anymore, you throw the exception, which you diligently catch.
In that case, you have all your ducks in a row, and know that there's nothing more you can do (because what was true became false, as if by itself), other than log the exception, send a notification to someone, and tell the customer politely that someone didn't sacrifice their virgins properly during the last full moon, and the problem will be fixed at the first possible moment.
less-enterprisey-suggestion-edit:
If you are doing this for your own pleasure (and the case seems to be this, see comments), I'd suggest returning a boolean instead. The usage would be something like this:
// ...
boolean success = transferTo(otherAccount, ONE_MILLION_DOLLARS_EXCLAMATION);
if (!success) {
UI.showMessage("Aww shucks. You're not that rich");
return; // or something...
} else {
profit();
}
// ...
My rule is
if statements for business logic errors (like your code)
cheched exceptions for environment errors where the application can recover
uncheched exception for environment errors where there is no recovery
Example for checked exception: Network is down for an application that can work offline
Example for uncheched exception: Database is down on a CRUD web application.
There is much documentation on the subject. You can find a lot by browsing the Hibernate
web pages since they changed all exceptions of Hibernate 2.x from checked to unchecked in version 3.x
I recently had a problem with exceptions, code threw NullPointerException and I had no idea why, after some investigation it turned out that real exception was swallowed(it was in new code, so its still being done) and method just returned null. If you do checked exceptions you must understand that bad programmers will just try catch it and ignore exception.
My feeling is that the checked exception is a useful contract that should be used sparingly. The classic example of where I think a checked exception is a good idea is an InterruptedException. My feeling is that I do want to be able to stop a thread / process when I want it to stop, regardless of how long someone has specified to Thread.sleep().
So, trying to answer your specific question, is this something that you absolutely want to make sure that everyone deals with? To my mind, a situation where an Account doesn't have enough money is a serious enough problem that you have to deal with it.
In response to Peter's comment: here's an example using InterruptedException as concrete case of an exception that should be handled and you need to have a useful default handler. Here is what I strongly recommend, certainly at my real job. You should at least do this:
catch (InterruptedException ie) {
Thread.currentThread().interrupt();
}
That handler will ensure that the code catches the checked exception and does exactly what you want: get this thread to stop. Admittedly, if there's another exception handler / eater upstream, it's not impossible that it will handle the exception less well. Even so, FindBugs can help you find those.
Now, reality sets in: you can't necessarily force everyone who writes an exception handler for your checked exception to handle it well. That said, at least you'll be able to "Find Usages" and know where it is used and give some advice.
Short form: you're inflicting a load the users of your method if you use a checked exception. Make sure that there's a good reason for it, recommend a correct handling method and document all this extensively.
From Unchecked Exceptions -- The Controversy:
If a client can reasonably be expected
to recover from an exception, make it
a checked exception. If a client
cannot do anything to recover from the
exception, make it an unchecked
exception.
Note that an unchecked exception is one derived from RuntimeException and a checked exception is one derived from Exception.
Why throw a RuntimeException if a client cannot do anything to recover from the exception? The article explains:
Runtime exceptions represent problems
that are the result of a programming
problem, and as such, the API client
code cannot reasonably be expected to
recover from them or to handle them in
any way. Such problems include
arithmetic exceptions, such as
dividing by zero; pointer exceptions,
such as trying to access an object
through a null reference; and indexing
exceptions, such as attempting to
access an array element through an
index that is too large or too small.
A checked exception means that clients of your class are forced to deal with it by the compiler. Their code cannot compile unless they add a try/catch block.
The designers of C# have decided that unchecked exceptions are preferred.
Or you can follow the C-style and check return values and not throw exceptions.
Exceptions do have a cost, so they shouldn't be used for control flow, as noted earlier. But the one thing they have going for them is that they can't be ignored.
If you decide that in this case to eschew exceptions, you run the risk that a client of your class will ignore the return value or fail to check the balance before trying to transfer.
I'd recommend an unchecked exception, and I'd give it a descriptive name like InsufficientFundsException to make it quite clear what was going on.
Simply put, use checked exception only as part of external contract for a library, and only if the client wants/needs to catch it. Remember, when using checked exception you are forcing yourself on the caller. With runtime exception, if they are well-documented, you are giving the caller a choice.
It is a known problem that checked exceptions are over-used in Java, but it doesn't mean that they are all bad. That's why it is such in integral part of the Spring philosophy, for example (http://www.springsource.org/about)
The advantage of checked exceptions is that the compiler forces the developer to deal with them earlier. The disadvantage, in my mind anyway, is that developers tend to be lazy and impatient, and stub out the exception-handling code with the intention of coming back and fixing it later. Which, of course, rarely happens.
Bruce Eckel, author of Thinking in Java, has a nice essay on this topic.
I don't think the scenario (insufficient funds) warrants throwing an Exception --- it's simply not exceptional enough, and should be handled by the normal control flow of the program. However, if I really had to throw an exception, I would choose a checked exception, by extending Exception, not RuntimeException which is unchecked. This forces me to handle the exception explicitly (I need to declare it to be thrown, or catch it somewhere).
IllegalArgumentException is a subclass of RuntimeException, which makes it an unchecked exception. I would only consider throwing this if the caller has some convenient way of determining whether or not the method arguments are legal. In your particular code, it's not clear if the caller has access to balance, or whether the whole "check balance and transfer funds" is an atomic operation (if it isn't then the caller really has no convenient way of validating the arguments).
EDIT: Clarified my position on throwing IllegalArgumentException.
Line is not always clear, but for me usually RuntimeException = programming errors, checked exceptions = external errors. This is very rough categorization though. Like others say, checked exceptions force you to handle, or at least think for a very tiny fraction of time, about it.
Myself, I prefer using checked exceptions as I can.
If you are an API Developer (back-end developer), use checked exceptions, otherwise, use Runtime exceptions.
Also note that, using Runtime exceptions in some situations is to be considered a big mistake, for example if you are to throw runtime exceptions from your session beans (see : http://m-hewedy.blogspot.com/2010/01/avoid-throwing-runtimeexception-from.html for more info about the problem from using Runtime excpetions in session beans).
At what point would you create your own exception class vs. using java.lang.Exception? (All the time? Only if it will be used outside the package? Only if it must contain advanced logic? etc...)
I think you need to ask yourself a slighly different question "What advantage does creating a new exception give me or developers who use my code?" Really the only advantage it gives you or other people is the ability to handle the exception. That seems like an obvious answer but really it's not. You should only be handling exceptions that you can reasonably recover from. If the exception you throw is a truly fatal error why give developers a chance to mis-handle it?
More in depth discussion: Custom exceptions: When should you create them?
Reason one:
Need to catch specific stuff. If calling code needs to deal with a specific exceptional condition, you need to differentiate your Exception, and Java differentiates exceptions with different types, so you need to write your own.
Basically, if someone has to write:
catch(ExistingException e) {
if({condition}) {
{ some stuff here}
}
else {
{ different stuff here}
}
}
You probably want to write a specific extension; catch Exception matching is clearer than conditionals, IMHO.
Remember: your new Exception can be a subclass of RuntimeException
Reason two:
API consolidation. If you write an interface and you have several implementations, it's possible that they will call different APIs with a whole bunch of different non-RuntimeExceptions thrown:
interface MyInterface {
void methodA();
}
class MyImplA {
void methodA() throws SQLException { ... }
}
class MyImplB {
void methodA() throws IOException { ... }
}
Do you really want MyInterface.methodA to throw SQLException and IOException? Maybe then it makes sense to wrap the possible exceptions in a custom Exception. Which again can be a RuntimeException. Or even RuntimeException itself...
I believe that:
catch (Exception e) {
...
}
... is an antipattern that should be avoided. You might want one centralized broad catch somewhere in your application, to log an error and prevent the whole application from terminating - but having them scattered around willy-nilly is bad.
Why:
try {
if(myShape.isHidden()) {
throw new Exception();
}
// More logic
} catch (Exception e) {
MyApp.notify("Can't munge a hidden shape");
}
So you try this, and due to a coding error, myShape is null. A NullPointerException gets thrown when the runtime tries to derefence myShape. This code reports a hidden shape, when it should be reporting a null pointer.
Either make your own exception, or find a suitably specialized exception in the API. It's not as if extending Exception or RuntimeException is onerous.
When I want to treat my exceptions differently from everybody else's. If I want to catch mine and propagate everyone else's, or if I want to catch someone else's and propagate mine, or if I want to catch both but treat them differently, then I will define a separate class for my exceptions. If I want to treat them all the same, either by propagating both or by catching both (and doing the same thing either way with the caught exceptions), the I will use the standard class.
IF there is an existing Exception with the language runtime or libraries, use it ELSE create your own, document it well and that should work in 99% of the cases.
Software captures meaning.
There are almost no reasons for throwing an existing exception: the JVM already does that for you. Your version of their exception isn't really accurate and throwing "Exception" isn't meaningful, either.
You might have a DataFormatException because of a parsing algorithm you wrote. This, however, is rare.
When your program encounters an exceptional situation, it's almost always unique to your program. Why force-fit your exceptional situation into an existing exception? If it's unique to your program, then... well... it's unique. Name it that way.
Do not, however, provide a unique exception class for each unique message. One exception class can have many variant messages and supporting details.
The Python rule of thumb, translated to Java, is to define any unique exceptions at the package level. [In Python, they suggest exceptions at the "module" level, something that doesn't precisely translate to Java.]
Start always by using the common exception classes and then when a need appears to handle it specially, change it.
When creating a method first time, just let exceptions go through.
If there are exceptions that must be handled, those can either be just defined in throws or wrapped to some runtime exception or wrapped own throws exception. I prefer runtime exceptions in many cases. Defining throws definition should be avoided until there is a need for it from API point of view.
Later when a need appears to do specific handling for an exception in some caller, come back and create new exception for it.
The point is to avoid doing extra work before knowing what is needed.
I can't imagine specifically throwing a java.lang.Exception if some object/class/method had a problem. It's too generic - if you're not going to create your own Exception class, seems to me like there ought to at least be a more specific Exception-type in the API.
I would use the exceptions from the Java API when the exception relates to the API. But if an exceptional situation arises that is unique to my own API then I will create an Exception for it. For example if I have a Range object with two properties min and max and the invariant min <= max then I will create an exception InvalidRangeException.
When I am writing code this helps because I know if the exception originates because I violated one of my own conditions or its something from the Java API.
In most cases it doesn't make sense to create your own exception class.
There is a tendency in novice programmers to create their own exception class just so they can use a name that is more indicative of the type of error. So you'll find classes like FTPInitializationException, DAOFactoryException etc. even though such exceptions are not being handled differently than standard exceptions. This is clearly an anti pattern that should be avoided.