Develop Reference

Is it okay to declare a checked exception for an exception thrown by a called method?

Consider this:
public void Do() throws Exception {
if (blah) throw new Exception(...);
Thingy thingy = ...;
Foo(thingy);
}
public void Foo(Thingy thingy) throws EmptyThingyException {
if (thingy == null ||
thingy.isEmpty()) throw new EmptyThingyException();
...
}
public class EmptyThingyException extends Throwable { ... }
In this case, is it okay to not handle EmptyThingyException inside Do and declare Do like so:
public void Do() throws Exception, EmptyThingyException {
or do I have to handle EmptyThingyException inside Do and throw it back again like so:
public void Do() throws Exception, EmptyThingyException {
try {
} catch (EmptyThingyException empty) {
throw empty;
}
...
}

The short answer to the question is:
Yes, it's correct to declare a checked exception thrown by a called method.
How a method achieves its purpose is an implementation detail and it shouldn't matter to the interface how much it does directly or how much it delegates to methods. The language rules about checked exceptions are carefully defined to make sure methods advertise all checked exceptions they may throw or methods they call throw (but are not handled by the method itself). Letting an unhandled exception get 'thrown through' a method is how things are supposed to work.
Indeed the answer is in the name of the construct "non-local exception handling" it was conceived to take effort out of endless error handling all the way up a call chain when the only real action is "that didn't work" at some point near the start.
To align to that method, you should only catch exceptions you're going to do something about.
Clean up code should be achieved with finally so the normal reasons to catch an exception are to log it and/or abandon a task at some point rather than letting the stack unwind further.
In this specific case the best answer would be to throw an IllegalArgumentException:
throw new IllegalArgumentException("thingy==null || thingy.isEmpty()");
That's unchecked and wisely so. Correct code shouldn't encounter illegal arguments and they should expect to be thrown rarely and be indicative of program flaw (either in the class, it's package or consumer code). External and user input should be validated directly and programs shouldn't rely on IllegalArgumentException.
In practice IllegalArgumentException and IllegalStateException should cover 'internal errors' meaning "You can't do this with that" or "You can't do that right now" respectively and should be commented to specify the fault.
The idea that you might sub-class those two because consumer code might respond differently to different illegal actions it might take is bodging pure and simple.
Program correctness includes that a program never makes an illegal call on some other part of the program or enters an invalid or corrupted state and exceptions only occur as a result of environmental failures that mean a program or sub-task in a program cannot be completed as intended.

if you want to do something after exception happen, then use try-catch, or you can just declare it on the method.
Beyond that, if EmptyThingyException is sub class of Exception, then it is no need to declare EmptyThingyException when you have declared Exception.

1- Declare the specific checked exceptions that your method can throw
public void foo() throws Exception { //Incorrect way
}
Always avoid doing this as in above code sample. It simply defeats the whole purpose of having checked exception. Declare the specific checked exceptions that your method can throw. If there are just too many such checked exceptions, you should probably wrap them in your own exception and add information to in exception message. You can also consider code refactoring also if possible.
2- Always catch only those exceptions that you can actually handle
catch (NoSuchMethodException e) {
throw e; //Avoid this as it doesn't help anything
}
Well this is most important concept. Don’t catch any exception just for the sake of catching it. Catch any exception only if you want to handle it or, you want to provide additional contextual information in that exception. If you can’t handle it in catch block, then best advice is just don’t catch it only to re-throw it.
3- Avoid using Throwable class
Throwable is the superclass of Exception and Error, as far as I know you need to use Throwable when you want to deal with both exceptions and errors, but it's definitely not your concern here, most of the java code deal with Exception and it's the way to go whenever you need to deal with checked exceptions http://docs.oracle.com/javase/tutorial/essential/exceptions/index.html.
**
Well if I was you I would do something like :
public void Do() throws BlahIsFoundException{
try {
if (blah) throw new BlahIsFoundException(...);
Thingy thingy = ...;
Foo(thingy);
} catch(EmptyThingyException exception) {
//Handle the exception correctly, at least log it
} finally {
//Do some clean up if needed, for example close a database connection or free some resources.
}
}
public void Foo(Thingy thingy) throws EmptyThingyException {
if (thingy == null ||
thingy.isEmpty()) throw new EmptyThingyException();
...
}
public class EmptyThingyException extends Exception { ... }
public class BlahIsFoundException extends Exception { ... }
Hope that helps, here are some good documents to read :
http://docs.oracle.com/javase/tutorial/essential/exceptions/index.html
http://howtodoinjava.com/best-practices/java-exception-handling-best-practices

How am I supposed to let an Unchecked Exception bubble up and log?

So quoting from this page, which is titled: Exception-Handling Antipatterns Blog and seems to be written (or at least to be approved) by Oracle..
An unchecked exception probably shouldn't be retried, and the correct response is usually to do nothing, and let it bubble up out of your method and through the execution stack. This is why it doesn't need to be declared in a throws clause. Eventually, at a high level of execution, the exception should probably be logged.
I am not sure if I understand this. How can I log an unchecked exception? If I have something like:
public static void main(String args) {
foo();
// How do I know what to log here? The method I am calling
// is not throwing an Exception.
// Do I just blindly catch(Exception ex)?
}
static void foo() {
bar();
}
static void bar() {
baz();
}
static void baz() {
// I will do nothing as Oracle suggests and let this exception bubble up.. I wonder who is going to catch it and how this is going to be logged though!
throw new NullPointerException();
}
Can you help me understand what Oracle is suggesting here? I do not see any direct (or clear) way to catch runtime exceptions (I do not understand why it is not just called unchecked exceptions..) in higher levels and I am not sure how this suggested practice is useful. To me it would make more sense if it were talking about checked exceptions. Something like..
If a checked exception is thrown in a method that is not reasonable to be re-tried, the correct response is to let it bubble up and log..

You can also register a global ExceptionHandler that will handle the Exceptions that were not caught by your code:
Thread.setDefaultUncaughtExceptionHandler
This exception handle could then log whatever occured.

First of all, this is a general advice and it depends on the context. The idea behind it is that when a runtime exception occurs (ex. NullPointerException), the system is usually in an indeterministic state, meaning the rest of the code is not be guaranteed to execute as expected, so it's better to stop everything.
In most cases, your code will run in a separate thread and the exception will only stop the current thread, while the rest of the program keeps running.
This is not the case in your example, because everything is executed in a single thread, so the uncaught exception will effectively stop the whole program. In this scenario you might want to catch the exception and handle it.
public static void main(String args) {
try {
foo();
catch(Throwable t) {
t.printStackTrace(); // log exception
// handle the failure
}
}
You can also catch the exception earlier on, log and rethrow it further.
static void bar() {
try {
baz();
catch (Throwable t) { // catch
t.printStackTrace(); // log
throw t; // rethrow further
}
}
Edit: catch Throwable instead of Exception, will also catch Error
Note: Catching throwable is usually a bad idea, and should only be done with a specific purpose, not in general case. See #RC.'s comment.

As I understand it the documentation is suggesting that you have a generic handler at a high level of your code that logs such 'unexpected' (unrecoverable?) exceptions just as the comments in your main method suggest. So it might look something like this
public static void main(String args) {
try {
foo();
}
catch (ArithmeticException aex) { //if it's arithmetic log differently
log("arith issue! "+aex.getMessage());
}
catch (Exception ex) { //Otherwise do the best we can
log("unknown issue! "+ex.getMessage())
}
}
So there is still no path to recovery but at least before the process ends you get a chance to log the issue. You can also use the methods of Exception (or throwable) to get the stack trace and first causal exceptions in many case - so there is is a lot of extra useful information that might be logged.

There is a very straightforward way to catch unchecked exceptions, since they are all subclasses of RuntimeException or Error:
public static void main(String[] args) {
try {
// your code
} catch (RuntimeException | Error e) {
// handle uncaught exceptions, e.g.
e.printStackTrace();
}
}

How do I know what to log here? The method I am calling is not throwing an Exception.
As Joshua Bloch recommends in the Effective Java
Use the Javadoc #throws tag to document each unchecked exception that
a method can throw, but do not use the throws keyword to include
unchecked exceptions in the method declaration
And if you are using method wrapping in multilayered app i can recommend use exception translation:
Higher layers should catch lower-level exceptions and, in their place, throw exceptions that can be explained in terms of the higher-level abstraction
See Effective Java item 61
So i think for your example actually you should use something like:
try {
bar();
} catch(NullPointerException e) {
throw new HigherLevelException(...);
}

The most important guideline regarding exceptions is that a method that couldn't sucessfully complete its task should throw an exception.
Only if you can guarantee successful completion of your method's task, you should catch an exception inside your method (without re-throwing this or another exception). From my experience that's only true in very specific situations, e.g. if you have an alternative way to try if some first attempt fails, or if you really really understand all possible causes of this specific Exception class that you are about to catch.
Speaking about RuntimeExceptions, there are so many different types of RuntimeException that you can hardly justify an assertion like "When such an exception arises in my code or a method called from inside my code, that won't affect the outcome of my method - I can continue just as if nothing happened." So, you should signal to your caller that you failed to fulfill your task, and the clearest way to do that is to let the exception ripple through, without try/catch block or throws declaration, just relying on Java's default behaviour.
In my opinion, the same reasoning applies to nearly all kinds of exceptions, not only RuntimeExceptions.
The difference with checked exceptions is that you have to declare them in the throws clause of your method. Then you have two choices: list the exception in the throws clause of your method (and all parent methods as well!) or catch the exception, wrap it in a new RuntimeException(ex), and throw that from your method.
With e.g. a typical GUI application, your users will be grateful if a problem in one menu function won't crash the whole application - probably other menu items might still work as expected. So, top-level commands or menu items are typically the places where to catch exceptions, tell the user something like "Oops!", log the exception to some file for later inspection, and allow the user to continue with another action.
In your main/foo/bar/baz application, I don't see a place where continuing after an exception makes sense. So the whole program should be aborted (which happens automatically in your case). If you want some error logging to a file, then establish an uncaught exception handler or wrap the body of main() in a try / catch(Throwable t) block. You'll probably want every exception logged, whatever type it is, so catch them all, and that's why I'm suggesting Throwable.

public static void main(String[] args) {
try {
foo();
}
catch(NullPointerException e){
System.out.println("NullPointerException in main.");
}
}
static void foo() {
bar();
}
static void bar() {
baz();
}
static void baz() {
// I will do nothing as Oracle suggests and let this exception bubble up.. I wonder who is going to catch it and how this is going to be logged though!
throw new NullPointerException();
}
OUTPUT :
NullPointerException in main.
Basically the error is expected at a higher level, so there is no need to catch it on the baz() method level. If I understood correctly.

You can catch them just like any other exception with try-catch block. But the benefit is that you don't have to.
Use cases can vary. To my mind, the most popular is when it doesn't make sense to catch the exception right in that place or the appropriate handling should be implemented several levels (in terms of methods) higher than the method, calling the one throwing the exception (sorry, if that is not clear enough).
For example, the typical web application layout in java is as follows: you have a layer of controllers, a layer of services and a layer of dao. First one is responsible for dispatching requests, the second one is for managing business logic and the last one makes actual calls to db. So here for example it often doesn't make much sense to catch the exception in service layer if something goes wrong on the dao level. Here unchecked exceptions can be used. You log an exception and throw an unchecked exception so it could be handled some levels above for a user to get valuable feedback of work of the application.
If in this case you throw a checked exception you will have to rethrow it every level above just to bubble up it to the place of the actual handling. So here the unchecked exception is better to use in order not to copy and paste all that ugly try-catch block, rethrowing an exception and add the throws clause to the method.

Effects of Try/Catch against Throwing Exceptions in a constructing class' constructor

I was playing around with some of my code and came across something I didn't fully understand. I have a class called SentimentClassifier, the constructor of which looks like this:
public SentimentClassifier(final int nGramToBeUsed) {
try {
classifier = (DynamicLMClassifier<?>) AbstractExternalizable.readObject(new File(etc));
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
I have another class which creates this one, like so:
public TwitterManager(final int nGramToBeUsed) {
sentimentClassifier = new SentimentClassifier(nGramToBeUsed);
}
If I run the code like this, everything works fine. But if I change the first class from using try/catch to throw the exception, like so:
public SentimentClassifier(final int nGramToBeUsed) throws ClassNotFoundException, IOException {
classifier = (DynamicLMClassifier<?>) AbstractExternalizable.readObject(new File(etc));
}
Suddenly the second class complains that the IOException isn't being handled. Why is this thrown only for the thrown exception and not for the try/catch?

When you call a method M1 from another method M2:
If some code in M1 raises some Checked Exception, and the method M1 itself handles it, rather than throwing it, you don't have to worry about the exception while calling it.
Now, if the exception raised in M1, is not being handled in M1 itself, rather it is propagated up the stack trace, then M1 must declare that exception in the throws clause. This is just for the convenience of the calling method to know that it should be ready to handle those exception in case they are thrown. This is only the case with Checked Exception.
But if the calling method M2, doesn't handle that exception, it has the option to re-declare that exception to be thrown in it's own throws clause, in which case the exception will be propagated further up the stack trace.
If method M2 does neither of the previous two task, you will get a compiler error. Because you haven't given any proper path or way to handle the exception that can be thrown.
Note all the above arguments are true for Checked Exception only. For Unchecked exception, you don't need to handle it yourself, neither you need to declare it in throws clause.
Suggested Read:
Java: checked vs unchecked exception explanation
Unchecked Exception controversies
JLS - The Kinds and Causes of Exceptions

In Java, if a method declares that throws an Exception (other than RuntimeException), callers must handle the exception. They can do this one of two ways: catch it, or declare that they themselves throw it.
You moved the handling of these two exceptions from the SentimentClassifier constructor to its callers.

If the constructor declares any exceptions, the calling code must handle them or declare them. After all, the constructor could throw/propagate these exceptions, and any code that calls it must handle them.

When you catch an exception, it means that you will deal with it on the catch block, and its consequences, so the external code can continue to progress without being warned about the internal exception.
If your exception is thrown, you are forcing by contract to any creator/invoker class to deal with any declared exception that could be produced during the initialization/execution process, as it can be critical for the business logic.
In this case, if the exceptions that can be generated during init are critical, and could stop the class from working properly, they should be thrown, as the creator class TwitterManager could have a disfuncional or partially initialized instance of the SentinelClassifier object, leading to unexpected errors.

Java custom exception class usage [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Throws or try-catch
I'm writing an API and I wish to write code such that I can raise an exception in a particular scenario. I've created an exception class as follows :-
public class InvalidSeverityException extends Exception {
private static final long serialVersionUID = 1L;
public InvalidSeverityException() {
// TODO Auto-generated constructor stub
}
}
In the codebase im having the following to call the code :-
throw new InvalidSeverityException();
However Eclipse suggests that I either use throws or enclose it within a try ... catch block. I feel that I shouldn't be catching this error and that the developers who use my API should enclose the code within try...catch.
Does that make sense? Am I doing something wrong?

When handling with exceptions in Java you must understand the concept of checked exceptions and unchecked exceptions.
In your case currently you are defining a checked exception, maybe you want an unchecked one.
Here's a brief description about each one of the types:
Checked Exceptions
This exceptions must be part of the method's signature that raises them (or that invokes one method that does) or you must catch them with a try catch block and deal with the problem. Usually checked exceptions are used when there is something that can be done about the error and also when you want the developer to be aware that such error may occur and that has to be handled.
In java java.lang.Exception is a checked exception and all its subclasses will also be checked.
Unchecked Exceptions
This exceptions on the other hand don't need to make part of the method signature, nor you have to wrap methods that throw new in a try catch block. It's just expected that somewhere in the call stack there will be a try catch to handle it, otherwise if it reaches the JVM it will nicely dump you a stacktrace.
In java java.lang.RuntimeException is an unchecked exception and so are all its subclasses.
My opinion
If you are defining an API my suggestion is to use checked exceptions, this is mostly because you explicitly inform the developers using your API that such an exception might occur (so they can handle it anyway they want).

You are correct, you should not catch it. As suggested by eclipse, you should use throws so that the developers will know that your method potentially throws that exception and can then catch it.
.... method() throws YourException{

The method where you have throw new InvalidSeverityException(); should define throws InvalidSeverityException
Example:
void yourMethod() throws InvalidSeverityException
{
........//Some code
throw new InvalidSeverityException();
}

Well then surely you follow the first suggestion by Eclipse and set your method to throw the exception.
public void myMethod() throws InvalidSeverityException {
//throw it somewhere in here so that other
//developer can catch it while calling your method
}

Is "throws Throwable" good practice

In the past I'd read tons of code with methods like:
public Object doSomething() throws Throwable {
...
}
Is it common practice to do that?
What are pros & cons?
throws Trowable seemed to me like the "Agent Orange" way of getting the Exception- matter done
EDIT
Handle expected Exceptions in the Method
Throw unexpected Exceptions (one by one)
Don't care of Errors
Is that the way to go?

You should not throw Throwable. Here's why.
Throwable is the top of the hierarchy of things that can be thrown and is made up of Exceptions and Errors. Since Errors by definition arise from unsalvagable conditions, it is pointless to include them in your method declaration. That leaves just Exception.
You should declare your method with throws Exception instead.
Note that the narrower the range of throws the better.
Declaring your method to be throws Exception is ok if your method doesn't generate the exceptions, but instead calls other code that is declared as throws Exception and you want exceptions to percolate up the call stack.
If your method is the generating the exception, then declare a narrower range, eg throws IOException, MyProcessingException, etc

That's a loaded question. This isn't so much about exception handling as it is about code readability.
It depends where you get your code samples from. Professionals prefer to be more specific when throwing out of a method. The main reason is that it keeps your APIs more readable. For example, if your method throws Throwable, that basically means anything could happen and your method doesn't want to deal with it, no matter what. But really, only a limited number of things could happen:
Whatever checked exceptions resulting from other calls you are making in your method
Whatever checked exceptions you are throwing on purpose based on your own assertions
Whatever unchecked exception you didn't plan for
Errors (java.lang.Error) that are more global to the JVM and the environment
By specifically stating the exceptions you want to throw, you are telling the users of your API about what they should beware of. For example, when you use InputStream, you'll notice most methods throw at least java.io.IOException, which gives you some useful information about what you should watch for.
When coding, as a general rule, you want to try to keep your APIs as expressive as possible. You've got essentially one line of code to show the public API of a method (i.e. its signature, annotations too I guess), so you want it completely expressive (return type, name, parameters, but also the thrown exceptions).
As far as catching the throwables and printing the stack trace, I'd say that you should not catch the exception unless you can do something about it. Instead, let it roll up the call stack until some class catches it to do something about it. Sometimes, it may roll all the way up to your main class, which I guess would have to catch it and print the stack trace as last resort. Basically, if you can't act upon the exception, then let it go up the call stack. Also it is extremely rare that you find yourself in a situation where you should silence an exception (i.e. catch it but do nothing about it). That's usually inviting problems when comes time to troubleshoot issues.
Here is a fun but interesting article around misuse of exception handling in general.

In some rare cases it is acceptable to throw Throwables. For example, #Around advices in Spring AOP are usually declared to throw a Throwable.
The following example is copied verbatim from Spring AOP docs:
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.ProceedingJoinPoint;
#Aspect
public class AroundExample {
#Around("com.xyz.myapp.SystemArchitecture.businessService()")
public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
// start stopwatch
Object retVal = pjp.proceed();
// stop stopwatch
return retVal;
}
}
Why is doBasicProfiling declared to throw a Throwable? Because the original method (i.e. the execution join point), might throw an Error, RuntimeException, or a checked exception. So it only makes sense to declare doBasicProfiling to throw a Throwable.

Functionally, it is equivalent with throws Exception, since errors are unchecked.
I see no reason to declare a method to throw Throwable. However, this doesn't mean that catch and printStackTrace is a good alternative.
Usually, you want to catch throwables where you can do something sensible with them.
Code that throws a throwable you don't expect should explode gloriously, so you can see the error and fix the bug.

Is it common practice to do that?
In the JDK it is rare. This is mostly used when it is not clear how to handle checked exceptions.
What are pros & cons?
The pros is that you get your code to compile without worrying about checked exception.s
The cons is that exception you should be handling are being ignored.
Isn't it better to catch and printStackTrace()?
Unhandled exception are usually printed anyway so catching them doesn't help much.
You should catch an exception when you can add some value by doing so and add the exception to the throws clause when you can't.

It is really debatable matter.
Having method throwing too many exceptions will result in lot of error handling code. Some times it is not intended.
But because I don't like too many exception in signature does not mean that Lets use Parent of all exceptions and we are done!! It will not work.
What one can do is categorise exceptions such as BusinessException,ServiceException so that if you have a business rule which says that minimum balance in account can not be less than say 100$ then InsufficientBalance exception will be generated which will be child of BusinessException
so you method will be like
public Object doSomething() throws BusinessException {
if(!hasMinimumbalance())
{
throw new InsufficientBalance(ErrorCode);
}
}
What this will do is club related exceptions together and whenever API user wants to detect exception specific error then he can do it, else generic error handling is possible.
The core point here is on the UI you should display to the user that You have run out of balance and you can not withdraw money
You can say on the larger aspect to display human readable form of error it is really necessary to have separation of exceptions.

Are you asking about Throwable specifically? If so, then it's not good practice. It doesn't provide any useful information to class (method) user.

Throwing (and catching) Throwable (or Exception) is generally bad practice because it 'blankets' any specific exceptions you might want to catch. Then you would have to resort to ugliness like below:
public void myMethod() throws Throwable {
if (x) {
throw new MyException1();
}
if (y) {
throw new MyException2();
}
}
public void callingMethod() {
try {
myMethod();
}
catch(Throwable t) {
if (t instanceof MyException1) {
// handle exception 1
}
else if (t instanceof MyException2) {
// handle exception 2
}
else {
// handle other exceptions
}
}
}
Which is error prone (and flagged by CheckStyle as a code violation). It is much preferrable to have code like this:
public void myMethod() throws MyException1, MyException2 {
if (x) {
throw new MyException1();
}
if (y) {
throw new MyException2();
}
}
public void callingMethod() {
try {
myMethod();
}
catch(MyException1 e) {
// handle exception 1
}
catch(MyException2 e) {
// handle exception 2
}
}
Handling an exception just by calling printStackTrace() is usually not a good idea. printStackTrace() sends the stacktrace to standard error, which may not be read at all. A better option is to use the application's logging facility (like log4j) to report the exception. Even then, just logging it might no be enough.
My rule of thumb is:
If you can handle an exception locally, do so. For example when parsing a String as an Integer you could catch the NumberFormatException and return a default value:
prvate int parseAmount(String amountValue) {
int amount;
try {
amount = Integer.parseInt(amountValue);
}
catch(NumberFormatException e) {
// default amount
amount = 0;
}
return amount;
}
If you cannot handle an exception locally, consider if you should expose the exception type that is being thrown. If this type is some obscure (implementation-dependent) type, then wrapping it in your own generic exception type is probably a good idea:
private Customer getCustomer(int customerId) throws ServiceException {
try {
return customerService.getCustomer(customerId);
}
catch(CustomerServiceSpaghettiTangledException e) {
throw new ServiceException("Error calling the customer service", e);
}
}
Here 'ServiceException' is a subclass of Exception created by you. Spring also offers an exception hierarchy specifically for this purpose.
By wrapping the exception you hide the implementation details, making your service layer much simpler to use.
If you decide to throw an exception from your method, you will need to handle it 'higher up' in the callstack. This can be a generic error page in your web application stating that something went wrong and possibly providing an error message or code. In some cases the higher level code can attempt a retry or possibly an alternative way to obtain the required result.

The only use case I can think of would be for test code like unit tests. But Adam's counterpoint still stands "If so, then it's not good practice. It doesn't provide any useful information to class (method) user."