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Rethrowing exceptions in Java without losing the stack trace

In C#, I can use the throw; statement to rethrow an exception while preserving the stack trace:

try
{
   ...
}
catch (Exception e)
{
   if (e is FooException)
     throw;
}

Is there something like this in Java (that doesn't lose the original stack trace)?

Is there anything like .NET's NotImplementedException in Java?

Is there anything like .NET's NotImplementedException in Java?

Should a retrieval method return 'null' or throw an exception when it can't produce the return value?

I have a method that is suppose to return an object if it is found.

If it is not found, should I:

1. return null
2. throw an exception
3. other

When to throw an exception? [closed]

I have exceptions created for every condition that my application does not expect. UserNameNotValidException, PasswordNotCorrectException etc.

However I was told I should not create exceptions for those conditions. In my UML those ARE exceptions to the main flow, so why should it not be an exception?

Any guidance or best practices for creating exceptions?

Can I catch multiple Java exceptions in the same catch clause?

In Java, I want to do something like this:

try {
    ...     
} catch (IllegalArgumentException, SecurityException, 
       IllegalAccessException, NoSuchFieldException e) {
   someCode();
}

...instead of:

try {
    ...     
} catch (IllegalArgumentException e) {
    someCode();
} catch (SecurityException e) {
    someCode();
} catch (IllegalAccessException e) {
    someCode();
} catch (NoSuchFieldException e) {
    someCode();
}

Is there any way to do this?

In Python, check if a directory exists and create it if necessary

What is the most elegant way to check if the directory a file is going to be written to exists, and if not, create the directory using Python? Here is what I tried:

filename = "/my/directory/filename.txt"
dir = os.path.dirname(filename)

try:
    os.stat(dir)
except:
    os.mkdir(dir)       

f = file(filename)

Somehow, I missed os.path.exists (thanks kanja, Blair, and Douglas). This is what I have now:

def ensure_dir(f):
    d = os.path.dirname(f)
    if not os.path.exists(d):
        os.makedirs(d)

Is there a flag for "open", that makes this happen automatically?

Catch multiple exceptions at once?

It is discouraged to simply catch System.Exception. Instead, only the "known" exceptions should be caught.

Now, this sometimes leads to unneccessary repetitive code, for example:

try
{
    WebId = new Guid(queryString["web"]);
}
catch (FormatException)
{
    WebId = Guid.Empty;
}
catch (OverflowException)
{
    WebId = Guid.Empty;
}

I wonder: Is there a way to catch both exceptions and only call the WebId = Guid.Empty call once?

The given example is rather simple, as it's only a GUID. But imagine code where you modify an object multiple times, and if one of the manipulations fail in an expected way, you want to "reset" the object. However, if there is an unexpected exception, I still want to throw that higher.

About the answer: Thanks everyone! For some reason, I had my mind set on a switch-case statement which does not support switching on GetType(). Now, there were two answers, one using "typeof" and one using "is". I first thought typeof() would be my function, because I thought "Hey, I only want to catch FormatException because that's the only thing I expect". But that's not how catch() works: catch also catches all derived exceptions. After thinking about it, this is really obvious: Otherwise, catch(Exception ex) would not work! So the correct answer is "is". Yay, learned two things with only one question \o/

Uncatchable ChuckNorrisException

Is it possible to construct a snippet of code in Java that would make a hypothetical java.lang.ChuckNorrisException uncatchable?

Thoughts that came to mind are using for example interceptors or aspect-oriented programming.

How do I check if a variable exists in Python?

I want to check if a variable exists. Now I'm doing something like this:

try:
   myVar
except NameError:
   # Do something.

Are there other ways without exceptions?

Catch multiple exceptions in one line (except block)

I know that I can do:

try:
    # do something that may fail
except:
    # do this if ANYTHING goes wrong

I can also do this:

try:
    # do something that may fail
except IDontLikeYourFaceException:
    # put on makeup or smile
except YouAreTooShortException:
    # stand on a ladder

But if I want to do the same thing inside two different exceptions, the best I can think of right now is to do this:

try:
    # do something that may fail
except IDontLIkeYouException:
    # say please
except YouAreBeingMeanException:
    # say please

Is there any way that I can do something like this (since the action to take in both exceptions is to say please):

try:
    # do something that may fail
except IDontLIkeYouException, YouAreBeingMeanException:
    # say please

Now this really won't work, as it matches the syntax for:

try:
    # do something that may fail
except Exception, e:
    # say please

So, my effort to catch the two distinct exceptions doesn't exactly come through.

Is there a way to do this?

Proper way to declare custom exceptions in modern Python?

What's the proper way to declare custom exception classes in modern Python? My primary goal is to follow whatever standard other exception classes have, so that (for instance) any extra string I include in the exception is printed out by whatever tool caught the exception.

By "modern Python" I mean something that will run in Python 2.5 but be 'correct' for the Python 2.6 and Python 3.* way of doing things. And by "custom" I mean an Exception object that can include extra data about the cause of the error: a string, maybe also some other arbitrary object relevant to the exception.

I was tripped up by the following deprecation warning in Python 2.6.2:

>>> class MyError(Exception):
...     def __init__(self, message):
...         self.message = message
... 
>>> MyError("foo")
_sandbox.py:3: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6

It seems crazy that BaseException has a special meaning for attributes named message. I gather from PEP-352 that attribute did have a special meaning in 2.5 they're trying to deprecate away, so I guess that name (and that one alone) is now forbidden? Ugh.

I'm also fuzzily aware that Exception has some magic parameter args, but I've never known how to use it. Nor am I sure it's the right way to do things going forward; a lot of the discussion I found online suggested they were trying to do away with args in Python 3.

Update: two answers have suggested overriding __init__, and __str__/__unicode__/__repr__. That seems like a lot of typing, is it necessary?

WPF global exception handler

sometimes, under not reproducible circumstances, my WPF application crashes without any message. The application simply close instantly.

Where is the best place to implement the global Try/Catch block. At least i have to implement a messagebox with: "Sorry for the inconvenience ..."

Exception messages in English?

We are logging any exceptions that happen in our system by writing the Exception.Message to a file. However, they are written in the culture of the client. And Turkish errors don't mean a lot to me.

So how can we log any error messages in English without changing the users culture?

Dealing with "java.lang.OutOfMemoryError: PermGen space" error

Recently I ran into this error in my web application:

java.lang.OutOfMemoryError: PermGen space

It's a typical Hibernate/JPA + IceFaces/JSF application running on Tomcat 6 and JDK 1.6. Apparently this can occur after redeploying an application a few times.

What causes it and what can be done to avoid it? How do I fix the problem?

How do you assert that a certain exception is thrown in JUnit 4 tests?

How can I use JUnit4 idiomatically to test that some code throws an exception?

While I can certainly do something like this:

@Test
public void testFooThrowsIndexOutOfBoundsException() {
  boolean thrown = false;

  try {
    foo.doStuff();
  } catch (IndexOutOfBoundsException e) {
    thrown = true;
  }

  assertTrue(thrown);
}

I recall that there is an annotation or an Assert.xyz or something that is far less kludgy and far more in-the-spirit of JUnit for these sorts of situations.