Catch a generic exception in Java?

Question

We use JUnit 3 at work and there is no ExpectedException annotation. I wanted to add a utility to our code to wrap this:

 try {
     someCode();
     fail("some error message");
 } catch (SomeSpecificExceptionType ex) {
 }

So I tried this:

public static class ExpectedExceptionUtility {
  public static <T extends Exception> void checkForExpectedException(String message, ExpectedExceptionBlock<T> block) {
     try {
        block.exceptionThrowingCode();
        fail(message);
    } catch (T ex) {
    }
  }
}

However, Java cannot use generic exception types in a catch block, I think.

How can I do something like this, working around the Java limitation?

Is there a way to check that the ex variable is of type T?

Answer 1

You could pass the Class object in and check that programatically.

public static <T extends Exception> void checkForException(String message, 
        Class<T> exceptionType, ExpectedExceptionBlock<T> block) {
    try {
       block.exceptionThrowingCode();
   } catch (Exception ex) {
       if ( exceptionType.isInstance(ex) ) {
           return;
       } else {
          throw ex;  //optional?
       }
   }
   fail(message);
}

//...
checkForException("Expected an NPE", NullPointerException.class, //...

I'm not sure if you'd want the rethrow or not; rethrowing would equally fail/error the test but semantically I wouldn't, since it basically means "we didn't get the exception we expected" and so that represents a programming error, instead of a test environment error.

Answer 2

Well, you could just catch Exception and rethrow if it's not an expected Exception. Though good coding practice usually dictates that the success path of code should not be defined by an Exception, so you might want to rethink your design.

Answer 3

I understand the impulse to try to simplify your exception-test idiom, but seriously: don't. Every possible choice you'll come up with is a cure that's worse than the disease. Especially JUnit 4's @ExpectedException nonsense! It is a too-clever frameworky solution, requiring everyone to learn how it works, as opposed to a plain self-evident bit of regular Java code. Worse, it gives you no way to wrap only the part of your test that you expect to throw the exception, so if an earlier setup step throws that same exception, your test will pass even though your code is broken.

I could write a long diatribe about this here (I'm sorry for not having enough time to), as we've had a lengthy discussion of this issue among Java engineers here at Google, and the consensus was that none of these crazy solutions are worthwhile. Get used to try/catch, it's really not that bad.

Answer 4

You can also use an IDE that supports live template ( like IntellJ IDEA for instance ) and assign a shortcut like ee -> [tab] that inserts the try/catch/ignore for your and let you type the correct one.