Why do you have to write throws exception in a class definition?

Question

Coming from C#, I just don't get this 'throws exception' that is written after a class/method definition:

public void Test() throws Exception

Do you have to write this? What if you don't? If I call a method that has this symbol, do I have to catch it?

Answer 1

You don't have to write it in all cases -- you just have to write it if your method throws a checked Exception (an exception that is a subclass of Exception and not a subclass of RuntimeException). This is because your method signature is a contract, and it's declaring to all the code which calls it that it has the potential for throwing the given exception. Because it's a checked exception -- an exception which can be anticipated -- the calling code needs to anticipate the potential of seeing the exception thrown, and needs to be able to handle it.

To answer your two specific questions:

• do you have to write it/what if you don't: if your method throws a checked exception, then yes, you have to declare it in your method signature. If you don't, then your code won't compile.
• do you have to catch it: you have to do something with it. The code calling the method can either catch it and handle it, it can catch it and re-throw it, or it can just pass it up the chain. In order to pass it up the chain, the code calling the method has to itself declare that it throws the same exception -- e.g., if method bar can throw SomeException, and method foo calls bar and doesn't want to catch the exception, the method signature for foo would declare that it too throws SomeException.

The Exceptions chapter of the Java lessons is very good at explaining this in detail, and JavaWorld has a good article about throwing and catching exceptions that I've always found to be a good reference to pass onto folks.