Why is it legal to re-throw a Throwable in certain cases, without declaring it?

Question

I would expect the following code to raise a compile-time error on throw t;, because main is not declared to throw Throwable, but it c

Answer 1

When you catch Throwable or Exception and the variable is effectively final you can rethrow the same variable and the compiler will know which checked exceptions you could have thrown in the try {} catch block.

Answer 2

This is covered by JLS 11.2.2 (emphasis mine):

A throw statement whose thrown expression is a final or effectively final exception parameter of a catch clause C can throw an exception class E iff:

• E is an exception class that the try block of the try statement which declares C can throw; and

• E is assignment compatible with any of C's catchable exception classes; and

(...)

In other words, E, the type referenced in the doc, is the type that can be thrown, not the type of the catch clause parameter that catches it (the catchable exception class). It just has to be assignment compatible to the catch clause parameter, but the parameter's type is not used in analysis.

This is why the go out of their way to say a final or effectively final exception parameter--if t in your example were reassigned, the analysis would go out the window.

Answer 3

Because the compiler is smart enough to know that a checked exception can not be thrown from the try block, and the caught Throwable is thus not a checked exception that must be declared.

Note that this is true since Java 7, if I'm not mistaken.