Why doesn't Java allow generic subclasses of Throwable?

java generics exception language-design
Hosam Aly picture Hosam Aly · Feb 1, 2009 · Viewed 30k times · Source

According to the Java Language Sepecification, 3rd edition:

It is a compile-time error if a generic class is a direct or indirect subclass of Throwable.

I wish to understand why this decision has been made. What's wrong with generic exceptions?

(As far as I know, generics are simply compile-time syntactic sugar, and they will be translated to Object anyway in the .class files, so effectively declaring a generic class is as if everything in it was an Object. Please correct me if I'm wrong.)

Answer

Torsten Marek picture Torsten Marek · Feb 1, 2009

As mark said, the types are not reifiable, which is a problem in the following case:

try {
   doSomeStuff();
} catch (SomeException<Integer> e) {
   // ignore that
} catch (SomeException<String> e) {
   crashAndBurn()
}

Both SomeException<Integer> and SomeException<String> are erased to the same type, there is no way for the JVM to distinguish the exception instances, and therefore no way to tell which catch block should be executed.

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