Generic Class May Not Extend Throwable

Another bit of Java trivia I just learned: A generic class may not implement Throwable. Interesting, huh? I guess Throwable and exceptions have a special meaning in Java and representation in the JVM, so there are probably some limitations, but I still find this irritating.

I was trying to write a wrapper exception that extended RuntimeException for the purpose of moving a checked exception out of a method that does not have a throws clause for that checked exception in a controlled fashion. This is what I tried:

public class WrappedException
extends RuntimeException {
private T _wrapped;
public WrappedException(String message, T cause) {
super(message, cause);
_wrapped = cause;
}
public WrappedException(T cause) {
super(cause);
_wrapped = cause;
}
public T wrapped() { return _wrapped; }
}


It would have allowed me to do the following:

public someMethod() { // no throws clause
try {
...
}
catch(IOException e) { // checked exception
throw new WrappedException(e);
}
}

public otherMethod() throws IOException {
try {
someMethod();
}
catch(WrappedException we) {
throw we.wrapped();
}
}


But I guess it all makes sense… It’s erasure at work. If you look at the catch clause, you just know this can’t work in Java: How will the JVM be able to distinguish a WrappedException<IOException> from a WrappedException<ClassNotFoundException>? It just can’t, with everything erased.