Java SneakyThrow of exceptions, type erasure

Question

Can someone explain this code?

public class SneakyThrow {


  public static void sneakyThrow(Throwable ex) {
    SneakyThrow.sneakyTh         


        

Answer 1

If you compile it with -Xlint you'll get a warning:

c:\Users\Jon\Test>javac -Xlint SneakyThrow.java
SneakyThrow.java:9: warning: [unchecked] unchecked cast
    throw (T) ex;
              ^
  required: T
  found:    Throwable
  where T is a type-variable:
    T extends Throwable declared in method sneakyThrowInner(Throwable)
1 warning

That's basically saying "This cast isn't really checked at execution time" (due to type erasure) - so the compiler reluctantly assumes you're doing the right thing, knowing that it won't actually be checked.

Now it's only the compiler which cares about checked and unchecked exceptions - it's not part of the JVM at all. So once you've got past the compiler, you're home free.

I'd strongly advise you to avoid doing this though.

In many cases there's a "real" check when you're using generics because something uses the desired type - but that's not always the case. For example:

List strings = new ArrayList();
List raw = strings;
raw.add(new Object()); // Haha! I've put a non-String in a List!
Object x = strings.get(0); // This doesn't need a cast, so no exception...