Exception and its checked Exception subclasses

[Bin Wang]

public class ExceptionTest {
public static void main(String args[]) {
try {
ClassNotFoundException e=null;
}
catch (ClassNotFoundException e) { //compile error
//catch (Exception e) { //compile and run
System.out.println(e);
}
finally {
System.out.println("Finally");
}
}
}
For the above program, catch (Exception e) compile and run, but
catch (ClassNotFoundException e) has the following compile error:
Exception java.lang.ClassNotFoundException is never thrown in the body of the corresponding try statement.
I think java.lang.Exception is never thrown in the body of the corresponding try statement too, but why it doesn't create the compile error?
Thanks.

[Manfred Leonhardt]

Hi Bin,
You might be getting lost in syntax here. The compiler is telling you that your try/catch/finally block is looking for an exception that is not being thrown.
What does that mean? It means that in order to catch an exception something inside the try section must throw it! In your case, your try block doesn't throw any exception, but you want to catch something with your catch statement.
The JLS only states that if you try and catch an exception it had better be thrown somewhere. It doesn't say to use a try/catch/finally block your code should throw an exception inside the try statement!
Think about a case where you want to perform something at the end of a method in most error cases. We could do the following:

public method()
{
  int i = 14;
  int j = 12;
  try
  {
    i = i / j;
    if( i < 5 ) return;
  }
  finally
  {
    System.out.println( "This always prints (even with return being executed)!" );
  }
}

Regards,
Manfred.
[This message has been edited by Manfred Leonhardt (edited June 18, 2001).]

[Bin Wang]

Hi Manfred:
Thanks for your explanation, but could you please explain why the following program compiles and runs fine although its try block doesn't throw any exception at all?
public class ExceptionTest {
public static void main(String args[]) {
try {
Exception e1=null;
ClassNotFoundException e2=null;
Exception e3=new Exception();
ClassNotFoundException e4=new ClassNotFoundException();
}
catch (Exception e) {
System.out.println(e);
}
finally {
System.out.println("Finally");
}
}
}

[Scott Appleton]

Manfred, can you point me to the reference in JLS that deals with trying to catch an Exception which is not thrown by the associated try block? I checked chapters 11 & 14 but couldn't find this information.
Alternatively, what exception could be thrown by the statement "ClassNotFoundException e=null;"? If this statement does indeed throw an exception of some sort, then I understand why "catch (Exception e)" compiles whereas "catch (ClassNotFoundException e)" fails to compile.

[Manfred Leonhardt]

Hi Bin,
The reason your second code compiles, is because you are catching Exception (the superclass of all exceptions). The compiler doesn't know if a run-time exception will happen at compile time so it MUST let the code compile. Whereas with ClassNotFoundException is a specific exception and the compiler can easily figure out if it is being thrown.
Regards,
Manfred.

[Bin Wang]

Manfred,
Thank you very much. I understand it now.