Disambiguating empty catch blocks

Bruce Chapman brucechapman at paradise.net.nz
Tue Apr 13 08:18:27 UTC 2010

Empty catch blocks are ambiguous and can mean one of

- I want to ignore this exception - the code works correctly when the 
catch block executes.
- I know that this exception will never actually occur in this 
particular case - the catch block never executes.
- The catch block is incomplete - a bug.

To enable programmers to disambiguate these situations I propose adding 
the following methods to Exception:

This method does absolutely nothing. Calling it in a catch clause can be 
used to explicitly indicate that the exception itself is being 
deliberately ignored.
public void ignore() {


     This method throws an AssertionError with this Exception as its
     @throws an AssertionError
     public void impossible() {
         AssertionError aPaddy =
             new AssertionError("The impossible happened");
         throw aPaddy;


     byte[] rawMessage= ...;
     String msg = null;
     try {
         msg = new String(rawMessage,"UTF-8");
     } catch (UnsupportedEncodingException e) {

These two methods provide a means to make the programmer's intent 
explicit for the first two meanings of an empty catch block and 
satisfies tools (IDEs and static anaylsis) that the catch block isn't 
just declaring an unused variable, thus enabling a more robust 
interpretation of an empty catch block (or unused exception variable in 
a catch block)  as a problem to be rectified.

Some questions:

Is Exception the right home for this, or is there any value in hoisting 
it into Throwable?

Any problems which I have overlooked?


With a little encouragement (like someone offering to mentor this), I 
will file an RFE for this, and start work on a changeset against 
openJDK7 to implement it. Yes I realise that the javadoc needs a little 
more development.


Bruce Chapman

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