PROPOSAL: Unchecked Exceptions as Subclasses of Checked Exceptions

[Alan Snyder]

Unchecked Exceptions as Subclasses of Checked Exceptions


AUTHOR: Alan Snyder

OVERVIEW

FEATURE SUMMARY:

This proposal would allow the definition of an unchecked exception class
that extends a checked exception class.

MAJOR ADVANTAGE:

Unchecked exception classes are useful when an exception must be thrown
through an existing interface
that does not declare any checked exceptions or an appropriate checked
exception.
If one can define both an unchecked exception class and a
corresponding checked exception class such that the unchecked exception
class extends the checked exception class,
then the unchecked exception will be caught by a try statement that
catches the checked exception.

MAJOR BENEFIT:

Programs do not have to explictly catch both the unchecked and checked
exceptions. Most developers need
only be aware of the checked exception.

MAJOR DISADVANTAGE:

A new marker interface (or equivalent) must be defined and the definition
of unchecked exceptions changed.

ALTERNATIVES:

The alternative is to always explicitly catch both the unchecked and
checked exceptions, which introduces
a likely source of programming errors.

EXAMPLE:

This proposal uses a marker interface as an alternative way (besides
subclassing RuntimeException and Error)
of indicating that an exception class defines an unchecked exception.

The following would define a checked exception and an unchecked exception
that extends the checked exception:

	public class TheCheckedException extends Exception {};

	public class TheUncheckedException extends TheCheckedException implements
UncheckedException {};

There is no advanced usage.

DETAILS

SPECIFICATION:

The JLS would be changed to add one more way to define an unchecked
exception.

JLS 11.2
The unchecked exceptions classes are the class RuntimeException and its
subclasses, and the class Error and its subclasses.
All other exception classes are checked exception classes.

would become

The unchecked exceptions classes are the class RuntimeException and its
subclasses, the class Error and its subclasses,
and those subclasses of Exception that implement the UncheckedException
interface.
All other exception classes are checked exception classes.

JLS 11.5
The subclasses of Exception other than RuntimeException are all checked
exception classes.

would become

The subclasses of Exception other than RuntimeException are checked
exception classes, unless they implement the UncheckedException interface.

The marker interface UncheckedException would have to be defined in some
java package, with java.lang being the obvious choice.

COMPILATION:

The compiler would have to be extended to recognize the new category of
unchecked exceptions.

I'm not aware of other changes.

COMPATIBILITY

The major compatibility issue arises from the introduction of a new name
in the class name space. That could potentially cause a program
to fail to compile if it imported a class with the name UncheckedException
using a wild card import statement.

There could also be programs that examine the class hierarchy using
reflection that would not recognize some exception classes as being
unchecked.