While some superficial similarities exist between Smalltalk and Java exception handling mechanisms, there are also some fundamental differences. So, deciding whether and how to integrate these mechanisms presented one of the more challenging problems for the design of Bistro. The following table compares the salient aspects of the Smalltalk and Java exception handling mechanisms.
|Table 7. Smalltalk and Java Exceptions|
Finding the common ground between Smalltalk and Java exceptions is
straightforward, but merging the mechanisms completely does not really make
sense. Some of the Smalltalk idioms can easily be mapped to Java. For
ZeroArgumentBlock class provides the standard ifCurtailed:,
ensure:, and on:do: methods for handling exceptions. However,
Bistro also provides a new idiom that the compiler maps directly to the
Java exception handling mechanism.
|Bistro Exception Idioms|
Finally, Bistro supports the declaration of exceptions thrown by methods and blocks. Both method and block signatures can include a throws: clause. If a Bistro method signature includes a throws: clause, the compiler translates it directly into the equivalent Java method signature - i.e., the Java method will also include a throws clause with the same list of exception classes. However, block exceptions require special handling by the compiler.
The signatures of the value methods in the block classes (e.g.,
ZeroArgumentBlock) do not include exception declarations. For this reason, the Bistro
compiler automatically generates wrapper methods for blocks that are declared
to throw exceptions. Each generated wrapper method catches any exception thrown
by a block and rethrows it as an
UnhandledJavaException is derived from
RuntimeException, the Java compiler will not complain about the exception
erasure. Exception erasure is a special kind of type erasure that
makes the Java exceptions transparent. The following table provides
examples of the syntax used to declare method and block exceptions.
|Methods with Exceptions||Description|
||This unary method can throw any exception.|
||This binary method can throw any exception.|
||This keyword method can throw an
|Blocks with Exceptions||Description|
||This block can throw any exception. However, if an
exception is thrown from within the block, it will be caught and rethrown as an
||This block can throw an
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Copyright 1999-2001 Nikolas S. Boyd.