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Thinking in Java
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History

Exception handling originated in systems like PL/1 and Mesa, and later appeared in CLU, Smalltalk, Modula-3, Ada, Eiffel, C++, Python, Java, and the post-Java languages Ruby and C#. The Java design is similar to C++, except in places where the Java designers felt that the C++ design caused problems.

To provide programmers with a framework that they were more likely to use for error handling and recovery, exception handling was added to C++ rather late in the standardization process, promoted by Bjarne Stroustrup, the language’s original author. The model for C++ exceptions came primarily from CLU. However, other languages existed at that time that also supported exception handling: Ada, Smalltalk (both of which had exceptions but no exception specifications) and Modula-3 (which included both exceptions and specifications).

In their seminal paper[44] on the subject, Liskov and Snyder note that a major defect of languages like C that report errors in a transient fashion is that:

“...every invocation must be followed by a conditional test to determine what the outcome was. This requirement leads to programs that are difficult to read, and probably inefficient as well, thus discouraging programmers from signaling and handling exceptions.”

Note that one of the original motivations of exception handling was to prevent this requirement, but with checked exceptions in Java we commonly see exactly this kind of code. They go on to say:

“...requiring that the text of a handler be attached to the invocation that raises the exception would lead to unreadable programs in which expressions were broken up with handlers.”

Following the CLU approach when designing C++ exceptions, Stroustrup stated that the goal was to reduce the amount of code required to recover from errors. I believe that he was observing that programmers were typically not writing error-handling code in C because the amount and placement of such code was daunting and distracting. As a result, they were used to doing it the C way, ignoring errors in code and using debuggers to track down problems. To use exceptions, these C programmers had to be convinced to write “additional” code that they weren’t normally writing. Thus, to draw them into a better way of handling errors, the amount of code they would need to “add” must not be onerous. I think it’s important to keep this goal in mind when looking at the effects of checked exceptions in Java.

C++ brought an additional idea over from CLU: the exception specification, to programmatically state in the method signature what exceptions may result from calling that method. The exception specification really has two purposes. It can say “I’m originating this exception in my code, you handle it.” But it can also mean “I’m ignoring this exception that can occur as a result of my code, you handle it.” We’ve been focusing on the “you handle it” part when looking at the mechanics and syntax of exceptions, but here I’m particularly interested in the fact that often we ignore exceptions and that’s what the exception specification can state.

In C++ the exception specification is not part of the type information of a function. The only compile-time checking is to ensure that exception specifications are used consistently; for example, if a function or method throws exceptions, then the overloaded or derived versions must also throw those exceptions. Unlike Java, however, no compile-time checking occurs to determine whether or not the function or method will actually throw that exception, or whether the exception specification is complete (that is, whether it accurately describes all exceptions that may be thrown). That validation does happen, but only at run time. If an exception is thrown that violates the exception specification, the C++ program will call the standard library function unexpected( ).

It is interesting to note that, because of the use of templates, exception specifications are not used at all in the standard C++ library. Exception specifications, then, may have a significant impact on the design of Java generics (Java’s version of C++ templates, expected to appear in JDK 1.5).
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire