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Exceptions
Syntax, semantics, and
pragmatics
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Exception create
If (some error){
throw new SomeException(”some message”);
}
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Exceptions catch
Try{
methods where exception can occur
} catch( SomeException se){
do the exception handling
}
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Finally
• The finally block is executed whether or not an
exception is thrown.
– Leaving the method you always execute the finally
block
• Used to release resources
– Example: Closing a connection to a network,
database, or file
– Coding idiom:
FileReader input = null;
try { … open input and use it … }
finally { if (input != null) { input.close(); } }
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Try with resource statement
• Java 7 language feature
• The coding idiom
– Declare … try { open + use } finally {close }
– Is now supported in the Java programming language
• New syntax
– Try (open + initialize) { use }
– The resource must implement the
java.lang.AutoCloseable interface
– Finally is no longer necessary in this case
– Further readings + examples
• The Java Tutorial
– http://download.oracle.com/javase/tutorial/essential/exceptions/try
ResourceClose.html
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Program your own exception
• Why?
– Technical exceptions like IOException,
SQLException, etc. should not be propagated to the
model layer.
– Instead you must define your own application specific
exception like LibraryException
• How? That’s very easy!
– Define a new class which extends the class Exception
– You probably need to define 3 constructors.
– Your exception class may have data + methods
• But you probably never need it.
– NetBeans can assist you.
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Item 57: Use exceptions only for
exceptional conditions
• Don’t loop over a collection until it throws
an exception.
• Only throw exceptions if the state is really
exceptional
– Searching for something without finding it, is
that exceptional? Probably not. Better to
return null.
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Item 58: Checked exceptions vs.
run-time exceptions
• Use checked exceptions for recoverable
conditions and run-time exceptions for
programming errors
– Use checked exception for conditions form which the
call can reasonably be expected to recover.
– Use run-time exceptions to indicate programming
error
• The caller made an error
– Most likely a violation the methods precondition
– Examples: IndexOutOfBoundException, NullPointerException
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Item 59: Avoid unnecessary use of
checked exceptions
• If the caller cannot handle the exception,
then throw a run-time exception.
• Provide check methods
– Example: StringTokenizer.hasMoreElements()
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Item 60: Favor the use of standard
exceptions
• Don’t use a home-made exception if you
can use a standard exception.
• Specially with run-time exceptions.
• Reusable standard run-time exceptions
– IllegalArgumentException
– IllegalStateException
– NullPointerException
– IndexOutOfBoundsException
– UnsupporteOperationException
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Item 61: Throw exceptions
appropriate to the abstraction
• Higher layers should catch lower-level
exceptions and throw exceptions appropriate for
the higher level
• Exception translation
– Catch (LowLevelException ex) { throw new
HighLevelException(message); }
• Exception chaining
– Catch (LowLevelException ex) { throw new
HighLevelException(ex); }
– The LowLevelException is “inside” the
HighLevelException
– New in Java 1.4: New constructor in class Throwable
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Item 62: Document all exceptions
thrown by each method
• For all your methods
– Document (using the Javadoc @throws tag)
all the exceptions the method might throw
– Including unchecked exceptions.
• NetBeans can assist you
– Mainly with checked exceptions.
– Don’t forget the run-time exceptions.
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Item 63: Include failure-capture
information in detail message
• The message in the exception is the only
information the receiver gets.
• The message in the exception should
include all values that “contributed” to the
exception
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Item 64: Strive for failure atomicity
• A failed method should invocation should
leave the object in the state that it was
prior to invocation.
– Easier to recover from exception.
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Item 65: Don’t ignore exceptions
• An empty catch block is highly suspicious
– If you really mean it, then write a comment in
the empty catch block.
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References
• Ken Arnold et al.: The
Java Programming
Language, 4th edition,
Addison Wesley, 2006
– Chapter 12: Exceptions
and Assertions, page 279303
• Joshua Bloch: Effective
Java, 2nd edition,
Addison Wesley, 2008
– Chapter 9: Exceptions,
page 241-258
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