Exceptions 1

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/

tryResourceClose.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 279-303

• Joshua Bloch: Effective Java, 2nd edition, Addison Wesley, 2008– Chapter 9: Exceptions,

page 241-258