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Exceptions

Syntax, semantics, and pragmatics

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Syntax, semantics, pragmatics

• Syntax– How it looks, i.e. how we have to program to

satisfy the compiler.• Semantics

– What it means / how it works• Pragmatics

– How to use it in the proper way.

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Introduction

• Exceptions are a part of the Java programming language– And other programming languages like C++,

C#, etc.• Exception related keywords in Java

– throw, throws, try … catch … finally

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More syntax: Multi catch

• General try … catch– try { do something dangerous} – catch (SomeException ex) { handle the

exception}• Multi catch

– Java 7 featureCatch (SomeExceptionType |

SomeOtherExceptionType ex) { Handle exception}

– Useful if you want the same handling for different exceptions

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Exception classes

RuntimeException

+getMessage() : String+getCause() : Throwable+toString() : String

-message : String-cause : Throwable

Throwable

Exception Error

IOException

NullPointerException

MyOwnException

Indicate problems inside the JVM

Checked exceptions

Uncheckedexceptions

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Exception class hierarchy

• The class hierarchy can get quite deep – java.lang.Object

• java.lang.Throwable – java.lang.Exception

» java.sql.SQLException » java.ql.SQLWarning

– More detailed exceptions• More detailed information on the problem• More detailed handling of the problem

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Checked vs. runtime exceptions• Checked exceptions

– Extends Exception, directly or indirectly.– Must be caught or declared to be thrown

• This is checked by the compiler• Called the “Catch or specify requirement”

– http://download.oracle.com/javase/tutorial/essential/exceptions/catchOrDeclare.html

– Can usually be recovered from at runtime

• Run-time exceptions– Extends RuntimeException, directly or indirectly– Can be caught or declared to be thrown

• This is not checked by the compiler

– Can usually not be recovered from at runtime

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The class Error

• By convention the class Error is reserved for use by the JVM to indicate problems in the JVM.– Don’t ever subclass Error– Don’t ever throw an Error

• Unless you are programming a JVM, of course!

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What happens when an exception is thrown?

• When an exception is thrown the current block ({…}) is popped of the call stack

• This popping continues until some block has a catch clause.

• If no block has a catch clause we will eventually end in main, which is then popped– And the program stops.

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Sequence of catch blocks

• If a block has more than one catch block the first catch block, that matches the exception type, is executed.

• General rule:– Special exception must be caught before

general exceptionsTry { … }

catch (FileNotFoundException ex) { …}

catch (IOException ex) { …}

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Different kinds of exception handling

• Ignore– Usually a bad programming habit used by 1st semester students

to make the compiler shut up!• Handle

– Only handle the exception if you really can.– Just printing something to the screen is usually a bad idea,

except if you are in the user interface layer.• Re-throw

– If you don’t know how to deal with the exception re-throw it.• Partly handle + re-throw

– Sometimes you want to partly handle the exception for example write to a log file, and then re-throw the exception.

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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(); } }Example: java7exceptionfeatures

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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– Example: java7exceptionfeatures– 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– Example: eventLayersExceptions

• 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.

• A well-designed API must not force the users to use exception for ordinary control flow

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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 (programmer) 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– Example

• throw new IllegalArgumentException(“….” + parameter)

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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.– In JUnit test you sometimes see an empty

catch block• But even here you can improve

catch (SomeException ex) {

assertEquals(“….”, ex.getMessage();

}

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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