CSC 520 – Advanced Object Oriented Programming, Fall, 2010 Thursday, September 2, Week 4 Design by Contract, Java Exceptions, Eventing, Finding the Classes, Design Patterns and the Kitchen Sink

Design by Contract Preconditions – constraints on the caller Postconditions – constraints on the called Invariants – constraints on an object’s state Preconditions may weaken for derived class methods that override a base class or interface method. Why??? Postconditions may strengthen for derived class methods that override a base class or interface method. Why???

Exception constructs already in use try { attempt to run a block of code } catch (Type_A_Exception taex) { Do something with taex, e.g., taex.getMessage() } catch (Type_A_Exception tbex) { Do something with tbex, e.g., print, then throw tbex } finally { Do this after all of the above, even after a return! }

Unchecked and checked exceptions javac forces client code to catch checked exceptions unchecked exceptions need not be caught; they can be – java.lang.Error java.io.IOError – maybe a disk drive goes off line – java.lang.RuntimeException java.lang.NullPointerException Other java.lang.Exceptions are checked

Throwing a new exception An explicit throw creates a new Exception. – public int deleteTiles(String tileset) throws ScrabbleException throw new ScrabbleException("Player " + name + " does not have " + tile.toString() + " to delete from set of tiles.");

Rethrowing an exception A rethrow catches and handles an Exception object, then throws it again. } catch (NumberFormatException nx) { System.err.println(“Exception: “ + nx.getMessage(); nx.printStackTrace(); // to System.err throw nx ; // This is the rethrow

An implicit throw An implicit throw allows a called method to throw an Exception via the calling method. – public String [][] move(String command) throws ScrabbleException Invokes “int used = players[nextPlayer].deleteTiles(validword); But move() does not catch deleteTiles’s exception: – public int deleteTiles(String tileset) throws ScrabbleException The remove() implicitly throws deleteTile’s exception.

A Chained Exception A chained Exceptions tacks a detail message onto an underlying cause Exception. – } catch (NumberFormatException nx) { – throw new Exception(“detail message”, nx); Used to prepend a context-specific message to an Exception thrown from a called method. The new Exception must have the appropriate constructor. It may be a custom Exception.

Custom Exceptions A custom Exception inherits, directly or indirectly, from java.lang.Exception. – public class ScrabbleException extends Exception public ScrabbleException(String text) – super(text); // Call to base class constructor. Client code can explicitly catch a custom Exception.

An event mechanism = publishers, subscribers & delivered events java.util.EventListener is a tagging interface that applications implement to create a listener whose class identity is unknown to the publisher java.awt.event.ActionListener for GUIs java.beans.beancontext.BeanContext for Java Bean tools that automated assembly of components via graphical tools games2010rev5.GameMoveEventListener

Publishers Publish Events Publisher classes support subscribe and unsubscribe methods via which their Event Listeners (see previous slide) can subscribe and unsubscribe for event delivery. Publishers later deliver events to subscribed listeners when the events occur. There is no standard interface or base class for publishers in the Java library. Any class that creates and delivers an event type is a publisher.

Event Objects Carry Data java.util.EventObject is the library base class getSource() returns Object source of the event – Subclasses use this base class to store the event publisher java.awt.event.ActionEvent for GUIs java.beans.beancontext.BeanContextEvent games2010rev5.GameMoveEvent – getDescription(), getGameState(), getMoveNumber() Publisher passes data contents to the event constructor – Event class provides getters for these data javax.sound.midi.MidiEvent does not implement it

UML Sequence Diagram for Event Subscription Lifecycle TwoDimensionalBoardGameGameMoveEventListener subscribeToGameMoveEvent notifyMoveEvent(event:GameMoveEvent) (0..* times) unsubscribeToGameMoveEvent

Observer Design Pattern Gang of Four calls this the Observer Pattern “Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.” The official design pattern does not show the event object, which typically passes as a parameter to the event listener-defined method.

Meyer and Design Patterns 22.3 General Heuristics for Finding Classes An analysis class describes a data abstraction directly drawn from the model of the external system. ScrabbleBoard An implementation class describes a data abstraction introduced for the internal needs of the algorithms in the software. private char [][] tiles = new char [RowHeadings.length][ColumnHeadings.length]; A design class describes an architectural choice. Design patterns.

How to Find the Classes Tell stories. Underline the nouns. Adjectives for deferred classes (comparable, serializable). Watch out for natural language nouns! Avoid useless classes. Avoid Blobs and Grand Central Stations. Elicit, then reject classes. (Edit!) Previous designs and class libraries. Meyer uses use cases only for validation, not analysis. He does not like CRC cards We use class diagrams.

Some other Design Patterns for Tonight -- Factories Abstract Factory – our map_interface The manufacturing interface is abstract. The manufactured object interface is abstract. A concrete class derived from the manufacturing interface constructs concrete objects derived from the manufactured object interface. Factory Method The manufacturing class has an abstract manufacturing method. Its subclasses manufacture concrete class objects.

Memento Pattern Without violating encapsulation, capture and externalize an object’s state so that the object can be restored to this state later. java.lang.Cloneable, java.io.Serializable See Scrabble class data field definitions. See ScrabbleGame save and restore methods.

Iterator Pattern Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. An iterator is an abstracted pointer. See interface java.util.Iterator and ListIterator. C++ Standard Template Library supports forward and reverse iterators.