Software Engineering Design Patterns

1. Software EngineeringDesign Patterns

2. Singleton • Single instance of class • Constructor is private • static final Class instance constructed when application loads • or loaded only when need (lazy initialization) • Examples of usage • to access database so that all threads go through one control point • Font class keeps memory load low

3. Singleton Example in Java public class DbaseConnector { private static final DbaseConnector instance=new DbaseConnector(); private DbaseConnector() { // database construction code….. } public static DbaseConnector getInstance() { return(instance); } }

4. Singleton Example(lazy initialization) public class DbaseConnector { private static DbaseConnector instance; private DbaseConnector() { // database construction code….. } public static DbaseConnector synchronized getInstance() { if (instance==null) { instance=new DbaseConnector(); } return(instance); } }

5. Wrapper/Adapter classes • Problem • Different external technologies to connect to • Example for database connection • ODBC (Microsoft) • JDBC (Java standard) • Other examples • External Credit card payment • Network connection (Java and Microsoft) • Data structure libraries

6. Wrapper classes • Problem with coding directly • Code will end up messy • Hard to port • Hard to understand • Benefits of wrapping code • easier to swap modules (e.g. CC function) • easier to implement standard functions (e.g. accountancy, error logs)

7. Wrapper example (unwrapped code) String sql="select * from customers"; try { java.sql.Statement s=dbConnection.createStatement(); int rows=s.executeUpdate(sql); } catch (Exception e) { status=sql+" "+e.toString(); };

8. Wrapped code public class SQLHelper { public void executeSQL(String sql) { try { java.sql.Statement s=dbConnection.createStatement(); int rows=s.executeUpdate(sql); } catch (Exception e) { status=sql+" "+e.toString(); }; } }

9. Adapter interface(top level) • Defines all common services and constants • ICardProvider, IEmailProvider • All methods are virtual (no body) • E.g. • takeCardPayment(String cardno,

10. Abstract base • Contains common code that will be used by all providers • CardProviderBase implements ICardProvider • Details validation • HTTP request code • EmailProviderBase implements IEmailProvider • Code to connect to standard SMTP server • Code to handle store, addresses, contents, attachements

11. Common functionality on base class • Name of service • Priority • Enabled or not • Cost of service • Blacklists (email or CC card) • Risk management (card payment) • Audit trails and logging

12. Implementation • The actual providers need to implement the code to connect to particular service • e.g. • MetachargeCardProvider extends CardProviderBase • GmailEmailProvider extends EmailProviderBase

13. Provider manager • This will be a class which creates instances of providers and handles the requests • Card payment • Might try more than 1 provider in turn until payment goes through • Email • Again, tries different providers, some providers might be on or offline at any time

14. Summary • Design patterns can help • code structure • code quality • Using design patterns • look at design patterns example code • adapt design patterns for your own code • reference design pattern texts when designing your system