1CMSC 433 – Programming Language Technologies and ParadigmsSpring 2007Façade PatternApr. 05, 20072What is it?• Frequently, as your programs evolve and develop, they grow in complexity. • Furthermore, there may be a number of complicated subsystems, each of which has its own complex interface.• The Façade pattern allows you to simplify this complexity by providing a simplified interface to these subsystems. • This simplification may in some cases reduce the flexibility of the underlying classes, but usually provides all the function needed for all but the most sophisticated users.• These users can still, of course, access the underlying classes and methods.3For Example• Java provides a set of classes that connect to databases using an interface called JDBC. • You can connect to any database for which the manufacturer has provided a JDBC connection class -- almost every database on the market. • Some databases have direct connections using JDBC and a few allow connection to ODBC driver using the JDBC-ODBC bridge class.4For Example• To connect to a database, you use an instance of the Connection class. • Then, to find out the names of the database tables and fields, you need to get an instance of the DatabaseMetadata class from the Connection. • Next, to issue a query, you compose the SQL query string and use the Connection to create a Statement class.• By executing the statement, you obtain a ResultSet class, and to find out the names of the column rows in that ResultSet, you need to obtain an instance of the ResultsetMetadata class.• Thus, it can be quite difficult to juggle all of these classes and since most of the calls to their methods throw Exceptions, the coding can be messy.5In Pictures!6A Facade• By designing a Façade consisting of a Database class and a resultSet class (note the lowercase “r”), we can build a much more usable system.7Database Class8resultSet9Summary• The Façade pattern shields clients from complex subsystem components and provides a simpler programming interface for the general user. • However, it does not prevent the advanced user from going to the deeper, more complex classes when
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