Structural Design Patterns # facade

Facade is a structural design pattern that provides a simplified interface to a library, a framework, or any other complex set of classes.

Problem

Imagine that you must make your code work with a broad set of objects that belong to a sophisticated library or framework. Ordinarily, you’d need to initialize all of those objects, keep track of dependencies, execute methods in the correct order, and so on.

As a result, the business logic of your classes would become tightly coupled to the implementation details of 3rd-party classes, making it hard to comprehend and maintain.

Solution

A facade is a class that provides a simple interface to a complex subsystem which contains lots of moving parts. A facade might provide limited functionality in comparison to working with the subsystem directly. However, it includes only those features that clients really care about.

Having a facade is handy when you need to integrate your app with a sophisticated library that has dozens of features, but you just need a tiny bit of its functionality.

For instance, an app that uploads short funny videos with cats to social media could potentially use a professional video conversion library. However, all that it really needs is a class with the single method encode(filename, format). After creating such a class and connecting it with the video conversion library, you’ll have your first facade.

Structure

Facade Design Pattern Example

We can have two helper interfaces, namely MySqlHelper and OracleHelper.

import java.sql.Connection;

public class MySqlHelper {
	
	public static Connection getMySqlDBConnection(){
		//get MySql DB connection using connection parameters
		return null;
	}
	
	public void generateMySqlPDFReport(String tableName, Connection con){
		//get data from table and generate pdf report
	}
	
	public void generateMySqlHTMLReport(String tableName, Connection con){
		//get data from table and generate pdf report
	}
}

import java.sql.Connection;

public class OracleHelper {

	public static Connection getOracleDBConnection(){
		//get Oracle DB connection using connection parameters
		return null;
	}
	
	public void generateOraclePDFReport(String tableName, Connection con){
		//get data from table and generate pdf report
	}
	
	public void generateOracleHTMLReport(String tableName, Connection con){
		//get data from table and generate pdf report
	}
	
}

Facade Design Pattern Interface

We can create a Facade pattern interface like below. Notice the use of Java Enum for type safety.

import java.sql.Connection;

public class HelperFacade {

	public static void generateReport(DBTypes dbType, ReportTypes reportType, String tableName){
		Connection con = null;
		switch (dbType){
		case MYSQL: 
			con = MySqlHelper.getMySqlDBConnection();
			MySqlHelper mySqlHelper = new MySqlHelper();
			switch(reportType){
			case HTML:
				mySqlHelper.generateMySqlHTMLReport(tableName, con);
				break;
			case PDF:
				mySqlHelper.generateMySqlPDFReport(tableName, con);
				break;
			}
			break;
		case ORACLE: 
			con = OracleHelper.getOracleDBConnection();
			OracleHelper oracleHelper = new OracleHelper();
			switch(reportType){
			case HTML:
				oracleHelper.generateOracleHTMLReport(tableName, con);
				break;
			case PDF:
				oracleHelper.generateOraclePDFReport(tableName, con);
				break;
			}
			break;
		}
		
	}
	
	public static enum DBTypes{
		MYSQL,ORACLE;
	}
	
	public static enum ReportTypes{
		HTML,PDF;
	}
}

Facade Design Pattern Client Program

Now lets see client code without using Facade pattern and using Facade pattern interface.

import java.sql.Connection;

public class FacadePatternTest {

	public static void main(String[] args) {
		String tableName="Employee";
		
		//generating MySql HTML report and Oracle PDF report without using Facade
		Connection con = MySqlHelper.getMySqlDBConnection();
		MySqlHelper mySqlHelper = new MySqlHelper();
		mySqlHelper.generateMySqlHTMLReport(tableName, con);
		
		Connection con1 = OracleHelper.getOracleDBConnection();
		OracleHelper oracleHelper = new OracleHelper();
		oracleHelper.generateOraclePDFReport(tableName, con1);
		
		//generating MySql HTML report and Oracle PDF report using Facade
		HelperFacade.generateReport(HelperFacade.DBTypes.MYSQL, HelperFacade.ReportTypes.HTML, tableName);
		HelperFacade.generateReport(HelperFacade.DBTypes.ORACLE, HelperFacade.ReportTypes.PDF, tableName);
	}

}