façade pattern

The façade pattern is an object-oriented design pattern.

A façade is an object that provides a simplified interface to a larger body of code, such as a class library. A façade can:

• make a software library easier to use and understand, since the façade has convenient methods for common tasks;
• make code that uses the library more readable, for the same reason;
• reduce dependencies of outside code on the inner workings of a library, since most code uses the façade, thus allowing more flexibility in developing the system;
• wrap a poorly designed collection of APIs with a single well-designed API (As per task needs).

An Adapter is used when our wrapper must respect a particular interface and must support a polymorphic behavior. On the other hand, a façade is used when one wants an easier/simpler interface to work with.

Structure

Façade

The façade class interacts Packages 1, 2 & 3 with the rest of the application.

Clients

The objects using the Façade Pattern to access resources from the Packages.

Packages

Software library / API collection accessed through the Façade Class.

Examples

Java

The following example hides the parts of a complicated calendar API behind a more user friendly façade. The output is:

Date: 1980-08-20
20 days after: 1980-09-09

<source lang="java"> import java.text.*; import java.util.*;

/** "Facade" */ class UserfriendlyDate {

   Calendar cal = Calendar.getInstance();
   SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
    
   public UserfriendlyDate (String isodate_ymd) throws ParseException {
       Date date = sdf.parse(isodate_ymd);
       cal.setTime(date);
   }
  
   public void addDays (int days) {
       cal.add (Calendar. DAY_OF_MONTH, days); 
   }

   public String toString() { 
       return sdf.format(cal.getTime()); 
   }

}

/** "Client" */ class FacadePattern {

   public static void main(String[] args) throws ParseException
   {  
       UserfriendlyDate d = new UserfriendlyDate("1980-08-20");   
       System.out.println ("Date: " + d.toString());   
       d.addDays(20);   
       System.out.println ("20 days after: " + d.toString());
   }

} </source>

External links

• Description from the Portland Pattern Repository
• Description from the Net Objectives Repository
• Facade pattern discussion with 1-page examples

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