Facade : Compiler Example 

Facade pattern comes under Structural design pattern. It provides a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

Behaviour & Advantages

• Simplifies the subsystem usage.
• Acts as entry point to the subsystem.
• Shields clients from subsystem components.
• The interaction and communication among subsystems is hidden from client.

• Facade
  Provides a simplified API to interact with subsystems. It knows how to handle a request and manages request processing with subsystems.
• Subsystem classes
  Complex set of classes designed to perform a specific task.
• Client
  One who uses Facade to interact with subsystems.

This example shows a Compiler Facade which shield the users from directly interacting with Subsystems such as Scanner, Parser, Assembler and Linker.

Compiler and subsystem classes are shown below,

File Name  :   com/bethecoder/tutorials/dp/facade3/Compiler.java

package com.bethecoder.tutorials.dp.facade3; public class Compiler {   Parser parser = new Parser();   Assembler assembler = new Assembler();   Linker linker = new Linker();      public void compile(String sourceFile) {     String orgSrcFile = sourceFile;     parser.parse(sourceFile);     sourceFile = compileInternal(sourceFile);     sourceFile = assembler.assemble(sourceFile);     sourceFile = linker.link(sourceFile);          System.out.println();     System.out.println("Successfully compiled " + orgSrcFile);     System.out.println("Final executable is " + sourceFile);   }      private String compileInternal(String sourceFile) {     sourceFile = sourceFile.toLowerCase().replaceAll(".cpp", ".asm");     System.out.println("Compiled to assembly " + sourceFile);     return sourceFile;   } }

Scanner subsystem.

File Name  :   com/bethecoder/tutorials/dp/facade3/Scanner.java

package com.bethecoder.tutorials.dp.facade3; public class Scanner {   public void scan(String sourceFile) {     System.out.println("Started scanning " + sourceFile);   } }

Parser subsystem.

File Name  :   com/bethecoder/tutorials/dp/facade3/Parser.java

package com.bethecoder.tutorials.dp.facade3; public class Parser {   private Scanner scanner = new Scanner();      public void parse(String sourceFile) {     scanner.scan(sourceFile);     System.out.println("Started parsing " + sourceFile);   } }

Assembler subsystem.

File Name  :   com/bethecoder/tutorials/dp/facade3/Assembler.java

package com.bethecoder.tutorials.dp.facade3; public class Assembler {   public String assemble(String sourceFile) {     sourceFile = sourceFile.toLowerCase().replaceAll(".asm", ".obj");     System.out.println("Translated to binary object code " + sourceFile);     return sourceFile;   } }

Linker subsystem.

File Name  :   com/bethecoder/tutorials/dp/facade3/Linker.java

package com.bethecoder.tutorials.dp.facade3; public class Linker {   public String link(String sourceFile) {     sourceFile = sourceFile.toLowerCase().replaceAll(".obj", ".exe");     System.out.println("Linked to executable " + sourceFile);     return sourceFile;   } }

Facade pattern usage is shown below,

File Name  :   com/bethecoder/tutorials/dp/facade3/Test.java

package com.bethecoder.tutorials.dp.facade3; public class Test {   /**    * @param args    */   public static void main(String[] args) {          /**      * The client deals with Compiler      * which generates the final executable.      *       * He does'nt need to know how      * Scanner, Parser, Assembler and      * Linker interact.      */     Compiler compiler = new Compiler();     compiler.compile("C:\\random.cpp");   } }

Started scanning C:\random.cpp
Started parsing C:\random.cpp
Compiled to assembly c:\random.asm
Translated to binary object code c:\random.obj
Linked to executable c:\random.exe

Successfully compiled C:\random.cpp
Final executable is c:\random.exe