Download Command Pattern

Command
Pattern
Prepared by
Amol Vaze
Command Pattern
 Command Pattern is used to encapsulate a
request as an object[command] & pass to an
invoker.
 Command pattern is behavioural design pattern.
 Invoker is unaware of how to service requests
uses the encapsulated command to perform the
action.
Command Pattern- Hotel
Example
 Command Pattern allows requests to be encapsulated




as objects, thereby allowing clients to be
parameterized with different requests.
The “Check” at a dinner is an example of a command
pattern.
In example, waiter takes an order from a customer &
encapsulates it by writing it on the check.
The order is queued for a shorter order cook.
Check pad is being used by waiter is independent of
the menu & hence it supports commands to cook
many different items.
Command Pattern – Menu
Example
 In menu example, we have menu options as invoker




object which does not know how to handle request.
Invoker object use the command object execute
method to perform the needed menu operations.
Also we have our document as receiver object.
Execute method calls receiver object to handle the
request.
Command objects in this example have following 3
commands: Open Command
 Close Command
 Save Command
Command Pattern- Class
Diagram
Command – Sequence
Diagram
Command – Objects
Creation & Flow
Command Pattern –
Implementation
Command Pattern – Code
We have following java classes for our Menu
real life example.
 WordDocument.java
 Command.java
 OpenCommand.java
 SaveCommand.java
 CloseCommand.java
 MenuOptions.java
 Client.java
WordDocment. Java code
This class serves as Receiver in our working example.
public class WordDocument
{
public void open()
{
System.out.println("Document Opened");
}
public void save()
{
System.out.println("Document Saved");
}
public void close()
{
System.out.println("Document Closed");
}
}
Command Interface Code
This is a command interface in our working example
which has a method called “execute()”.
Command.java
public interface Command
{
public void execute();
}
OpenCommand.Java code
This class is responsible for opening document and since it implements Command
interface it has to override implemented “execute” method.
public class OpenCommand implements Command
{
private WordDocument wordDocument;
public OpenCommand( WordDocument wordDocument )
{
this.wordDocument = wordDocument;
}
@Override
public void execute()
{
wordDocument.open();
}
}
SaveCommand.Java code
This class is responsible for saving document and since it implements Command
interface it has to override implemented “execute” method.
public class SaveCommand implements Command
{
private WordDocument wordDocument;
public SaveCommand( WordDocument wordDocument )
{
this.wordDocument = wordDocument;
}
@Override
public void execute()
{
wordDocument.save();
}
}
CloseCommand.Java code
This class is responsible for closing document and since it implements Command
interface it has to override implemented “execute” method.
public class CloseCommand implements Command
{
private WordDocument wordDocument;
public CloseCommand( WordDocument wordDocument )
{
this.wordDocument = wordDocument;
}
@Override
public void execute()
{
wordDocument.close();
}
}
MenuOptions. Java Code
public class MenuOptions
{
private Command openCommand;
private Command saveCommand;
private Command closeCommand;
public MenuOptions( Command open, Command save, Command close )
{
this.openCommand = open;
this.saveCommand = save;
this.closeCommand = close;
}
public void clickOpen()
{
openCommand.execute();
}
public void clickSave()
{
saveCommand.execute();
}
public void clickClose()
{
closeCommand.execute();
}
}
Client.Java Code
This is main client class which has objects of all other classes and it invokes respective class
methods to perform the actions.
public class Client
{
public static void main( String[] args )
{
WordDocument wordDocument = new WordDocument();
Command openCommand = new OpenCommand(wordDocument);
Command saveCommand = new SaveCommand(wordDocument);
Command closeCommand = new CloseCommand(wordDocument);
MenuOptions menu = new MenuOptions(openCommand, saveCommand, closeCommand);
menu.clickOpen();
menu.clickSave();
menu.clickClose();
}
}
Output For Menu Code
Hence, finally this java code is able to produce the
following output using Command design pattern.
Output 
Document Opened
Document Saved
Document Closed
Thank You!