Design Patterns in Java (Behavioral) - Command Pattern

Command Pattern

• Encapsulate request as an object
• Object-oriented callback
• Decouple sender from processor
• Often used for "undo" action

Design

• Object per command
• Command interface
• execute method
• undo method
• Use reflexion

Main

public static void main(String[] args) {



  //Receiver - (Action)

  LightReceiver lc = new LightReceiver();





  //OnCommand = concrete command

  Command c = new OnCommand(lc);






  //Invoker - (Execute)

  SwitchInvoker si = new SwitchInvoker();

  si.storeAndExecute(c); 

 }

Receiver

public class LightReceiver {



 public void on(){

  System.out.println("Light is on");

 }

 public void off(){

  System.out.println("Light is off");

 }

}

Invoker

public class SwitchInvoker {



  public void storeAndExecute( Command command){

   command.execute();

  }

}

 

Command

public interface Command {



 public void execute();

}

Specific Command

public class OnCommand implements Command {



 LightReceiver lg;

 

 public OnCommand(LightReceiver lg){

  this.lg = lg;

 } 

 

 @Override

 public void execute() {

  lg.on();

 }

}

Adding STATE

Specific Command with State

In this case is another command but the Invoker state equals

Receiver

public class ToogleLightReceiver {






boolean isOn = false;

 

 public boolean isOn(){

  return isOn;

 }

 

 public void toogle(){

  if(isOn)

   off();

  else

   on();

  isOn = !isOn;

 }

 

 public void on(){

  System.out.println("Light is on");

 }

 

 public void off(){

  System.out.println("Light is off");

 }

}

New Command with state

public class ToggleCommand implements Command {

 

 ToogleLightReceiver lg;

 

 public ToggleCommand(ToogleLightReceiver lg){

  this.lg = lg;

 } 

 

 @Override

 public void execute() {

  lg.toogle();

 }

}

Main

 public static void main(String[] args) {




  //Receiver



  ToogleLightReceiver lc = new ToogleLightReceiver();







  //Invoker



  SwitchInvoker si = new SwitchInvoker();



  



  //OnCommand = concrete command



  Command c = new ToggleCommand(lc);



  



  si.storeAndExecute(c);



  si.storeAndExecute(c);



  



 }