Command Pattern

Tags
Design Pattern, Java

Introduction

• Purpose
  • Encapsulates a request as an object
  • This allows the request to be handled in traditionally object based relationships such as queuing and callbacks.
• Use When
  • Requests need to be specified, queued, and executed at variant times or in variant orders.
  • A history of requests is needed.
  • The invoker should be decoupled from the object handling the invocation.

How to Use (Example)

• Command Interface

  public interface Command {
      public void execute();
  }
  

• Implementing a command

  public class MyCommand implements Command {
      MyReceiver myRecevier;
  
      public MyCommand(MyReceiver myReceiver) {
          this.myReceiver = myReceiver;
      }
  
      public void execute() {
          // Call a method of myReceiver
          // ex. myReceiver.work();
      }
  }
  

• Building Invoker

  public class MyInvoker {
      Command slot;
  
      public MyInvoker() {}
  
      public void setCommand(Command command) {
          slot = command;
      }
  
      // Use your own method
      public void buttonPressed() {
          slot.execute();
      }
  }
  

• Client Program

  public class Main {
      public static void main(String[] args) {
          // Create invoker
          MyInvoker myInvoker = new MyInvoker();
  
          // Create receiver
          MyReceiver myReceiver = new MyReceiver();
  
          // Create command
          MyCommand myCommand = new MyCommand(myReceiver);
  
          // linking the invoker with the command
          myInvoker.setCommand(myCommand);
  
          myInvoker.buttonPressed();
      }
  }