Command Design Pattern

Command Design Pattern Rick Mercer and Rice University

Command Design Pattern • The Command Pattern encapsulates a request as an object, thereby letting you parameterize other objects with different requests, queue or log requests, and support undoable operations.

General Form

Example for Heads FirstNote: RemoteLoader loads commands into slots of the remote control

Command Pattern • One object can send messages to other objects without knowing anything about the actual operation or the type of object • Polymorphism lets us encapsulate a request for services as an object • Establish a method signature name as an interface • Vary the algorithms in the called methods

Java Examples • Sun used the Command pattern to improve the event model in Java 1.1 • one example method signature: public void actionPerfomed(ActionEvent e) • JButtons and JTextFields send actionPerformed messages to "command" objects (the listeners) without knowing what will happen • Event generators — buttons, text fields, mouse — have listener objects (actually a Vector of listener objects)

Uses • The Command object can also be used when you need to tell the program to execute the command later. • In such cases, you are saving commands as objects to be executed later • You could also sending command objects over the network (in new project) or save them in a collection class such as a Stack for undo/redo operations

Example we saw before was Command • Make 3 command classes • Log instances by writing the objects to a file • See SaveWorkCommands .java in the Eclipse project CommandPattern (command.zip) • Like RemoteLoader in HFSP or Client in gen. form • Read the objects later and execute them • See ExecuteSavedCommands .java in the Eclipse project CommandPattern (command.zip) • Like Light on HFSP or Receiver in gen. form

A UML View of the Sample

import java.io.Serializable; // Command design pattern - Decoupling producer from consumer. publicinterface WorkCommand { void execute(); } class DomesticEngineer implements WorkCommand, Serializable { publicvoid execute() { System.out.println("Take out the trash."); } } class Politician implements WorkCommand, Serializable { publicvoid execute() { System.out.println("Take money from the rich, take votes from the poor."); } } class Programmer implements WorkCommand, Serializable { publicvoid execute() { System.out.println("Sell the bugs, charge extra for the fixes."); } }

Code Demo • Need these three files • WorkCommand.java • CommandClient.java • CommandServer.java

Summary • The Command design pattern encapsulates the concept of a command into an object • A command object could be sent across a network to be executed elsewhere or it could be saved as a log of operations

References • [Adelson and Soloway] B. Adelson and E. Soloway. The Role of Domain Experience in Software Design. IEEE Trans. on Software Engineering, V SE-11, N 11, 1985, pp. 1351-1360. • [Linn and Clancy] M. Linn and M. Clancy, The Case for Case Studies of Programming Problems. Communications of the ACM V 35 N 3, March 1992, pp. 121-132. • [Soloway and Ehrlich] E. Soloway and K. Ehrlich, Empirical Studies of Programming Knowledge, IEEE Transactions on Software Engineering V SE-10, N 5, September 1984. • [Curtis] B. Curtis, Cognitive Issues in Reusing Software Artifacts. In Software Reusability, V II. ed. T. Biggerstaff and A. Perlis, Addison Wesley 1989, pp. 269-287. • [Sierra and Bates ], Heads First Design Patterns • http://www.exciton.cs.rice.edu/JavaResources/DesignPatterns/command.htm

Command Design Pattern