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Unit 22Command

Summary prepared by Kirk Scott

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Bronze cast of the furcula of "Sue" the Tyrannosaurus, Field Museum

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Design Patterns in JavaChapter 24Command

Summary prepared by Kirk Scott

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The Introduction Before the Introduction

• The ordinary way for a client to cause a method to execute is to call it (on an object), in-line

• There may be other times when the client can’t specify exactly when or where a particular method should be called, but would like to be able to pass the method to another piece of code which will call it when needed

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• This can be accomplished by writing a class which contains the desired method

• Then at run time an instance of that class can be constructed

• That object can have the method called on it• Passing the method can be accomplished by

passing the object to another piece of code as an explicit parameter

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• Book definition:• The intent of the Command pattern is to encapsulate a

request in an object.• Comment mode on:• Notice that this is what has always happened when

writing a class definition and putting a public method in it.

• You learned to pass objects as parameters a long time ago, and within receiving code it is possible to call methods on the parameters

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A Classic Example: Menu Commands

• Menus and listeners work together using the Command design pattern

• They illustrate the idea that the application writer can’t predict in advance when a user will select a menu item, but can write the code to deal with it when it happens

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• In schematic form, these are the steps in creating a menu in code:

• JMenuBar myMenuBar = new JMenuBar();

• JMenu myMenu = new JMenu(“File”);

• myMenuBar.add(myMenu);

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• These steps are followed by the steps for adding items to the menu:

• JMenuItem myExitMenuItem = new JMenuItem(“Exit”);• ExitListener myExitListener = new ExitListener();• myExitMenuItem.addActionListener(myExitListener);• myMenu.add(myExitMenuItem);

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• The ExitListener is implemented as an inner class of the frame which has the menu

• This is what its code looks like:

• private class ExitListener implements ActionListener• {• public void actionPerformed(ActionEvent event)• {• System.exit(0);• }• }

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• The ExitListener class has an actionPerformed() method in it

• The actionPerformed() method is executed when the Exit item is clicked in the menu

• In the constructor for the frame, where the menu is being created, it is impossible to predict when the Exit item will be clicked

• In other words, it is impossible to predict when the actionPerformed() method will be called

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• That’s all right• The following line of code shows how

actionPerformed() is linked to the menu item• It is this line of code that illustrates the

Command design pattern• myExitMenuItem.addActionListener(myExitListener);

• The object myExitListener is passed to myExitItem as an explicit parameter in the call to addActionListener

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• The actionPerformed() method can be called on myExitItem in the future in response to a mouse click on myExitItem

• This is mildly cryptic because programmer written code does not contain the call myExitListener.actionPerformed()

• The actual call is made by the system through Java’s event handling mechanism

• Even so, the Command design pattern is evident in the way the item and the listener are linked

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• The book notes that once again, polymorphism is at work in the way menus function

• Each menu item can have a different listener which takes different actions

• Each listener has a method with the same name, actionPerformed()

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• Which action is performed depends on which kind of listener the system calls actionPerformed() on

• The system only needs to know that in each case, actionPerformed() should be called, and polymorphism makes sure that the right action is taken for each different kind of listener

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• In CS 202, for example, event handling is simply described as the system calling the actionPerformed() method

• It is important to note that the system calls the actionPerformed() method on the listener object

• The system has access to the objects that are declared as listeners in a program

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Menu Commands and the Mediator Pattern

• Challenge 24.1• The mechanics of Java menus make it easy to

apply the Command pattern but do not require that you organize your code as commands.

• In fact, it is common to develop an application in which a single object listens to all the events in a GUI.

• What pattern does that follow?

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• Comment mode on:• This may be an interesting question, but the

authors are spinning out into space now• This question is unrelated to understanding

the design pattern in question.

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• Solution 24.1• Many Java Swing applications apply the

Mediator pattern, registering a single object to receive all GUI events.

• This object mediates the interaction of the components and translates user input into commands for business domain objects.

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• Notice that the approach just described could be a mess to implement for menus and it could be equally messy for other situations

• If you had a single listener for all menu items, the contents of the actionPerformed() method would degenerate into a series of if/else statements

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• These if/elses would have to determine which item had been clicked

• Their bodies would consist of blocks of code implementing the appropriate actions

• Notice how this is exactly the opposite of having many different listeners and relying on polymorphism and dynamic binding to take care of which action to take

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The Command and Strategy Patterns

• If you think back to the Strategy design pattern, for example, that was the kind of mess you were hoping to avoid

• Instead of having if/else code, the solution was to have different classes for each strategy (in this case, each menu item)

• Each class would have a method with the same name but containing the implementation of a different strategy (in this case, a different action to be performed)

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• In other words, the handling of menus in Java exhibits the characteristics of two design patterns

• The fact that there are different listener classes, each with an actionPerformed() method illustrates the Strategy design pattern

• The fact that listeners are added to items so that their actionPerformed() method can be called in the future when needed illustrates the Command design pattern

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• Recall also, that with the Strategy design pattern you ended up with lots of little classes, each containing just a single method, and which you only needed one instance of

• The same thing happens with (standard) menus• There is a different kind of listener for each menu item• Each listener just contains an implementation of

actionPerformed()• Only one instance of each listener is needed

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Anonymous Classes

• The book observes that this is the kind of situation where anonymous classes can be used

• I still don’t like anonymous classes, but it is important to get used to them because other programmers use them

• Recall that in CS 202, it was also a listener which was used as an anonymous class example

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• The book next presents code with the anonymous listeners missing

• It is not reproduced here because it is immediately followed by a challenge to fill in the missing listeners

• It’s easier to just go directly to the complete example with all of the pieces included

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• Challenge 24.1• Fill in the code for the anonymous subclasses

of ActionListener, overriding the actionPerformed() method. Note that this method expects an ActionEvent argument.

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• Solution 24.2• Your code should look something like this:• Comment mode on:• In the book, the solution code is followed by

some additional remarks• Instead of putting them at the end, they are

given up front here, followed by a comment on them, followed by the code

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• Book’s remarks on the solution code:• Although the actionPerformed() method requires

an ActionEvent argument, you can safely ignore it. • The menus() method registers a single instance of

an anonymous class with the Save menu item and a single instance of another anonymous class with the Load menu item.

• When these methods are called, there is no doubt about the source of the event.

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• Comment mode on:• There are two things to say about this:• First of all, the book’s statement forewarns us

that the creation of the menu in the solution code occurs in a separate method in the application, named menus()

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• Secondly, there is nothing new in the remarks about the ActionEvent parameter

• We have always ignored it when writing menu listeners• We’ve seen examples in CS 202 where the event

parameter is of interest• For example, it was necessary to get the (x, y)

coordinates of a mouse click to see if it occurred in a cup

• But quite often, there has been no need to do anything with the event parameter in a listener

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• The book’s solution code is given on the following overheads

• Keep in mind the purpose of the challenge and the code shown

• It is to illustrate how menu listeners (in this case, anonymous ones) make use of the Command design pattern

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• public class Visualization2 extends Visualization {• public static void main(String[] args) • {• Visualization2 panel = new Visualization2(UI.NORMAL);• JFrame frame = SwingFacade.launch(panel, "Operational Model");• frame.setJMenuBar(panel.menus());• frame.setVisible(true);• }

• public Visualization2(UI ui) • {• super(ui);• }

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• public JMenuBar menus() • {• JMenuBar menuBar = new JMenuBar();

• JMenu menu = new JMenu("File");• menuBar.add(menu);

• JMenuItem menuItem = new JMenuItem("Save As...");• menuItem.addActionListener(new ActionListener() • {• public void actionPerformed(ActionEvent e) • {• save();• }• });• menu.add(menuItem);

• menuItem = new JMenuItem("Restore From...");• menuItem.addActionListener(new ActionListener() • {• public void actionPerformed(ActionEvent e) • {• restore();• }• });• menu.add(menuItem);

• return menuBar;• }

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• public void save() • {• try • {• mediator.save(this);• } • catch (Exception ex) • {• System.out.println("Failed save: " + ex.getMessage());• }• }

• public void restore() • {• try • {• mediator.restore(this);• } • catch (Exception ex) • {• System.out.println("Failed restore: " + ex.getMessage());• }• }• }

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Using Command to Supply a Service

• In the previous example, half the work was done by the Java API

• All that was necessary was to plug a command into an existing context

• In other words, the addActionListener() machinery is provided by the API

• It is also possible for a programmer to provide both the context and the command

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The Next Example

• The next example shows how to time how long it takes to execute some method, methodA()

• There will be a timing method, methodB(), that takes as its input parameter an object which can have methodA() called on it

• Inside methodB() the call to methodA() on the input parameter will be sandwiched between timing code

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Setting up the Command to be Passed

• Let there be an abstract class named Command which includes this abstract method declaration:

• public abstract void execute();• A concrete command class would extend

Command and implement execute()• The abstract method, execute(), defines the

interface for using a command object

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Setting up the Code that will Receive the Command

• In addition to a concrete command class, the example includes a class named CommandTimer

• This class contains a static method named time()• The time() method takes as an input parameter

an instance of a command class• execute() is called on that command class object

in the body of the time() method

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• Calls to methods that make it possible to keep track of the passage of time can be placed before and after the call to execute() in the time() method

• Code for the CommandTimer class and its time() method are given on the next overhead

• Keep in mind that the method is static.

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• public class CommandTimer• {• public static long time(Command command)• {• long t1 = System.currentTimeMillis();• command.execute();• long t2 = System.currentTimeMillis();• return t2 – t1;• }• }

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• The book next presents code to test the setup, with the call that causes the execution missing

• The code with the missing step is not reproduced here because it is immediately followed by a challenge to fill in the missing line of code

• It’s easier to just go directly to the complete example with all of the pieces included

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• The authors are trying to illustrate the use of a JUnit test framework

• This is mentioned in Appendix C, on the source code, and also at various points in the text

• You don’t have to worry about it• It just adds a little stuff at the end of the example

code which you can ignore• They also use the anonymous class syntax when

creating the command

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• Challenge 24.3• Complete the assignment statement that sets

the value for actual in such a way that the doze command is timed.

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• Solution 24.3• The testSleep() method passes the doze command

to the time() utility method.• Comment mode on:• The code creates an instance of Command, named

doze• It then calls the static time() method, passing doze

as a parameter• See the code on the next overhead.

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• public class TestCommandTimer extends TestCase • {• public void testSleep() • {• Command doze = new Command() • {• public void execute() • {• try • {• Thread.sleep(2000 + Math.round(10 * Math.random()));• } • catch (InterruptedException ignored) • {• }• }• };

• long actual = CommandTimer.time(doze);

• long expected = 2000;• long delta = 5;• assertTrue("Should be " + expected + " +/- " + delta + " ms", • expected - delta <= actual• && actual <= expected + delta);• }• }

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• The point of the foregoing example code was that it created an object of the Command class named doze

• This object contained an execute() method that caused it to sleep for a random amount of time

• The line of code shown below made use of the command machinery given previously to time how long it takes for doze.execute() to run

• long actual = CommandTimer.time(doze);

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Command Hooks

• This section builds on material that was presented in the chapter on the Template Method

• You may recall that in that chapter I restricted myself to the discussion of sorting as implemented in the Java API

• I did not pursue the other examples• Therefore, it is not possible to pursue this

example in this chapter• You are not responsible for it

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Command in Relation to Other Patterns

• The book states that command is similar to a pattern in which a client knows when an action is required but doesn’t know exactly which operation to call.

• This may be true, but this is not exactly what the examples in this chapter have illustrated.

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• In the menu example, there was uncertainty about the timing of the call (sometime in the future) but which actionPerformed() method was linked to which item was well defined

• In the timing example there was also no doubt about what to do—just a need to encase a call to it in other predefined blocks of code

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• Challenge 24.5• Which pattern addresses the situation in

which a client knows when to create an object but doesn’t know which class to instantiate?

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• Comment mode on:• This challenge introduces yet another slight

difference between the thing they’re talking about and the examples they’re giving:

• Now we’re talking about the creation of an object.

• Since we don’t know what to create, this must mean calling a “creational” method rather than calling a constructor.

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• Solution 24.5• In Factory Method, a client knows when to

create a new object but not what kind of object to create.

• Factory Method moves object creation to a method that isolates a client from knowing which class to instantiate.

• This principle also occurs in Abstract Factory.

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• Comment mode on:• The foregoing observation is certainly true• After a while all of the patterns begin to look

similar• In particular, you observe a “meta pattern” that

there are some patterns that work with vanilla methods, and there are other analogous patterns that work with methods that create objects

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An Example using Command with other Patterns

• The Command design pattern can be used with other design patterns

• The book gives code showing Command and Mediator being used together in a MVC design for their Visualization program

• MVC hasn’t been covered yet.• Still, the example can be covered as a

combination of mediator and command.

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• In this example the mediator class is responsible for returning instances of different kinds of listeners, as needed, for the buttons created in the application

• The example code which will be given next shows part of the controller portion of an MVC design

• It’s the creation of a graphical button in a visualization with a listener attached to it

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• The code contains a call like this: mediator.undoAction()

• This doesn’t “undo an action” on the mediator• It returns an “undo listener” which the

mediator is responsible for keeping track of

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• The listener returned is used as an explicit parameter in this call: undoButton.addActionListener()

• This is the point where the command pattern is in evidence

• Attaching a listener to a button is analogous to attaching a listener to a menu item

• This example is analogous to the first example in the unit except that the listener comes from the mediator

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• protected JButton undoButton() • {• if (undoButton == null) • {• undoButton = ui.createButtonCancel();• undoButton.setText("Undo");• undoButton.setEnabled(false);• undoButton.addActionListener(mediator.undoAction());• }• return undoButton;• }

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Other Code for this Example

• If you go to the book’s Web site you can find the code for the VisMediator class

• Details of that class are not of interest at the moment

• All you need to know is that it contains implementations of methods like undoAction(), which return listeners

• Methods of this kind with different names return different kinds of listeners

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actionPerformed() is the Command that is Packaged in a Listener

• When the listener is added to the button, the command that is being packaged up in the object is actionPerformed()

• Different versions of actionPerformed() contain calls to methods (commands) that do different things

• These commands are ultimately what is triggered by this use of the Command design pattern

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Another Example

• The other example for this unit is built on the examples for the previous two patterns, factory method and abstract factory

• It is available on the course Web page if you want to take a look at it

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UML for the Pattern

• A progression of UML diagrams for the pattern is shown on the following overheads

• The first diagram shows the simple roots of the pattern

• The second shows the direct use of a command• The third shows that in reality, a given context

could make similar use of many different commands

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Lasater’s UML for the Pattern

• Lasater’s diagram attempts to show that a generic execute() command is really just a wrapper for a call to some action that is significant within the problem domain or context.

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Summary

• The Command design pattern makes it possible to encapsulate a request for service in an object

• In other words, calls to methods can be managed by passing around objects which can receive the calls

• The actual calls can be made at the time needed or when necessary conditions have been met

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• The Command design pattern is illustrated by the way menus are constructed in Java

• The listener for an item is an object which can have the actionPerformed() method called on it

• The listener is passed to the item as an explicit parameter in a method call

• actionPerformed() is actually executed only at the time that the item in the menu is selected

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• The second example illustrated how the Command design pattern can be used to embed a call between blocks of code

• The third example in the book was not pursued, and you are not responsible for it

• That example happened to illustrate that the command pattern can be used to accomplish things that the Template Method design pattern can also be used to accomplish

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• The idea of passing an object as a parameter and then calling methods on the object in the body of the receiving method is very basic

• You have written code that works this way many times without thinking about it or trying to classify it as some sort of pattern

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• It takes on the meaning of a design pattern depending on the context in which it’s used

• In practice, the command pattern doesn’t necessarily appear in isolation

• It may appear in a more complex design in combination with the Mediator, Memento, and other design patterns

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The End