Building Command Objects in Java

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Building Command Objects
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There are several ways to go about building Command objects for a program like this, and each has some advantages We start with the simplest: deriving new classes from the MenuItem and Button classes and implementing the Command interface in each Following are examples of extensions to the Button and Menu classes for our simple program
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class btnRedCommand extends Button implements Command { public btnRedCommand(String caption) { super(caption); //initialize the button } public void Execute() { psetBackground(Colorred); } } //------------class flieExitCommand extends MenuItem implements Command public fileExitCommand(String caption) { super(caption); //initialize the menu } public void Execute() { Systemexit(0); } }
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This certainly lets us simplify the calls made in the actionPerformed method, but it requires that we create and instantiate a new class for each action that we want to execute
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mnuOpenaddActionListener(new fileOpen()); mnuExitaddActionListener(new fileExit()); btnRedaddActionListener(new btnRed());
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We can circumvent most of the problem of passing needed parameters to those classes by making them inner classes This makes the Panel and Frame objects available directly However, inner classes are not such a good idea as commands proliferate because any that access any other GUI components must remain inside the main class This clutters up the code for this main class with a lot of confusing little inner classes Of course, if we are willing to pass the needed parameters to these classes, they can be independent Here, we pass in the Frame object and a Panel object
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mnuOpen = new fileOpenCommand("Open ", this); mnuFileadd(mnuOpen); mnuExit = new fileExitCommand("Exit"); mnuFileadd(mnuExit); p = new Panel();
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add(p); btnRed = new btnRedCommand("Read", p); padd(btnRed);
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In this second case, the Menu and Button command classes can then be external to the main class and even stored in separate files if we prefer
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The Command Pattern
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Now, while it is advantageous to encapsulate the action in a Command object, binding that object into the element that causes the action (such as the menuitem or button) is not exactly what the Command pattern is about Instead, the Command object should be separate from the invoking client sothat we can vary the invoking program and the details of the command action separately Rather than having the command be part of the menu or button, we make the Menu and Button classes containers for a Command object that exists separately We thus make UI elements that implement a CommandHolder interface
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public interface CommandHolder public void setCommand(Command comd); public Command getCommand(); }
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This interface indicates that there is a method to put a Command object into theinvoking object and a method to fetch that Command object and call its Executemethod Then we create the cmdMenu class, which implements this interface
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public class cmdMenu extends JmenuItem implements CommandHolder { protected Command menuCommand; //internal copies protected JFrame frame; public cmdMenu(String name, JFrame frm) { super(name); //menu string frame = frm; //containing frame } public void setCommand(Command comd) { menuCommand = comd; //save the command } public Command getCommand() { return menuCommand; //return the command } }
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This actually simplifies our program we don't have to create a separate menu class for each action that we want to carry out We just create instances of the menu and pass them different Command objects
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mnuOpen = new cmdMenu("Open ", this); mnuFileadd(mnuOpen); mnuOpensetCommand (new fileCommand(this)); mnuExit = new cmdMenu("Exit", this); mnuExitsetCommand (new ExitCommand());
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mnuFileadd(mnuExit);
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Creating the cmdButton class is analogous
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btnRed = new cmdButton("Red", this); btnRedsetCommand (new RedCommand (this, jp)); jpadd(btnRed);
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We still must create separate Command objects, but they are no longer part ofthe user interface classes For example, the FileCommand class is the following:
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public class fileCommand implements Command { JFrame frame; public fileCommand(JFrame fr) { frame = fr; } public void Execute() { FileDialog fDlg = new FileDialog(frame, "Open file"); fDlgshow(); //show file dialog } }
Then our actionPerformed method needs to obtain the actual Command objectfrom the UI object that caused the action and then execute that command
public void actionPerformed(ActionEvent e) { CommandHolder obj = (CommandHolder)egetSource(); objgetCommand()Execute(); }
This is only slightly more complicated than our original routine and again keeps the actionPerformed method from having to perform a series of if tests We can see the relationships between these classes and interfaces clearly in the UML diagram in Figure 172 Figure 172 A class structure for three different objects that implement the Command interface and two that implement the CommandHolder interface
Here you see that cmdButton and cmdMenu implement the CommandHolder interface and that there are two instances of cmdMenu in the UI class fullCommand The diagram also shows the classes ExitCommand, RedCommand, and fileCommand, which implement the Command interface and are instantiated in the fullCommand UI class This is, finally, the complete implementation of the Command pattern that we have been inching toward