Java By Example PHẦN 7 ppt

Example: Adding Components to a Window Frame windows are just like any other window you see when you create an applet.. Creating and Setting a MenuBar Object The first step in adding a m

public class FrameApplet2 extends Applet

Tell Java that the applet uses the classes in the awt package.

Tell Java that the applet uses the classes in the applet package

Derive the FrameApplet2 class from Java's Applet class

Declare the custom frame-window and button objects

Override the init( ) method

Create the custom frame window

Create the button component

Add the button to the applet

Override the action( ) method

Determine whether the window is visible

If the window is visible

Hide the window

Change the button's label to "Show Window."

Else if the window is hidden

Show the window

Change the button's label to "Hide Window."

Tell Java that the message was handled okay

Derive the CustomFrame class from Java's Frame class

Define the class's constructor

Pass the title string on to the Frame class

Override the window's paint( ) method

Resize the window

Display a message in the window

NOTE

When you compile FrameApplet2, notice that, although both the FrameApplet2 and CustomFrame classes are defined in the same file, the Java compiler creates two class files called

FrameApplet2.class and CustomFrame.class

Example: Adding Components to a Window

Frame windows are just like any other window you see when you create an applet That is, you can add components organized into a variety of layouts and respond to the user's selections of these components

In fact, adding layouts and components to a frame window is not unlike doing the same thing with your applet's main window, which you did in the previous chapter First you create and set the layout manager, and then you add the components as appropriate for the layout manager you've chosen

Listing 23.3 is an applet called FrameApplet3 that not only creates a custom frame window, but also creates a simple layout for the window This layout contains only a single button; however, you can

create as sophisticated a layout as you like Feel free to experiment further with this applet Figure 23.3 shows FrameApplet3 running under Appletviewer, after the user has displayed the frame window As you can see in the figure, the window has a single button labeled "Close Window." When you click this button, the frame window's action( ) method responds by calling the dispose( ) method, which not only removes the window from the screen, but also destroys the window in memory

Figure 23.3 : This is FrameApplet3 running under Appletviewer

Listing 23.3 FrameApplet3.java: Adding Components to a Window.

frame = new CustomFrame("Custom Frame Window");

button = new Button("Show Window");

add(button);

}

public boolean action(Event evt, Object arg)

else

{

frame.show();

button.setLabel("Hide Window"); }

Table 23.1 shows some useful methods you can use to manipulate a frame window Some of these

methods are defined in the Frame class, whereas others are inherited from the class's superclasses, such

as Window and Container

Table 23.1 Useful Frame-Window Methods.

int getCursorType( ) Returns the window's cursor type

Image getIconImage( ) Returns the window's icon object

LayoutManager getLayout( ) Returns the window's layout manager

MenuBar getMenuBar( ) Returns the window's menu bar object

String getTitle( ) Returns the window's title

Boolean isResizable( ) Returns true if the window is

resizable

void removeAll( ) Removes all components from the

Sets the window's icon object

void setLayout( ) Sets the window's layout manager

void setMenuBar(MenuBar mb) Sets the window's menu bar

void setResizable(boolean Sets the window's resizable

resizable)attribute

void setTitle(String title) Sets the window's title

Using Menu Bars

Most Windows applications have menu bars, which enable the user to more easily locate and select the various commands and options supported by the program The frame windows you create from within your applets can also have menu bars To create a menu bar in a window, you must follow a series of steps:

1 Create an object of the MenuBar class

2 Call the window's setMenuBar( ) method to give the menu bar to the window

3 Create objects of the Menu class for each menu you want in the menu bar

4 Call the MenuBar object's add( ) method to add each menu object to the menu bar

5 Create objects of the MenuItem or CheckboxMenuItem classes for each item you want to appear in the menus

6 Call the menus' add( ) methods in order to add each item to its appropriate menu

Each of the above steps is covered in the sections that follow

Creating and Setting a MenuBar Object

The first step in adding a menu bar to a frame window is to create the MenuBar object that'll hold all the menus and commands The menu bar in a window is the horizontal area near the top that contains the names of each of the menus in the menu bar To create the MenuBar object, call the MenuBar class's constructor, like this:

MenuBar menuBar = new MenuBar( );

As you can see, the MenuBar( ) constructor requires no arguments

After you've created the MenuBar object, you have to tell Java to associate the menu bar with the frame window You do this by calling the window's setMenuBar( ) method:

setMenuBar(menuBar);

At this point, you have an empty menu bar associated with the window In the next steps, you add menus

to the menu bar

Adding Menus to a Menu Bar

A menu bar is the horizontal area near the top of a window that contains the names of the menus

contained in the menu bar After creating and setting the MenuBar object, you have the menu bar, but it

contains no menus To add these menus, you first create objects of the Menu class for each menu you want in the menu bar, like this:

Menu fileMenu = new Menu("File");

Menu editMenu = new Menu("Edit");

Menu optionMenu = new Menu("Options");

The Menu class's constructor takes a single argument, which is the string that'll appear as the menu's name on the menu bar The example lines above create three menus for the menu bar

After creating the Menu objects, you have to add them to the menu bar, which you do by calling the MenuBar object's add( ) method, like this:

Figure 23.4 : This window's menu bar contains three empty menus

Adding Menu Items to Menus

You may have empty menus at this point, but you're about to remedy that problem To add items to your menus, you first create objects of the MenuItem or CheckboxMenuItem classes for each menu item you need To add items to the Options menus you made previously, you might use Java code something like this:

MenuItem option1 = new MenuItem("Option 1");

MenuItem option2 = new MenuItem("Option 2");

MenuItem option3 = new MenuItem("Option 3");

The MenuItem constructor takes as its single argument the string that'll be displayed in the menu for this item

If you're thinking that, after you create the menu items, you must call the appropriate Menu object's add( ) method, you're be exactly right Those lines might look like this:

Example: Using a Menu Bar in a Frame Window

Now that you have this menu bar business mastered, it's time to put what you've learned to work Listing 23.4 is an applet called MenuBarApplet This applet displays a single button, which, when selected, displays a frame window with a menu bar This menu bar contains a single menu with three items The first two items are regular MenuItem objects The third item is CheckboxMenuItem, which is a menu item that can display a check mark Figure 23.6 shows MenuBarApplet with its frame window displayed and the Test menu visible (Notice the menu separator above the checked item.)

Figure 23.6 : This is MenuBarApplet's frame window and menu bar

Listing 23.4 MenuBarApplet.java: An Applet That Uses a Menu Bar.

frame = new MenuBarFrame("MenuBar Window");

button = new Button("Show Window");

frame.hide();

button.setLabel("Show Window"); }

else

{

frame.show();

button.setLabel("Hide Window"); }

Menu menu = new Menu("Test");

str = "You selected Command 1";

else if (arg == "Command 2")

str = "You selected Command 2";

else if (arg == "Check")

str = "You selected the Check item";

Tell Java that the applet uses the classes in the awt package.Tell Java that the applet uses the classes in the applet package.Derive the MenuBarApplet class from Java's Applet class.Declare the custom frame-window and button objects.

Override the init( ) method

Create the custom frame window

Create and add the button component

Override the action( ) method

Determine whether the window is visible

If the window is visible

Hide the window

Change the button's label to "Show Window."

Else if the window is hidden

Show the window

Change the button's label to "Hide Window."

Tell Java that the message was handled okay

Derive the MenuBarFrame class from Java's Frame class.Declare the class's menu bar and string objects

Define the class's constructor

Pass the title string on to the Frame class

Create and set the menu bar

Create and add the Test menu

Create and add two regular menu items

Create and add a menu separator

Create and add a checkmark menu item

Initialize the class's display string and font

Override the window's paint( ) method

Resize the window

Show the display string in the window

Override the action( ) method

if a menu item was selected

Respond to the selected menu

Repaint the window, so the new string is displayed

Return true if the message was handled

Or else return false so Java knows the event is unhandled

NOTE

To determine the state (checked or unchecked) of a CheckboxMenuItem object, you can call its getState( )method This method returns true if the item is checked and false if the item is unchecked In addition, you can set the item's state by calling its setState( ) method

As you can see from MenuBarApplet's source code, you respond to menu-item selections in the same way you respond to other events in applets This time, however, you have overridden two action( )methods The first is in the MenuBarApplet class and handles the applet's single button The second overridden action( ) method, which is the one that handles the menu items, is in the

MenuBarFrame class

Summary

Although it's an ability you may not frequently take advantage of, Java applets can display windows The Frame class makes this possible, by providing the functionality for frame windows, which can be sized, moved, used to display components, and much more A frame window can, in fact, even have a full-featured menu bar, just like the menu bars you see in many Windows applications Creating a menu bar, however, requires knowing how to create and manipulate MenuBar, Menu, MenuItem, and

CheckboxMenuItem objects Luckily, you learned about those classes in this chapter, so you're all ready to amaze the world with your Java frame windows

Review Questions

1 How do you create a frame window?

2 How do you display a frame window after you create it?

3 How can you determine whether a frame window is currently visible?

4 What's the difference between MenuItem and CheckboxMenuItem objects?

5 Which Java class must you extend to create a custom frame-window class?

6 How do you ensure that a custom frame-window class has properly initialized its superclass?

7 How do you draw text or graphics in a frame window?

8 What are the six steps that must be completed in order to add a menu bar to a frame window?

9 How do you add components, such as controls, to a frame window?

10 How do you respond to selected menu items?

11 How do you create a menu separator object?

Review Exercises

1 Write an applet that displays a frame window as soon as the applet runs

2 Write an applet that displays a frame window containing a 2x2 grid of buttons

3 Modify the applet you wrote in exercise 2 so that the frame window contains a menu bar with two menus Each menu should have a single menu item

4 Modify the MenuBarApplet so that the menu bar has an additional menu called View This menu should contain a single checkmarked option called Window that determines whether a second frame window is visible on the screen When the user selects the Window command, the

command should be checkmarked and the second window should appear When the user clicks this command a second time, the second window should disappear and the command should be unchecked Figure 23.7 shows the resultant applet in action (You can find the solution to this exercise in the CHAP23 folder of this book's CD-ROM.)

Figure 23.7 : This is MenuFrameApplet running under Appletviewer

Chapter 24

Dialog Boxes

CONTNETS

● Using a Dialog Box

❍ Creating the Dialog Box

❍ Creating the Dialog Box's Layout

❍ Displaying the Dialog Box

❍ Removing the Dialog Box

❍ Methods of the Dialog Class

❍ Example: A Dialog Box for Text Input

requirement limits their usefulness, but you still may want to use a dialog box at one time or another In this chapter, you'll see how

Using a Dialog Box

To create, display, and handle a dialog box, you must perform the following steps:

1 Create the dialog box object

2 Create and set a layout manager for the dialog box

3 Create controls and add them to the dialog box

4 Call the dialog's show() method to display the dialog box

5 When the user clicks the OK or Cancel button, call the dialog's hide() method to remove the dialog box from the screen

6 Extract and process the data, if any, entered into the dialog box's controls

The following section discuss the above steps in greater detail

Creating the Dialog Box

Java's dialog boxes are objects of the Dialog class So, to create a dialog box object, you first call the Dialog class's constructor, like this:

Dialog dialog = new Dialog(frame, title, modal);

The constructor's three arguments refer to a frame window, the dialog box's title, and boolean value indicating whether the dialog box is modal (true) or modeless (false) A modal dialog box, which is the most common of the two types, retains the focus until the user dismisses it This forces the user to respond to the dialog box before continuing with the program A modeless dialog box can lose the focus

to another window, which means that the user can switch to another window even while the dialog box is still on the screen

NOTE

Although Java claims to support both modal and modeless dialog boxes, the constructor's argument doesn't seem to make any difference In my experience, every Java dialog box is modeless

Maybe this inconsistency will be corrected by the time you read this book

Creating the Dialog Box's Layout

Once you have the dialog box object created, you must give it a layout manager If you fail to do this, any components you try to place in the dialog box will not appear You perform this step exactly as you would for any other type of window or applet, by creating and setting the layout object:

FlowLayout layout = new FlowLayout();

dialog.setLayout(layout);

The next step is to create and add whatever controls you want to appear in the dialog box You'll always have at least an OK button, with which the user can dismiss the dialog box:

Button button = new Button("OK");

dialog.add(button);

Displaying the Dialog Box

Just like a frame window, a dialog box doesn't appear on the screen until you call its show() method, like this:

dialog.show();

Once the dialog box is on the screen, the user can manipulate its controls in order to enter information into the dialog box's fields or to dismiss the dialog box from the screen

Removing the Dialog Box

When the user clicks a dialog box's OK or Cancel buttons, that's your applet's signal to remove the dialog box from the screen, which you do by calling its hide() method:

dialog.hide();

The hide() method removes the dialog box from the screen, but the dialog box and its controls remain

in memory so that you can access them in order to extract whatever information the user may have

entered into the dialog box

After you've removed the dialog box from the screen, you can use a control's methods to extract

whatever information the user may have entered into the dialog's controls For example, to get the entry from a text field control, you'd call the control's getText() method

Methods of the Dialog Class

Like any class, Dialog provides a set of public methods that you can use to control the dialog box

Dialog also inherits many methods from its superclasses, Window and Container Table 24.1 lists the most useful methods of the Dialog class, including those methods inherited from the Window and Container classes.

Table 24.1 Useful Methods of the Dialog Class (Including Inherited).

Component add(Component

comp)

Adds a component to the dialog box

LayoutManager getLayout() Returns the dialog's layout manager

String getTitle() Returns the dialog box's title

Sets the dialog's layout manager

void setResizable(boolean Sets the resizable

attribute.resizable)void setTitle(String title) Sets the dialog box's title

Example: A Dialog Box for Text Input

Your last task in this chapter is to put your newly acquired knowledge of dialog boxes to work Listing 24.1 is an applet that enables you to display a frame window From the frame window's menu bar, you can select a command that displays a dialog box This dialog box contains an OK button for dismissing the dialog box and a text field for entering information When you dismiss the dialog box, the text you

entered into the text field control appears in the frame window Figure 24.1 shows the applet, the frame window, and the dialog box

Figure 24.1 : This is DialogApplet running under Appletviewer

Listing 24.1 DialogApplet.java: An Applet That Displays a Dialog Box.

frame = new DialogFrame("Dialog Window");

button = new Button("Show Window");

add(button);

}

public boolean action(Event evt, Object arg)

{

boolean visible = frame.isShowing();

if (visible)

{

frame.hide();

button.setLabel("Show Window"); }

else

{

frame.show();

button.setLabel("Hide Window"); }

if (evt.target instanceof MenuItem)

textField = new TextField("", 20); Button button = new Button("OK"); dialog.add(button);

Override the init() method

Create the frame window

Create and add the button component

Override the action() method

Determine whether the frame window is visible

If the window is visible

Hide the window

Change the button's label to "Show Window."

Else if the window is hidden

Show the window

Set the window's size

Change the button's label to "Hide Window."

Tell Java that the message was handled okay

Define the frame window class

Declare objects needed by the class

Define the class's constructor

Initialize the superclass (Frame)

Create the window's menu bar.

Initialize the display string

Override the paint() method

Resize the frame window

Draw the display text in the window

Override the action() method

If the "Dialog Box" command was selected, create the dialog

Else if the OK button was selected

Hide the dialog box

Get the contents of the dialog's test field

Tell Java to repaint the frame window

Tell Java that the event was handled

Define the ShowDialogBox() method

Create the new dialog box and set its layout

Create and add components to the dialog box

Display and resize the dialog box

NOTE

In addition to normal dialog boxes, Java supports file dialog boxes for loading and saving files The file dialogs are represented by the FileDialog class However, because Java applets have

extremely limited access to files, file dialog boxes are not covered here

Summary

You probably won't have much call for dialog boxes in your Java applets, but it's always good to know they're there when you need them Using dialog boxes in conjunction with a frame window, you can inform the user of critical problems, as well as obtain information from the user without cluttering your main window with controls Because dialog boxes are much like other display windows in Java, you can set up layout managers, add components, and control the dialog box using the many methods defined in the Dialog class or inherited from the class's superclasses

Review Questions

1 What are the three arguments accepted by the Dialog class's constructor?

2 How do you display and hide a dialog box?

3 Why are frame windows important to dialog boxes?

4 What is the one control every dialog box should have?

5 What's the difference between modal and modeless dialog boxes?

6 How do you add components to the dialog box?

Review Exercises

1 Write the code needed to create and display a dialog box with the title "Test Dialog" and with two button controls

2 Write an applet that can display a dialog box with a 2×2 grid containing four buttons

3 Modify DialogApplet so that the dialog box has both an OK and a Cancel button If the user clicks the Cancel button, the dialog should be removed from the screen, but the string that was entered into the text field control should be ignored Figure 24.2 shows the resultant applet, called

DialogApplet2 (You can find the solution to this exercise in the CHAP24 folder of this book's CD-ROM.)

Figure 24.2 : This is the DialogApplet2 applet run-ning under Appletviewer

❍ Handling Mouse Clicks

❍ Example: Using Mouse Clicks in an Applet

❍ Handling Mouse Movement

❍ Example: Responding to Mouse Movement in an Applet

● The Keyboard

❍ Responding to Key Presses

❍ Predefined Key Constants

❍ Key Modifiers

❍ Example: Using Key Presses in an Applet

● Handling Events Directly

❍ Example: Overriding handleEvent() in an Applet

The Event Object

In order to understand how to respond to various types of events, you need to know more about Java's

Event class, an object of which is passed to any event-handling method When you want to respond to a