Java Programming for absolute beginner- P12 pdf

Using Frames The Frame class is a GUI component that defines a top-level window. A Frame includes a border and a title bar. The Frame class extends the Window class, which is a top-level window without a border or title bar. The Window class subclasses the Container class, which is a component that can contain other components. Because Frame inherits from these other classes, it is itself a container able to hold other components. Table 6.2 summarizes some of the Frame class’s methods. Remember also that Frame inherits from the Component class, so the methods shown in Table 6.1 are also present in the Frame class. The UselessFrame Application The UselessFrame class extends the Frame class and doesn’t do much except set its size and display it. To write this application, first you must import the Frame class: import java.awt.Frame; Then you declare the UselessFrame class and indicate that it is a subclass of the Frame class: public class UselessFrame extends Frame { 178 J a v a P r o g r am m i n g f o r t h e A b s o l ut e B e gi n n e r Method Description Frame() Constructs a new Frame that is initially not visible. Frame(String) Constructs a new invisible Frame with the given String title. Image getIconImage() Returns this Frame’s icon Image object. MenuBar getMenuBar() Returns this Frame’s MenuBar. String getTitle() Returns this Frame’s String title. boolean isResizeable() Returns a boolean that indicates whether this Frame can be resized by the users. setIconImage(Image) Sets the Frame’s icon image to the given Image object. setMenuBar(MenuBar) Sets this Frame’s menu bar to the given MenuBar object. setResizeable(boolean) Determines whether this Frame can be resized. setTitle(String) Sets this Frame’s title to the given String. setVisible(boolean) Makes this Frame visible if the given parameter is true. If it is false, it makes the Frame invisible. TABLE 6.2 SUMMARY OF F RAME M ETHODS JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 178 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Frames are initially not visible and are automatically sized to their minimum size, which is quite small. So small, in fact, that you might not even notice them once they are displayed in the upper-left corner of your screen. The constructor takes care of creating a new UselessFrame with the title, Useless Frame, by call- ing its superclass’s Frame(String) constructor. Then it sets the size to 300 pixels wide by 200 pixels high, by calling the setSize(300, 200) method defined in the Component class. Finally, the constructor shows the UselessFrame by calling setVisible(true). The main() method simply instantiates a new UselessFrame because the constructor takes care of the rest. The source code for Useless- Frame.java is listed here: /* * UselessFrame * A Frame that does absolutely nothing aside from merely existing */ import java.awt.Frame; public class UselessFrame extends Frame { public UselessFrame() { super(“Useless Frame”); setSize(300, 200); setVisible(true); } public static void main (String args[]) { UselessFrame uf = new UselessFrame(); } } When you run this at the command prompt by typing java UselessFrame, a win- dow will pop up with the title Useless Frame. You can see the window in Figure 6.3. I ran this from a Microsoft Windows 98 environment at a screen resolution of 800×600. Your window might look different if you ran it from a different oper- ating system and/or screen resolution. The UselessFrame initially appears in the top-left corner of the computer screen, but I moved it by clicking the title bar and dragging it to a new location before I created the screen shot. When you run this you can go ahead and play around with it. Move it, minimize it, maximize it, deactivate it by clicking another window or on the desktop, reactivate it by click- ing it, and so on. The one thing you can’t do is close the window by clicking the x. That’s because you haven’t handled the window-closing event yet. You do this in the next section. When you create a window that won’t close, or any Java program that hangs without any activity, try pressing Ctrl+C at the command prompt to stop the pro- gram from running if you are using a Windows environment. Another thing you TRICK 179 C h a p t e r 6 C r e a t i n g a G U I U s i n g t h e A b s t r a c t W i n d o w i n g T o o l k i t JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 179 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. can try is pressing Ctrl+Alt+Delete to end the task from the task manager. Be careful when using Ctrl+Alt+Del because after you press this combo once, pressing it again will cause your computer to reboot and you will lose any unsaved files. 180 J a v a P r o g r am m i n g f o r t h e A b s o l ut e B e gi n n e r FIGURE 6.3 This is an example of a frame that does nothing other than display itself. Learning about Containers The Container class defines a Component that can contain other Components. The Container maintains a list of the Components it contains. You can add Components to it by calling its add(Component) method. Containers arrange their Components by using a layout manager. Layout managers are fully covered in Chapter 7, but in this chapter you do use the FlowLayout class, the simplest of the layout man- agers in this chapter to display multiple Components within a Frame. Using the WindowListener Interface The WindowListener interface handles WindowEvents. The UselessFrame window doesn’t close when you click the close button because it does not implement the WindowListener interface. When you implement this abstract interface in your classes, you are required to define all the following methods: public void windowClosing(WindowEvent) public void windowActivated(WindowEvent) public void windowClosed(WindowEvent) public void windowIconified(WindowEvent) JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 180 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. public void windowDeiconified(WindowEvent) public void windowDeactivated(WindowEvent) public void windowOpened(WindowEvent) These methods are defined in Chapter 7. In this chapter, you are only interested in the windowClosing(WindowEvent) method. This is what is called when the user clicks your window’s close button to close it. They are all listed here because you have to define them in the ComponentTestFrame class because you want to be able to use the interface to actually close the window without having to manually intervene and halt your program. The ComponentTestFrame class extends Frame and implements the WindowLis- tener interface. How do you get it to do that? You just tell it the way it is: public class ComponentTestFrame extends Frame implements WindowListener { You use the extends word normally, followed by the superclass, Frame, and then you follow that with another keyword implements and the interface, WindowLis- tener . As soon as you do this, you know that you have to define the methods shown previously. Take a look at the source code for ComponentTestFrame.java and then make sure you take the time to write it out and compile it. You need to use it throughout the rest of this chapter. /* * ComponentTestFrame * A Simple Frame to use for testing components */ import java.awt.*; import java.awt.event.*; public class ComponentTestFrame extends Frame implements WindowListener { public ComponentTestFrame(String title) { super(title); setBackground(SystemColor.control); setSize(400, 300); setLocation(200, 150); setLayout(new FlowLayout()); addWindowListener(this); } // the only WindowListener method I care about public void windowClosing(WindowEvent e) { dispose(); System.exit(0); } // the rest of them that must be declared public void windowActivated(WindowEvent e) { } public void windowClosed(WindowEvent e) { } 181 C h a p t e r 6 C r e a t i n g a G U I U s i n g t h e A b s t r a c t W i n d o w i n g T o o l k i t JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 181 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. public void windowIconified(WindowEvent e) { } public void windowDeiconified(WindowEvent e) { } public void windowDeactivated(WindowEvent e) { } public void windowOpened(WindowEvent e) { } } The only method you care about here is the windowClosing(WindowEvent) method, so that’s the only one you need to add statements to. You add two state- ments to the body of this method that take care of closing the window and exit- ing the program. dispose() releases the native resources owned by the Frame and those of its components as well (closes and disposes of the window’s resources). The System.exit(0) statement terminates the program itself. The rest of the WindowListener methods are there only because you have to define them. In this case, you don’t add any statements in the methods, so they don’t actually do anything. The WindowListener interface is defined within the java.awt.event package, so you need to import that package. It also defines the WindowEvent class. When you import a package using the asterisk character *, it indicates that only those classes that are referenced within the program should be imported. Actually, nothing is imported into the object code; remember that the import statement just indicates where to find the classes you’re using. This ComponentTestFrame component performs some other operations of interest as well. It sets its background color to SystemColor.control, which is the back- ground color currently set for your operating system’s control objects, such as windows, dialog boxes, buttons, and so on. This color is not the default color for Frames; however, so you need to set it explicitly if you want it to have that partic- ular background color. It also sets its location to the (x, y) pixel coordinates ( 200, 150) and sets its layout to FlowLayout. Basically, this layout manager lays its Com- ponent s out in a center-aligned row (by default) until there is no more room, and then wraps around to the next row and continues to do this until there are no more Components to align. Using Components In this section, you test the AWT Components by adding them to a ComponentTest- Frame instance of the class you defined in the previous section. Because the lay- out is already set to FlowLayout, you create the Components and then add them to the ComponentTestFrame, using the add(Component) method. You learn about the different specifics of these Components by creating multiple instances of the same Component using different states. You then call different methods and compare their appearances and behaviors. TRICK 182 J a v a P r o g r am m i n g f o r t h e A b s o l ut e B e gi n n e r JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 182 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. The Label Component A Label contains read-only text that you display within a container. Like all other Components, you can change its background and foreground colors. You can also set the alignment of text within the Label’s area. Table 6.3 shows some of the more common fields and methods of the Label class. In the LabelTest program, you use the ComponentTestFrame to display four dif- ferent Labels. You create the l1 object by using the Label(String) constructor, which builds a Label with the given String that is left aligned by default. The l2 label demonstrates that the font can be changed using the setFont(Font) method. You use the Label() constructor with no arguments to construct the l3 Label , and then set the text using the setText(String) method and also call setEnabled(false) to disable the Label. As you can see in Figure 6.4, the graph- ics for a disabled Label are grayed out. The l4 Label’s foreground color is set to green and the background color is set to black. Its text is right-aligned because the constructor was called using Label.RIGHT. Next, you create the ComponentTestFrame. Because that class itself does most of its own work, you only need to create a ComponentTestFrame object, frame, by pass- ing a String title to its constructor. After you do that, you can add the Label com- ponents you created earlier by calling frame.add(Component) and passing the label as its Component parameter. Next, you call frame.setVisible(true) to show 183 C h a p t e r 6 C r e a t i n g a G U I U s i n g t h e A b s t r a c t W i n d o w i n g T o o l k i t Field or Method Description static int CENTER Indicates the Label should be center-aligned. static int LEFT Indicates the Label should be left-aligned. static int RIGHT Indicates the Label should be right-aligned. Label() Constructs a blank Label. Label(String) Constructs a Label with the given String text. Label(String, int) Constructs a Label with the given String text and int alignment. int getAlignment() Returns an int representation of this Label’s alignment. String getText() Returns the String object that holds this Label’s text. setAlignment(int) Sets the alignment of this object to the given int. setText(String) Sets the text of this Label to the given String. TABLE 6.3 FIELDS AND M ETHODS OF THE L ABEL C OMPONENT JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 183 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. the window and that’s all you need to do. The window-closing event is already handled in the ComponentTestFrame class itself, so you don’t have to worry about that here. Notice that if you resize the window, the Labels can be realigned. If you maximize the window, they will all form one row, but if you make the window narrow, the labels will all line up in one column. That fact further emphasizes how the FlowLayout layout manager works. Here is a listing of the source code: /* * LabelTest * Tests the Label Component */ import java.awt.*; public class LabelTest { public LabelTest() { //Make the Labels Label l1 = new Label(“Label”); Label l2 = new Label(“I am a Label”); l2.setFont(new Font(“Timesroman”, Font.BOLD, 18)); Label l3 = new Label(); l3.setText(“I am disabled”); l3.setEnabled(false); Label l4 = new Label(“Colored, Right aligned”, Label.RIGHT); l4.setForeground(Color.green); l4.setBackground(Color.black); //Make the Frame and add the labels to it ComponentTestFrame frame = new ComponentTestFrame(“Label Test”); frame.add(l1); frame.add(l2); frame.add(l3); frame.add(l4); frame.setVisible(true); } public static void main(String args[]) { LabelTest lt = new LabelTest(); } } 184 J a v a P r o g r am m i n g f o r t h e A b s o l ut e B e gi n n e r FIGURE 6.4 Four Labels are displayed in the Component- TestFrame . JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 184 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. The Font and Color classes of the java.awt package are used here to change the font and color associated with the Labels. They are used throughout this chap- ter to emphasize the flexibility you have in changing the appearance of Compo- nent s. The Font class defines a font face associated with a Component that is present on the system. In this chapter, you use the Font(String, int, int) con- structor to build a Font. The first argument is the name of the Font, and the sec- ond argument is the style of the Font, which can be Font.PLAIN, Font.BOLD, Font.ITALIC, or Font.BOLD + Font.ITALIC. The third argument is the point size for the Font. In this chapter you also use Color constants to specify colors for your Components. Some of these constants are Color.black, Color.blue, Color.cyan, Color.red, and Color.yellow. The Font class and the Color class are revisited in Chapter 7 when you learn about graphics programming. The Button Component The Button class defines a labeled button. Buttons typically trigger some action when the user clicks them. There are two constructors. One accepts no arguments and just creates an empty Button. You can set its label later on using the setLa- bel(String) method. The other constructor accepts a String argument specified to be its label. Table 6.4 shows some of the other common Button methods. In the ButtonTest program, you create four Button objects to get a feel for how to create and use the Button component. You construct b1 with the Button(String) constructor to set its label to “Button”. You call the Button() constructor to instantiate the b2 Button object, creating an empty Button. Then you call two of its methods to set the label and change the font: b2.setLabel(“Press me!”); b2.setFont(new Font(“Timesroman”, Font.BOLD, 18)); 185 C h a p t e r 6 C r e a t i n g a G U I U s i n g t h e A b s t r a c t W i n d o w i n g T o o l k i t Method Description Button() Constructs a Button with no text label. Button(String) Constructs a Button with the given String text label. addActionListener(ActionListener) Adds an ActionListener to this Button. String getLabel() Returns the String label of this Button. setLabel() Sets this Button’s label to the given String. removeActionListener(ActionListener) Removes the specified ActionListener from this Button. TABLE 6.4 B UTTON M ETHODS JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 185 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. When looking at Figure 6.5 from left to right, you can see that the font of the sec- ond Button is bigger, bold, and Times Roman. (The first Button is a standard one added for comparison.) You can also see that the third Button, b3 (called “Can’t press me”), is not enabled. The label is grayed out and when you actually run this, you can see that you cannot click it. Disabling a Component has a visual and func- tional effect. Its appearance is altered and the users cannot interact with it. The fourth Button, b4 (called “Colors”), has a different appearance than the other but- tons because you set its background color and foreground colors differently by calling its setBackground(Color) and setForeground(Color) methods. Here is a listing of ButtonTest.java. /* * ButtonTest * Demonstrates the Button Component */ import java.awt.*; public class ButtonTest { public ButtonTest() { //Make the Buttons Button b1 = new Button(“Button”); Button b2 = new Button(); b2.setLabel(“Press me!”); b2.setFont(new Font(“Timesroman”, Font.BOLD, 18)); Button b3 = new Button(“Can’t press me”); b3.setEnabled(false); Button b4 = new Button(“Colors”); b4.setForeground(Color.green); b4.setBackground(Color.black); //Make the Frame and add the buttons to it ComponentTestFrame frame = new ComponentTestFrame(“Button Test”); frame.add(b1); frame.add(b2); frame.add(b3); frame.add(b4); 186 J a v a P r o g r am m i n g f o r t h e A b s o l ut e B e gi n n e r FIGURE 6.5 This is a test of the Button component. JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 186 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. frame.setVisible(true); } public static void main(String args[]) { ButtonTest bt = new ButtonTest(); } } When you run the ButtonTest application, try traversing the buttons using Tab and Shift+Tab. You can see the appearance of the Button currently in focus is dif- ferent and stands out as the one that has user input focus. A Component is said to have focus when it is immediately ready to accept user input. For instance, while traversing through the Buttons, press the spacebar. The Button that currently has focus will be clicked. Also take note that the disabled Button never receives user input focus. The TextField Component There are two text components in the AWT. They subclass the TextComponent superclass. Table 6.5 shows some of the TextComponent methods inherited by 187 C h a p t e r 6 C r e a t i n g a G U I U s i n g t h e A b s t r a c t W i n d o w i n g T o o l k i t Method Description int getCaretPosition() Returns the caret position of this TextComponent. String getSelectedText() Returns the selected (highlighted) text in this TextComponent. int getSelectionStart() Returns the position in this TextComponent where the selected text begins. int getSelectionEnd() Returns the position in this TextComponent where the selected text ends. String getText() Returns the String that represents the full text of this TextComponent. select(int, int) Causes the text between the first and the second argument positions to become selected. setCaretPosition(int) Sets the position of the caret (the blinking cursor where text is inserted). setEditable(boolean) Sets whether the users can edit the text within this TextComponent. setSelectionStart(int) Sets the beginning index of the selected text. setSelectionEnd(int) Sets the ending index of the selected text. setText(String) Sets this TextComponent’s text to the given String. TABLE 6.5 T EXT C OMPONENT M ETHODS JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 187 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... methods of the TextArea class JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 192 Java Programming for the Absolute Beginner 192 The TextAreaTest application is pretty straightforward You create four TextArea objects and display them in the ComponentTestFrame The first one on the left, ta1, is constructed with no text, 10 rows, and 20 columns You can see in Figure 6.7 that the default for the scroll bars is TextArea.SCROLLBARS_BOTH,... ChoiceTest .java /* * ChoiceTest * Demonstrates the Choice Component */ TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Creating a GUI Using the Abstract Windowing Toolkit FIGURE 6.7 Chapter 6 public static void main(String args[]) { TextAreaTest tat = new TextAreaTest(); } JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 194 194 Java Programming. .. that are typed For example, a password field might use an asterisk as the echo character, so when a user types a password, the field will only display ******** setText(String) Sets this TextField’s text TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 189 189 import java. awt.*; public...JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 188 Java Programming for the Absolute Beginner 188 both TextField and TextArea The TextField class defines a text component that gives the users the capability to enter and edit a single line of... TextField(“username”, 8); TextField tf9 = new TextField(“password”,8); tf9.setEchoChar(‘*’); Chapter 6 /* * TextFieldTest * Demonstrates the TextField Component */ JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 190 Java Programming for the Absolute Beginner 190 FIGURE 6.6 This shows different implementations of the TextField component shows that when a TextField is constructed this way, it appears as a... item to be selected import java. awt.*; public class ChoiceTest { public ChoiceTest() { //Make the Choices Choice c1 = new Choice(); c1.add(“Soup”); c1.add(“Salad”); Choice c2 = new Choice(); c2.add( Java ); c2.add(“C++”); c2.add(“HTML”); c2.add(“JavaScript”); c2.add(“COBOL”); c2.add(“FORTRAN”); Choice c3 = new Choice(); c3.add(“One”); c3.add(“Two”); c3.add(“Three”); c3.setForeground(Color.red); c3.setBackground(Color.black);... frame.setVisible(true); Chapter 6 c3.setFont(new Font(“Courier”, Font.PLAIN, 16)); Choice c4 = new Choice(); c4.add(“Not Enabled”); c4.add(“Nope”); c4.setEnabled(false); JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 196 Java Programming for the Absolute Beginner 196 ously The user can select from “Milk”, “Eggs”, and “Bread” All of them can be selected, or none of them Also one or two can be selected as well... ComponentTestFrame(“TextArea Test”); frame.add(ta1); frame.add(ta2); frame.add(ta3); frame.add(ta4); frame.setVisible(true); } TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 193 193 } Here are four TextArea components The Choice Component The Choice component allows the users... c3 = new Choice(); c3.add(“One”); c3.add(“Two”); c3.add(“Three”); c3.setForeground(Color.red); c3.setBackground(Color.black); TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 195 195 } public static void main(String args[]) { ChoiceTest ct = new ChoiceTest(); } } FIGURE 6.8 You... actual value of the TextArea, which is useful when a user is entering a password and doesn’t want anyone to see it on the screen TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 191 191 The TextArea Component T EXT A REA F I E L D S AND METHODS Field or Method Description SCROLLBARS_NONE . SUMMARY OF F RAME M ETHODS JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 178 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Frames. of the rest. The source code for Useless- Frame .java is listed here: /* * UselessFrame * A Frame that does absolutely nothing aside from merely existing */ import java. awt.Frame; public class. C r e a t i n g a G U I U s i n g t h e A b s t r a c t W i n d o w i n g T o o l k i t JavaProgAbsBeg-06.qxd 2/25/03 8:52 AM Page 179 TEAM LinG - Live, Informative, Non-cost and Genuine! Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. can