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Java Unleashed Second Edition

Chapter 18 -- Programming Applets

Chapter 18

Programming Applets

by Rogers Cadenhead


CONTENTS




The proliferation of windowing software in the past five years, led by Microsoft Windows and Apple Macintosh systems, has created an expectation among users that software should make use of these features. In Java, windowing and other graphical user interface functions are handled by the Abstract Windowing Toolkit (AWT).

The AWT is one of the most useful packages included with the Java API. The AWT is a set of classes used to build a graphical user interface for Java applications and applets. It enables programmers to easily create the following features for programs:

The AWT also includes classes to handle graphics, fonts, and color in Java programs, and an Event class to enable programs to respond to mouse clicks, mouse movements, and keyboard input.

Today, the Java Developers Kit (JDK) is not the only choice available to programmers. There are numerous ways to develop a graphical user interface for Java programs, such as SunSoft Java WorkShop and Rogue Wave JFactory. Java WorkShop includes a Visual Java tool for the drag-and-drop development of windows, dialog boxes, and other elements that are common to windowing systems. JFactory is an interface builder that can be used with different Java programming environments.

However, there are several reasons to begin learning about interface development by using only the AWT. First, it's still the most commonly used interface designer for Java, so the majority of source code that's publicly available uses AWT classes to control its interface. Second, many of the other windowing design systems use the AWT as the underlying code to handle the interface.

This chapter focuses on how the AWT can be used in the development of applets. However, the same methods apply to the creation of applications. Many of these features and additional aspects of Java interface programming are covered in Chapter 16, "The Windowing (AWT) Package."

Simple Components

When you use the Abstract Windowing Toolkit, you make up the graphical user interface with components. Each element you can manipulate with a windowing program is represented by its own component. There are components for buttons you can press, components for text fields you can type into, and components for scroll bars you can control. There also are components for some things you cannot directly manipulate, such as labels-lines of text used on an interface, such as the words Enter password: next to a text field on a login window.

To use these components in a program, you must put them into some kind of container. A container is a blank slate where graphical user interface components can be put. All containers in Java are subclasses of the Container class.

There are two basic types of containers:

A Panel container is not visible when it is added to an applet. Its purpose is to provide a way to organize components when they are being laid out in a window.

Listing 18.1 is the full source code for an applet that has a new Panel added to its surface. It does not produce any output other than a blank window. However, it is useful as a template for the components you will learn to add throughout this chapter. This file-and all the other listings in this chapter-are included on the CD-ROM that accompanies this book in the /SOURCE/CHAP18 subdirectory.


Listing 18.1. The source code of Sample.java.

1: import java.awt.*;
2:
3: public class Sample extends java.applet.Applet {
4:     Panel p = new Panel();
5:
6:     public void init() {
7:         add(p);
8:     }
9: }

The method call that puts the Panel object p on the applet window is the statement add(p);. The add() method is used whenever a component of any kind is added to a container.

By replacing the line Panel p = new Panel() with the variable declarations of components you will create, and replacing the line add(p); with the components to be added to a container, you can use this source code to try each component out.

By default, components are added to a container in left-to-right, top-to-bottom order. If a component does not fit on a line, it is placed at the leftmost edge of the next line. You can control how components are organized by using a layout manager, as you learn later in this chapter. Until then, the examples use the default layout style.

Buttons

The Button component is a rectangular button that can be pushed by clicking it with a mouse. Figure 18.1 shows an example of a simple one-button applet.

Figure 18.1: A Button Component.

The following code is used to create a Button component and add it to an applet window:

Button b = new Button("Cancel");add(b);

The add(b) method does not refer to a specific container object, so it defaults to the applet itself, adding the button to the applet surface. If you wanted to create a new Panel and add a new Button component to that panel, use the following code:

Panel p = new Panel();
Button b = new Button("Send");
p.add(b);

Text Fields

The TextField component is an input box in which a user can type a single line of text. The number of characters that can be visible in the text field is configurable. Figure 18.2 shows an example of an applet with a text field.

Figure 18.2: A TextField component.

The following code is used to create a TextField component and add it to an applet window:

TextField t = new TextField(12);
add(t);

The parameter 12 in the constructor TextField(12) sets up the text field so that around 12 characters can be displayed in the field at one time. The user can type more characters than that, but only some of the characters will be displayed.

If a string is specified as a parameter to the constructor, as in the statement TextField t = TextField("your name"); the text field is created with the string as the default text in the input area of the field.

You can specify default text and a width at the same time by using a statement such as the following:

TextField country = new TextField("United States", 20);

Labels

The Label component is a string of text displayed on the container; it cannot be modified by the user. Figure 18.3 shows an example of an applet with a label next to a text field.

Figure 18.3: A Lable component to the left of a text field.

The following code is used to create a Label component and add it to an applet window:

Label l = new Label("E-mail address: ");
add(l);

The parameter in the constructor Label("E-mail address: ") identifies the text to be displayed.

Checkboxes

The Checkbox component is a toggle box that can be either selected-marked with a checkmark-or unselected. The checkbox usually has a line of text next to it explaining what the box signifies. Figure 18.4 shows an example of an applet with a checkbox.

Figure 18.4: A checkbox component.

The following code is used to create a Checkbox component and add it to an applet window:

Checkbox c = new Checkbox("Never Mind");
add(c);

The parameter in the constructor CheckBox("Never Mind") identifies the text to be displayed. If you omit this parameter, a checkbox is displayed without any text next to it.

A checkbox is often used with a group of related checkboxes so that only one of the boxes can be selected at one time. To group checkboxes together, you use the CheckboxGroup class. The setCheckboxGroup() method associates a checkbox with a particular group; the setCurrent() method of CheckboxGroup can be used to make one of the boxes the selected box. The following code shows the use of CheckboxGroup:

CheckboxGroup cbg = new CheckboxGroup();
Checkbox c1 = new Checkbox("To-MAY-to");
c1.setCheckboxGroup(cbg);
add(c1);
Checkbox c2 = new Checkbox("To-MAH-to");
c2.setCheckboxGroup(cbg);
add(c2);
Checkbox c3 = new Checkbox("Let's call the whole thing off.");
c3.setCheckboxGroup(cbg);
cbg.setCurrent(c3);
add(c3);

Figure 18.5 shows this applet with the third checkbox selected.

Figure 18.5: A group of checkbox components.

Choice List

The Choice component is a pop-up list of strings from which a single string can be selected. Its function is similar to a group of checkboxes because a choice list provides a group of options and enables one to be selected. Figure 18.6 shows an example of an applet with a choice list.

Figure 18.6: A choice component.

The addItem() method of the Choice class is used to build the choice list. The following code is used to create a choice list, add items to it, and then add the list to an applet window:

Choice c = new Choice();
c.addItem("Victor");
c.addItem("Hugo");
c.addItem("Laverne");
add(c);

Organizing the Interface

Up to this point, all the components have been added to a container in a manner similar to the way HTML elements are arranged on a Web page. Things are loosely organized from left to right and top to bottom, and the look of the container is highly dependent on the size of its display area.

This approach fits Java's requirement to be multiplatform. Because the language must work on any system that has a Java implementation, the windowing environment must be flexible. The Java applet you program looks dramatically different when shown on a Windows 95 system, a Macintosh system, and a SPARC workstation.

However, Java has a system of organizing user interface components that makes it possible for your applet's interface to work on each of the platforms-and to look close enough in appearance on different platforms to operate reliably. These organization tools are called layout managers.

When you added components to a container-the main applet window-in each of the previous examples, you were using the default layout manager: a class called FlowLayout. There are three other layout managers you can use as you organize your interface: BorderLayout, GridLayout, and GridBagLayout.

To start using a layout manager for a container, call the setLayout() method before you add any components. This approach is further described in the following sections.

The FlowLayout Class

The FlowLayout class is the default layout manager for all Panels-including the Applet class-and it is the one that's the simplest to use. Components placed under the rules of FlowLayout are arranged in order from left to right. When a component is too big to be added to the current row, a new line of components is begun below the first line.

Each row of components can be aligned to the left, right, or centered. The only parameter used with the add() method is the name of the object to add.

The following setLayout() statement sets up a container to use the FlowLayout manager:

setLayout( new FlowLayout() );

Figure 18.7 shows an example of an applet with all components arranged according to the rules of the FlowLayout manager.

Figure 18.7: Components arranged by the FlowLayout manager.

The BorderLayout Class

The BorderLayout class is the default layout manager for all Window, Dialog, and Frame classes. In a border layout, components are added to the edges of the container and the center area is allotted all the space that's left over. The add() method takes an additional parameter-a string that can be North, South, East, West, or Center. This parameter specifies the location in the border layout for the component. For example, the following statements create five buttons and add them to a container laid out with the BorderLayout manager:

Button b1 = new Button("Climb");
Button b2 = new Button("Dive");
Button b3 = new Button("Left");
Button b4 = new Button("Right");
Button b5 = new Button("Fire!");
setLayout( new BorderLayout() );
add("North", b1);
add("South", b2);
add("West", b3);
add("East", b4);
add("Center", b5);

The setLayout( new BorderLayout() ); statement shows the use of the setLayout() method to select a layout manager for the container. Figure 18.8 shows the resultant applet. Be sure to correctly capitalize the directional parameter to the add() method -unlike other aspects of the language that are not case sensitive, BorderLayout requires the directions to be capitalized consistently as North, South, East, West, and Center.

Figure 18.8: Components arranged by the BorderLayout manager.

The GridLayout Class

The GridLayout class puts each component into a spot on a grid and makes each component equal in size. The grid is given specific dimensions when created, and each component is added to the grid in order, starting in the upper-left corner. This is similar to the way that components are added according to the rules of FlowLayout, but with GridLayout, components are given equal amounts of space on the container.

Components placed under the rules of GridLayout are arranged in order from left to right. When there are no more grids remaining on a row, the next component to be added is placed in the leftmost grid on the next line, if one is available. The only parameter used with the add() method is the name of the object to add.

The following setLayout() statement sets up a container to use the grid layout manager with four rows and six columns:

setLayout( new GridLayout(4, 6));

Figure 18.9 shows an example of an applet with all components arranged according to the rules of the GridLayout manager.

Figure 18.9: Components arranged by the GridLayout manager.

The GridBagLayout Class

The GridBagLayout class is similar to the GridLayout manager except that GridBagLayout provides much more control over how the grid is organized and how components are presented. Cells in the grid are not required to take up the same amount of space, and components can be aligned in different ways in each grid cell.

A special GridBagConstraints object is used to determine how a component will be placed into a cell and how much space that cell will occupy. Unlike the FlowLayout and GridLayout managers, with the GridBagLayout manager, components can be added to the grid in any order.

The first step needed to use a GridBagLayout is to set up the layout and the GridBagConstraints object, as shown here:

GridBagLayout gl = new GridBagLayout();
setLayout ( gl );
GridBagConstraints gb = new GridBagConstraints();

Before a component can be added to the container, instance variables of the GridBagConstraints object are used to determine the component's location and alignment within its grid cell.

The following GridBagConstraints variables can be set:

Following is an example that places a component using the GridBagLayout manager:

// declare variables
GridBagLayout gl = new GridBagLayout();
GridBagConstraints gb = new GridBagConstraints();
Label l1 = new Label("Full e-mail address:");
// choose a layout manager
setLayout ( gl );
// set up the constraints
gb.gridx = 1;
gb.gridy = 2;
gb.gridwidth = 2;
gl.setConstraints(l1, gb);
// set up the component
l1.setAlignment(Label.RIGHT);
// add the component
add(l1);

In this example, the component is placed in row 1 and column 2 of the grid, and the component takes up two cells in width.

Figure 18.10 shows an example of an applet with all components arranged according to the rules of the GridBagLayout manager.

Figure 18.10: Components arranged by the GridBagLayout manager.

Nested Panels

Although the Abstract Windowing Toolkit offers several different kinds of layout managers, there are many times when one specific manager does not fit the user interface you are trying to create.

The solution to this problem is to nest one type of container inside another so that the nested container can use one type of layout manager, and the larger container can use another.

You can nest containers as many times as needed: containers can contain other containers, and each can use its own layout manager.

The following code creates a container that uses the BorderLayout manager and then adds a panel of choice boxes to that container. Because no layout manager is specified for the buttons, they default to the FlowLayout manager:

setLayout( new BorderLayout() );
Button b1 = new Button("Purchase");
add("North", b1);
Button b2 = new Button("Exit");
add("West", b2);
Button b3 = new Button("Help");
add("East", b3);
Button b4 = new Button("Browse Catalog");
add("South", b4);
Panel p = new Panel();
// add check boxes to panel
Checkbox c1 = new Checkbox("Brazil: $9.95");
p.add(c1);
Checkbox c2 = new Checkbox("Time Bandits: $12.95");
p.add(c2);
Checkbox c3 = new Checkbox("12 Monkeys: $39.95");
p.add(c3);
// add panel to main container
add("Center",p);

Figure 18.11 shows the applet that uses this code. Using nested panels and different types of layout managers makes it possible to create many different types of user interface windows.

Figure 18.11: A container organized by the BorderLayout manager with a nested panel in the center organized by the Flowlayout manager.

Advanced Components

In addition to the components you have learned to create up to this point, the AWT offers some more sophisticated elements such as text areas, scrolling lists, and scroll bars.

Text Areas

The TextArea component is an extended input box that enables more than one line of text to be entered. The number of lines and the number of characters per line that are visible in the text area are configurable. Figure 18.12 shows an example of an applet with a text area.

Figure 18.12: A TextArea component.

The following code is used to create a TextArea component and add it to an applet window:

TextArea t = new TextArea(5,20);
add(t);

The parameters 5 and 20 specify the text area's number of lines and the number of characters per line, respectively. If the text extends beyond the borders of the text area, a scroll bar appears to enable the user to scroll to the different sections of the text area.

If a string is specified as a parameter to the constructor, as in the statement TextArea t = TextArea("It was a dark and stormy night.", 7, 25); the text area is created with the string as the default text in the input area of the field. To cause parts of this text to start at the beginning of the next line, use a newline character (\n) in the text of the parameter. For example, to put the text stormy night. at the beginning of a new line, use the following statement:

TextArea t = TextArea("It was a dark and\nstormy night.", 7, 25);

Scrolling Lists

The List component is a scrolling list of strings from which one or more strings can be
selected. Figure 18.13 shows an example of an applet with a scrolling list.

Figure 18.13: A List component.

The addItem() method of the List class is used to build the scrolling list. The following code is used to create a scrolling list, add items to it, and then add the list to an applet window:

List l = new List(3,true);
l.addItem("Captain America");
l.addItem("Superman");
l.addItem("Cerebus");
l.addItem("Nexus");
add(l);

The List() constructor can be used with no parameters or with two parameters; the List l = new List(3, true); statement uses two parameters. The first parameter indicates the number of list items to display in the list window; the second parameter determines how many list items can be selected. If the second parameter is true, multiple items can be selected. Otherwise, only one choice from the list can be made.

Scroll Bars

The Scrollbar component is an up-down or left-right slider that can be used to set a numeric value. The component can be used by clicking the mouse on an arrow or grabbing the box on the slider. Figure 18.14 shows an example of an applet with a scroll bar.

Figure 18.14: A Scrollbar component.

The first parameter of the Scrollbar constructor determines whether it is a vertical or horizontal scroll bar. If you want the bar to be vertical, use the constant Scrollbar.VERTICAL. Otherwise, use the constant Scrollbar.HORIZONTAL. There are four other parameters that follow the alignment parameter. These parameters determine the slider's initial setting, the size of the scrollable area, the minimum value, and the maximum value.

Consider the following statement:

Scrollbar sb = new Scrollbar(Scrollbar.HORIZONTAL, 50, 30, 10, 90);

This creates a horizontal Scrollbar component with the slider initially set to 50. The visible part of the scrollable area has a value of 30 from one end to the other. The minimum value when the slider is all the way to the left is 10, and the maximum value when the slider is all the way to the right is 90.

The following code is used to add a scroll bar to an applet window:

Scrollbar s = new Scrollbar(Scrollbar.HORIZONTAL,50,100,1,100);
add(s);

Canvases

The Canvas component is a section of a window used primarily as a place to draw graphics. In that respect, it is more similar to a container than a component-however, a Canvas cannot be used as a place to put components. Figure 18.15 shows an example of a canvas, which appears as a white area on a darker applet background.

Figure 18.15: A canvas Component.

The following code is used to create a Canvas component, resize it to 50-by-50 pixels, set the background of the canvas to the color white, and add the canvas to an applet window:

Canvas c = new Canvas();
c.resize(50,50);
c.setBackground(Color.white);
add(c);

Event Handling

At this point, you have the tools necessary to build an impressive graphical user interface for a Java applet. You can use text fields to enter characters, click buttons, move scroll bars from side to side, and so on.

However, the interface has no way to respond to any of this user input. In a windowing environment such as the one provided by the Abstract Windowing Toolkit, user input-whether from the mouse, keyboard, or other means-generates an event.

An event is a way for a program to communicate that something has taken place. Events generate automatic calls to methods in the same way that a paint() method can be called automatically when an applet window has to be redrawn, or an init() method is called automatically when an applet is first run.

Events can involve a user interface element-such as a button that has been clicked. Events can also be something unrelated to the interface-such as an error condition that causes the program to stop execution.

In Java, the class java.awt.Event handles all events related to the user interface.

For the purpose of controlling the user interface components you can create for use with applets, there are two kinds of events to consider: action events and scroll-list events.

Action Events

An action() event is generated by most user interface components to signify that an event has taken place. This means different things depending on the component:

The action() method of the Event class takes the following form:

public boolean action(Event e, Object o) {
    // method code here
}

All user interface components that generate an action event do so by calling the action() method. To determine which component generated the event-and to gather some other information about what occurred-there are two parameters to the action() method: an instance of the Event class and an Object.

The Event class has several instance variables that provide information about the event that has taken place. The one you use most with action() events is the target variable, which indicates which component generated the event.

The code in Listing 18.2 creates three buttons on an applet window and sets a TextField in response to the button that was pressed. You can also find this code on the CD-ROM that accompanies this book. Windows 95 and Windows NT 4 users will find the code for this chapter in \WIN95NT4\SOURCE\CHAP18. Macintosh users will find code in \SOURCE\CHAP18. Windows NT 3.51 users must install the source code to their hard drives or select the files from the zipped source code on the CD-ROM.


Listing 18.2. The full source code of Buttons.java.

 1: import java.awt.*;
 2: 
 3: public class Buttons extends java.applet.Applet {
 4:     Button b1 = new Button("Swing Away");
 5:     Button b2 = new Button("Bunt");
 6:     Button b3 = new Button("Renegotiate Contract");
 7:     TextField t = new TextField(50);
 8: 
 9:     public void init() {
10:         add(b1);
11:         add(b2);
12:         add(b3);
13:         add(t);
14:     }
15: 
16:     public boolean action(Event e, Object o) {
17:         if (e.target instanceof Button) {
18:             String s = (String)o;
19:             if ( s.equals("Swing Away") )
20:                 t.setText("A long fly to center ... caught.");
21:             else if ( s.equals("Bunt") )
22:                 t.setText("You reach on a throwing error!");
23:             else
24:                 t.setText("You're now America's newest multimillionaire!");
25:         }
26:         return true;
27:     }
28: }

To test the applet, create an HTML file with the source code from Listing 18.3 (this code is also found on the CD-ROM that accompanies this book).


Listing 18.3. The HTML source code of Buttons.html.

1: <html>
2: <body>
3: <applet code="Buttons.class" height=100 width=475>
4: </applet>
5: </body>
6: </html>

Figure 18.16 shows this applet being run.

Figure 18.16: The Buttons applet.

Scroll Bar Events

The action() method is generated by every component described in this chapter except for one: Scrollbar. The Scrollbar component uses the handleEvent() method.

The handleEvent() method of the Event class takes the following form:

public boolean handleEvent(Event e) {
    // method code here
}

Unlike the action() method, handleEvent() takes only one parameter: an instance of the Event class. Using the target variable of the Event class, you can determine which component generated the event and respond to it.

The following example sets a value in a text field based on user input to a scroll bar. To try it out, type the code in Listing 18.4 into a text editor and save it as Scroller.java (alternatively, you can copy the code from the CD-ROM that accompanies this book).


Listing 18.4. The full source code of Scroller.java.

 1: import java.awt.*;
 2:  
 3: public class Scroller extends java.applet.Applet {
 4:     Scrollbar s = new Scrollbar(Scrollbar.HORIZONTAL,
 5: 			      50,100,0,100);
 6:     Label l = new Label("Choose Your Own Tax Rate: ");
 7:     TextField t = new TextField("50%", 3);
 8:  
 9:     public void init() {
10:         add(s);
11:         add(l);
12:         add(t);
13:     }
14:  
15:     public boolean handleEvent(Event e) {
16:         if (e.target instanceof Scrollbar) {
17:             int taxrate = ((Scrollbar)e.target).getValue();
18:             t.setText(taxrate + "%");
19:             return true;
20:         }
21:     return false;
22:     }
23: }

Compile the file and create a Web page to put the applet on. Enter the code in List-
ing 18.5 and save it as Scroller.html (you can also find this code on the CD-ROM that accompanies this book).


Listing 18.5. The HTML source code of Scroller.html.

1: <html>
2: <body>
3: <applet code="Scroller.class" height=200 width=200>
4: </applet>
5: </body>
6: </html>

When you use a Web browser or the applet viewer JDK utility to view the page, it should resemble Figure 18.17.

Figure 18.17: The Scroller applet.

Summary

This introduction to applet programming and the Abstract Windowing Toolkit showed you that a lot of Java's functionality is built into its class libraries. For many applets, creating the user interface represents the bulk of the work because a lot of the code needed to control the interface has already been written.

Each of the user interface components has methods you can use to retrieve or change their values, enable or disable their operation, and perform other tasks. The full reference to these methods is available in the Java API documentation that comes with the Java Developers Kit.

Once you have acquired some experience developing applets with the AWT, you will be in a much better position to take advantage of the other visual programming tools that have been introduced for Java.