by Michael Girdley
This chapter looks into the use and description of the java.AWT concepts of containers and components. Part of this chapter covers the implementation of some of the components. The graphical components of the AWT are covered in this chapter, while others are left for later chapters. Here is a list of the components that are covered in this chapter:
Buttons
Checkboxes
Radio buttons
Choice menus
Lists
Scrolling lists
Sliders and scrollbars
Notice that the component Canvas is left out of this chapter. It is a graphic component, but for organizational purposes it is included in Chapter 9, "The Order Entry System: Adding Graphics and a Logo."
This chapter also covers the implementation of handling user inputs, or events. This concept is touched on in the preceding chapter, but to sum it up, events are basically what their name says, "something happening." Some events are mouse clicks, keyboard presses, and events created by your own program to "fake" an event. All these events are covered in the "Handling Events" section of this chapter.
Finally, this chapter applies the concepts of components, applets, the AWT, and events to the Order Entry system. In building the Order Entry system applet, this chapter exercises these features in the process of coding a larger program. This is the area in which many instructional books are lacking.
But first, let's get into some components.
If you remember from the preceding chapter, the component class is the keystone to the AWT. The vast majority of the available classes in the AWT derive from this class. This includes the container class, which is one of the subclasses of the component class. This is the main reason that you can place components in containers in containers, and so on. This feature is useful when you manage the layout of the Order Entry system in Chapter 8, "The Order Entry System: Managing the Applet Layout," but for now, just appreciate the power of components and the AWT as a whole.
Components come in many shapes and sizes. As I stated before, this chapter covers the components that are graphics-oriented. Of course, all the components are graphical in nature. They are drawn just as any other GUI item is, but the components covered in this chapter are those that are not text-oriented, meaning not designed for the output or input of text. Placing all the components in one large section may seem to be the most logical, but dividing them into two groups keeps you from being bombarded by 12 explanations at once and also facilitates reference use of the book. Text-oriented components are covered in Chapter 7, "The Order Entry System: Entry Fields, Labels, and Validation."
The simplest components are buttons, which are covered next.
In the AWT, buttons are like the buttons found on virtually every GUI system. Listing 6.1 is an applet that demonstrates the insertion of buttons in an applet.
Listing 6.1. The heyabutton applet code.
1.// Import all of the java.awt package and the applet package.
2. import java.awt.*;
3. import java.applet.*;
4. // Declare the heyabutton class.
5. public class heyabutton extends Applet
6. {
7.
8. // Create a variable aButton of the Button class.
9. private Button aButton;
10. // Overriding the default init() method.
11. public void init()
12. {
13. // Allocate space for the Button.
14.. aButton = new Button("Longhorn");
15. // Add the button to the Applet panel.
16.. add(aButton);
17. }
18. }
The heyabutton class, when viewed from the Appletviewer utility, is shown in Figure 6.1.
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Figure 6.1. The heyabutton applet when viewed from the Appletviewer utility.
Line 1 makes all the classes from the java.awt package available to you. Line 2 does all the same with the applet package. Line 3 says that this program extends the applet class in the library. In other words, it is a subclass of the applet class and shares all functionality of it. You are, of course, overriding some of the inherited methods with your own declarations. In this applet, the code in line 6 is doing just that. This line declares a void that overrides the standard init() method with your own code.
But before that, line 5 declares a private button, which means that, for the purposes of object-oriented design and data encapsulation, the variable is local to this class and not accessible from outside. The button type is contained in the file java.awt.Button.
Line 8 uses the new method, "make aButton a new instance of the Button class." Now, aButton is your very own instance of the java.awt.Button class from the library.
So what do you do with it? You have a button; now add the component to the applet using Java's add() method. The method add() is common to all containers that place a component in the container. (Exactly where it goes is covered with the applet layout in Chapter 8.) For now, it's enough to know that it puts its parameter in the container from which it is called. Here's an example:
add(aPanel);
This line adds the aPanel component to the current container. The following code adds the aButton component to the container somecontainer:
somecontainer.add(aButton);
That is the end of the functional part of the applet and the coverage of buttons and how to add components.
After you have a good bit of Java knowledge, it's a good idea to go into the libraries and check out the contents. If you have experience with object-oriented design programs, such as C++, the library structure should be familiar. If not, Java is a good language to start with when you are trying to get into object-oriented design.
The process of looking at the methods, looking at the data structures, and going through the implementation, does a great deal for your debugging and code design efforts.
A checkbox is like its name sounds: it's a box you put a check in. Checkboxes are either on or off, selected or unselected. The mouse click (or a tab down) on the checkbox toggles its value, making a checked box unselected and vice versa. You use checkboxes when you are creating an interface that needs to get data that is either yes or no. For example, "Would you like to be included on our mailing list?"
Checkboxes are created using the class java.awt.Checkbox of the AWT.
The line numbers in the following example are for ease of reference only.
Here is the standard constructor for creating a checkbox:
1. private Checkbox aCheckbox; 2. aCheckbox = new Checkbox();
Of course, you can combine these two statements:
1. Checkbox aCheckbox = new Checkbox();
Checkboxes, like most classes implemented in Java, are overloaded to be able to accept different combinations of input. This is a feature shared with C++ and some other languages designed to facilitate object-oriented programming.
To create a checkbox and set the label, use the following format:
Checkbox aCheckboxpluslabel = new Checkbox("Label here!");
To set the checkbox's initial value, you can include another parameter:
Checkbox anotherCheckbox = new Checkbox("Label here!", null, true);
In this constructor, the first parameter is the label, and the third parameter is the initial starting value. The second parameter places the checkbox into a CheckboxGroup. This is used to create radio buttons, which are a group of checkboxes in which only one of them can be selected at the same time. Listing 6.2 is an applet that creates and inserts four checkboxes into an applet.
Listing 6.2. The heysomecheckboxes applet code.
1. import java.awt.*;
2. import java.applet.*;
3. public class heysomecheckboxes extends Applet
4. {
5. // Override the initialize method.
6. public void init()
7. {
// Declare the four checkboxes and set their labels.
8. Checkbox JonBox = new Checkbox("Jon");
9. Checkbox IanBox = new Checkbox("Ian");
10. Checkbox NateBox = new Checkbox("Nate");
11. Checkbox MichaelBox = new Checkbox("Michael");
12. // Add the four checkboxes to the applet panel.
13. add(JonBox);
14. add(IanBox);
15. add(NateBox);
9. add(MichaelBox);
10. }
11. }
Lines 1 through 6 do the same standard stuff: declare the applet, import classes, for example. Lines 8 through 11 create four checkboxes, each with a different label. In this case, four different names are used. This is helpful if you use an applet (or application) to report work done on a project and you need to say who has worked on or contributed to the project. Lines 13 through 16 add these boxes to the applet. Figure 6.2 shows this applet displayed in the Appletviewer utility.
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Figure 6.2. The heysomecheckboxes applet when viewed from the Appletviewer utility.
There are four checkboxes with corresponding labels on the applet. Easy enough, right? Sometimes, though, you may want only one checkbox in a group of checkboxes to be true at any one time. This situation is covered in the section on radio buttons.
Here are some of the methods available for controlling instances of checkboxes:
getState() This method is a function that returns the current state of the calling instance of a checkbox. Remember, the state of checkboxes is either true or false.
setState(boolean) This method allows you to set the state of the calling instance.
getLabel() This method returns the label of an instance of the checkbox class. The label is of type String.
setLabel(string) This method allows you to set the label of a checkbox to a new string.
The preceding section covered the addition of checkboxes to applets. The preceding example had the checkboxes operate independently of each other. But often you want only one checkbox to be true at a time. This behavior is characteristic of radio buttons in Java.
There isn't a specific type RadioButton. Instead, radio buttons are created by creating a group of checkboxes and specifying the initial checkbox to be selected. This group is defined by a class called (surprisingly) CheckboxGroup. The class CheckboxGroup allows you to group checkboxes and then manages the states of each checkbox in the group so that only one is selected at a time. Here is a sample declaration of a CheckboxGroup:
private CheckboxGroup NameBoxGroup; NameBoxGroup = new CheckboxGroup();
The process of adding checkboxes to the group is easy. When you declare a checkbox, you add the group name as another parameter. For example, you declare a checkbox and want to include it in the group declared above, NameBoxGroup. You also want it to be the box that starts with its value as true. Here is how you do it:
JonBox = new Checkbox("Jon", NameBoxGroup, true);
Do this for each checkbox you want in the group and you're ready to go. Listing 6.3 takes the previous example, the heysomecheckboxes applet, and extends it to include the four names to act as radio buttons. The new applet is the HeyaCheckBoxGroup applet.
Listing 6.3. The HeyaCheckBoxGroup applet code.
1. // import all of the classes in the java.awt and applet packages
2.. import java.awt.*;
3. import java.applet.*;
4. // declare the class HeyaCheckBoxGroup.
3. public class HeyaCheckBoxGroup extends Applet
4. {
5. // Overriding the standard init() void.
6. public void init()
7. {
8. // Declare the NameGroup as a CheckboxGroup().
8. CheckboxGroup NameGroup = new CheckboxGroup();
9. // Declare the JonBox as a new Checkbox(), and
do the same for four more boxes.
// Also, set the JonBox to be initially true while
the rest are false.
10. Checkbox JonBox = new Checkbox("Jon",NameGroup,true);
11. Checkbox IanBox = new Checkbox("Ian",NameGroup,false);
12. Checkbox NateBox = new Checkbox("Nate",NameGroup,false);
13. Checkbox MichaelBox = new Checkbox("Michael",NameGroup,false);
14.
14. // Add all of the Checkboxes to the applet panel.
15. add(JonBox);
16. add(IanBox);
17. add(NateBox);
18. add(MichaelBox);
19. }
20. }
This applet declares an instance of CheckboxGroup in line 8. Then lines 10 through 13 declare four checkboxes and insert them into the NameGroup with the JonBox being set initially true. Then the four boxes are added to the applet using the add method in lines 15 through 18. Figure 6.3 shows this applet.
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Figure 6.3. The HeyaCheckBoxGroup applet when viewed from the Appletviewer utility.
Other methods are available in checkboxes that deal with checkbox groups:
getCheckboxGroup() This allows you to get the group to which a checkbox belongs.
setCheckboxGroup(CheckboxGroup cbg) This allows you to change the group to which a checkbox belongs. It takes an instance of CheckboxGroup as a parameter.
Also, the setCurrent(Checkbox) and getCurrent() methods of the CheckboxGroup class allow you to set and get the currently chosen checkbox.
Pop-up choice lists allow you to combine many different choices into one area. A large number of radio buttons can become unwieldy, so this type of choice list puts many different choices into one compact area. Figure 6.4 is an applet that features a pop-up choice list viewed from the Appletviewer utility:
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Figure 6.4. A choice list.
Choice boxes come from the java.awt.Choice class. To declare a new choice box, you can use the following code:
Choice names = new Choice();
To insert items into the list, you use the addItem() method of the Choice class. The addItem() method takes a string as a parameter. Here is the code to place the four names--Jon, Ian, Nate, and Hal--into the choice list called names:
names.addItem("Jon");
names.addItem("Ian");
names.addItem("Nate");
names.addItem("Hal");
The items are listed in the pop-up choice list in the order in which they are added. The selection in the box is the current selection of the choice list. To change it, use the method select(). It is overloaded to accept both integers and strings. Should you want to select the third choice in a list initially, use the code:
names.select(3);
To choose the string "Jon", you use the following:
names.select("Jon");
The getItem() method allows you to get the string at a certain position if you know the index. Simply give it an integer, and it returns the string at that position. Also, after you put an item in the list, it is there for good; there is no way to get it out.
On some occasions, you want to have all the selections available for viewing at one time. You can do so by using scrolling lists. They allow more than one item to be selected at a time. Figure 6.5 shows an applet that contains a scrolling list.
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Figure 6.5. A scrolling choice list.
Lists are declared using the following constructors:
List() This is the default constructor. Only one choice may be selected at a time.
List(int, boolean) This creates a scrolling list of the size defined in the integer parameter, and the Boolean flag sets the ability to have multiple selections. True means that it can have multiple selections; false means it can't.
Here is an example construction of a list that specifies six items able to be viewed at one time and allows multiple selections at one time:
List Names = new List(6, true);
As with pop-up choice lists, you use the addItem() method of lists to insert items into the list:
Names.addItem("Hal");
Names.addItem("Nate");
Names.addItem("Evan");
Another instance of the addItem method allows you to set where you want the next item inserted in the list. Here is an example that places another name at position four in the list:
Names.addItem("Ignacio",4);
You can also use the select() method to set which items you want initially selected. The following two lines set the first and fourth items in the list to be highlighted:
Names.select(1); Names.select(4);
Figure 6.6 shows the preceding declaration plus a few more added into an applet viewed through the Appletviewer utility.
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Figure 6.6. The ScrollListExample applet viewed through the Appletviewer utility.
Listing 6.4 shows the ScrollListExample applet code.
Listing 6.4. The ScrollListExample applet code.
1. // import all of the classes in the java.awt and applet packages
2. import java.awt.*;
3. import java.applet.*;
4. // declare the class HeyaCheckBoxGroup.
5. public class ScrollListExample extends Applet
6. {
7. // Overriding the standard init() void.
8. public void init()
9. {
10. List Names = new List(6,true);
11.
12. Names.addItem("Hal");
13. Names.addItem("Nate");
14. Names.addItem("Ignacio,4");
15. Names.addItem("Yi");
16. Names.addItem("Wilson");
17. Names.addItem("Robb");
18. Names.select(1);
19. Names.select(4);
20. add(Names);
21. }
22.}
A number of other useful methods are available in the List() class:
clear() This removes all the items from the list.
getItem(integer) This returns the string at the index specified by the parameter.
countItems() This returns an integer specifying the number of items currently in the list.
replaceItem(string, integer) This replaces the item at the index specified by the integer with the string.
getSelectedIndex() This returns an integer specifying the position in a list of the selected item.
getSelectedIndexes() This returns an array of integers each of which specify the indexes of all the selected items.
getSelectedItems() This returns an array of strings that are all the selected items at the current time.
getSelectedItem() This returns the string that is currently selected.
Scrollbars are common to every graphical user interface system. In the AWT, they are used to scroll areas such as lists and sliders, which are a graphical means to set values. Figure 6.7 shows a small applet that includes a slider.
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Figure 6.7. An applet with a slider.
As sliders, scrollbars are used to input a value to the program from the user. The programmer can set the initial minimum value, the maximum value, the orientation, and the visible area of the scrollbar. You also can set how the scrollbar changes per different user actions. This activity is described later in this chapter when the process of handling events in the AWT is covered.
This section covers the construction of instances of the Scroll() type and their implementation. Chapter 8 covers using scrollbars to scroll through a window. There are two major ways to construct a scrollbar.
Scrollbar(int) constructs a scrollbar and sets the initial orientation (vertical or horizontal) of the scrollbar. The int you send in is either Scrollbar.HORIZONTAL or Scrollbar.VERTICAL. These are the only values currently available to you. You can then later use the setValues(int initial, int visiblearea, int min, int max) method to set the initial value, the visible area of the window, the minimum, and the maximum values for the scrollbar, respectively.
Scrollbar(int orientation, int visiblearea, int initial, int min, int max), takes five parameters (in this order): the orientation (either Scrollbar.HORIZONTAL or Scrollbar.VERTICAL), the visible area value, the initial value, the minimum value, and the maximum value.
The page value is the amount that the value of the scrollbar changes when a user clicks on the area between the end arrow and the current position. To set the page increment value, use the setPageIncrement(int pageincrement) method, which takes an integer parameter and makes that value the page value.
The line increment is the amount that the scrollbar changes when a user clicks on the end arrow of the scrollbar. The method to change or set this value is the setLineIncrement(int lineincrement) method of the Scrollbar class. As with the setPageIncrement(int pageincrement) method, this method accepts an integer.
Listing 6.5 is a sample applet that contains two scrollbars, using both of the Scrollbar() constructors discussed previously.
Listing 6.5. The TwoScrollBars applet code.
1. import java.awt.*;
2.
3. public class TwoScrollBars extends java.applet.Applet
4. {
6. public void init()
7. {
8. // Declare the Vertical Bar as a horizontal scrollbar..
9. Scrollbar VertBar = new Scrollbar(Scrollbar.VERTICAL);
10. // Set the values to be: initial value 25,
visible, area to 35, min to 0, max to 200.
11. VertBar.setValues(25, 25, 0, 200);
12. // Add the scrollbar to the applet panel.
13. add(VertBar);
14.
15. // Declare the HorizBar as a horizontal scroll bar,
with initial 16. value 10, visible area of 50,
17. //minimum of 0, and maximium of 200.
18. Scrollbar HorizBar = new Scrollbar
19. (ScrollBar.HORIZONTAL, 10, 50, 0, 200);
20. // Add the HorizBar to the applet panel.
21. add(HorizBar);
22. }
23.}
Lines 1 through 6 do the standard stuff: declare the applet, import classes. Line 9 declares the scrollbar and sets the initial orientation of the VertBar variable. Then the setValue() method is used to set the initial value, the visible area, the minimum value, and the maximum value. The scrollbar is then added to the applet panel. In lines 15 through 20, basically the same thing happens, except that all the setup done in setting and adding the VertBar is done in lines 17, 18, and 20.
There are actually more constructors for creating scrollbars. However, in the end, they all produce the same product and are just as easy. By providing you with the "all in one" constructor that takes five parameters, and also including a minimalist constructor that takes only one parameter, I hope that you are getting a feel for the whole range of constructors.
Here is a summary of the Scrollbar methods and declarations:
Scrollbar(int orientation, int visiblearea, int initial, int min, int max) The parameters set the orientation, visible area, initial value, minimum, and maximum respectively.
setPageIncrement(int) As discussed previously, this sets the page increment. This is the amount that the scrollbar changes when the user clicks between the current position and the endpoint.
setLineIncrement(int) As discussed above, this sets the line increment. The line increment is the amount that the scrollbar changes when the user clicks on the end arrow of the scrollbar.
setValues(int initial, int visible area, int min, int max)[em]This is the method to set the initial value, the visible area of the window, the minimum, and the maximum values for the scrollbar respectively.
int getValue() This returns the current value, an integer, of the scrollbar. This is the main method used later when making scrollbars active.
In any of the examples discussed in this chapter, you may notice that something is wrong. The insertion and setup of these different components of the AWT are covered but not how to handle when something happens with them. For example, nothing happens when you click on the buttons that have been created. Handling user actions like button selection is covered later in this chapter. But first, let's get started on creating the Order Entry system.
This section covers how to create and insert the different components to be used onto the applet panel. Don't worry about the positioning of the buttons. This is taken care of when you manage the applet layout in Chapter 8. For now, just worry about creating the components and slapping them down.
Also, notice that the Order Entry system exists as a stand-alone window. This also is covered in Chapter 8. Finally, the logo is left out for now because it is covered in Chapter 9.
Ideally, you want to use every component to give yourself a feel for them. So give it a shot, even though it may complicate matters. For the time being, you are going to implement the components covered in this chapter.
To start off, import all the AWT classes and also the Applet package as follows:
import java.awt.*; import java.applet.*;
And then declare the applet class like this:
public class OrderEntrySystem extends Applet {
The customary place to initialize and set up your applet is in the init() method. So, declare the init() method and insert and declare your first component, a button that reads Submit. Here is the code segment that declares the button and inserts it onto the applet panel:
Button SubmitButton = new Button("Submit");
add(SubmitButton);
Continue doing the same tasks for two more buttons labeled Quit and Clear, in the same manner. Next, insert a checkbox that asks if the person using the system is a repeat customer:
Checkbox RepeatCustCheckBox = new Checkbox("Repeat Customer?");
add(RepeatCustCheckBox);
You also should query the users as to how they are to be contacted. The choices are phone, e-mail, or U.S. mail. A radio button setup is ideal for this query since you want only one button to be active at one time. The first step in creating radio buttons is to create an instance of the CheckboxGroup class that coordinates the behavior of all the instances of the Checkbox class. Add the following:
private CheckboxGroup ContactMethodGroup; ContactMethodGroup = new ContactMethodGroup();
Next, create the buttons you're going to put in the group, using the declaration method specified previously:
Checkbox EmailBox = new Checkbox("Email",ContactMethodGroup,true);
Checkbox PhoneBox = new Checkbox("Phone",ContactMethodGroup,false);
And so on for the rest of the buttons. Since you'd rather e-mail, set the Email button to be initially true while the rest are false. Also, remember to add() all the buttons. There are a number of products to choose from in the applet, so set a means to choose from a number of products. Chapter 11, "Reading and Writing with Java," touches on how you could read in a file that contains all the products and prices. For now, set all the products in the program. First, construct the ProductList class as an instance of the List class and add some of the products,
List ProductList = new List(4, false);
ProductList.addItem("Oscar");
ProductList.addItem("Lionhead");
ProductList.addItem("Jack Dempsey");
ProductList.addItem("Angelfish");
.
.
The first parameter sets the list to have four items visible at a time. The second sets the list so that only one item may be selected at a time. There's also a pop-up choice box to let the user select the size of the product, in this case, fish, that he or she wants. To declare a choice box, you use much the same code as above.
And finally, the last component to add is a slider. This is used to allow the user to input the amount of each product that he or she wants to order. Here is the declaration for the slider:
Scrollbar OrderAmountSlider = new Scrollbar(Scrollbar.VERTICAL, 0, 0, 0, 100); add(OrderAmountSlider);
The scrollbar is declared to be vertically oriented, with a visible area of zero, an initial value of zero, a minimum of zero, and a maximum of 100.
Listing 6.6 is the complete code listing, showing what you should have so far in building the Order Entry system.
Listing 6.6. The OrderEntrySystem applet code so far.
import java.awt.*;
import java.applet.*;
public class OrderEntrySystem extends Applet {
public void init()
{
// Add and create three buttons.
Button SubmitButton = new Button("Submit");
add(SubmitButton);
Button ClearButton = new Button("Clear");
add(ClearButton);
Button QuitButton = new Button("Quit");
add(QuitButton);
// Add and create the repeat customer checkbox.
Checkbox RepeatCustCheckBox = new Checkbox("Repeat Customer?");
add(RepeatCustCheckBox);
// Declare the CheckboxGroup, and allocate space.
CheckboxGroup ContactMethodGroup;
ContactMethodGroup = new CheckboxGroup();
// Create some checkboxes to put in the group.
Checkbox EmailBox = new Checkbox("Email",ContactMethodGroup,true);
Checkbox PhoneBox = new
Checkbox("Phone",ContactMethodGroup,false);
Checkbox MailBox = new
Checkbox("US Mail",ContactMethodGroup,false);
// Add the checkboxes into the applet panel.
add(EmailBox);
add(PhoneBox);
add(MailBox);
// Create the list, 4 items visible, no multiple
// selections.
List ProductList = new List(4, false);
// AddItems to the List.
ProductList.addItem("Oscar");
ProductList.addItem("Lionhead");
ProductList.addItem("Jack Dempsey");
ProductList.addItem("Angelfish");
// Add the List to the Applet panel.
add(ProductList);
// Create the Choice box.
Choice SizeChoice = new Choice();
// AddItems to the List.
SizeChoice.addItem("Jumbo");
SizeChoice.addItem("Large");
SizeChoice.addItem("Medium");
SizeChoice.addItem("Small");
// Add the Choice to the Applet panel.
add(SizeChoice);
// Create a vertical slider, initial value of 0,
// minimum value of 0, maximum value of 144.
Scrollbar OrderAmountSlider = new Scrollbar(Scrollbar.VERTICAL, 0, 0, 0, 100);
// Insert the slider to the Applet panel.
add(OrderAmountSlider);
}
}
Figure 6.8 shows what you see when you view this applet through the Appletviewer utility.
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Figure 6.8. The start of the Order Entry system applet.
If you run the Order Entry system applet so far, you will notice something a little peculiar: nothing happens. You can click on the buttons, move the slider, and whatever else, and nothing happens. These user actions are called events in the Abstract Windowing Toolkit. Your program has to be able to deal with these events and give the proper response. For example, if a user clicks the up arrow on a slider, your program has to react and increment a variable. If a Clear button is clicked, your program has to reset all the data that has been input to default values. The next section covers the implementation and concepts behind events and handling them. It also deals with the specialized action() method used for different types of component events. And after that, it sets up much of the dealing with events in the Order Entry system. Also, you may think, "it looks terrible." Well, you're right. Hold on because the solutions are coming up in future chapters.
Some events in the AWT are mouse drags, window resizing, and slider. Your programs that use the AWT are set to react to each event. This makes programs based on the AWT very different from typical programs. When you're creating a windowing system, one of the tasks you do is enable your system to act on an event-by-event basis as opposed to a linear basis. The AWT takes this responsibility out of your hands.
Events in Java are declared using the Event class. Different types of events are identified by a different constant held in Event.id. Each different type of event, such as mouse clicks and window resizing is identified by a different constant. For example, if a mouse button is pressed, an event is generated whose id is equal to the constant MOUSE_DOWN. Table 6.1 explains all the mouse and keyboard event types. Table 6.2 contains events related to windows. Table 6.3 contains the miscellaneous events such as the event ids specific to the List class and scrolling events, and Table 6.4 lists the different values contained in instances of the Event class.
A few more event constants than these are available. However, the event.ids covered in this chapter are effective for almost all purposes.
Also, the when member of the Event class is used for time stamping. You generally don't worry about this because the vast majority of programs you deal with are event-driven and not linear.
After an event occurs, the Java runtime constructs an instance of the Event class and sets its id field appropriately. It then sends that instance to the handleEvent() method of your Java program that overrides the default handleEvent() method. This method deals with the event appropriately.
The handleEvent() method is a Boolean function. Three possible results can be returned: true, false, or super.handleEvent(Event). A returned true value signifies that your handleEvent() method has taken care of the event. A false value signifies that your event has not been handled. This is not desirable. The proper path for events to be handled is first through a class and then through its parent. A false value has the event sent to the parent container, and you don't want that. Instead, return super.handleEvent(Event), which sends the event to a parent of a class (the class it has been derived from) to be dealt with. To summarize, either return true, saying that your handleEvent() took care of the event, or return super.handleEvent(Event) to show that you didn't handle the event and your parent class should.
Notice that some events are missing in the previous lists. What about events resulting from instances of the Choice class? Well, these kinds of events are more easily handled with a Java method called the action() method. With the action() method, events work a little differently. The action() method deals with events that are generated by the AWT components themselves. The different components all create specific action events that your AWT programs can override to deal with specific events. The action method has the following structure:
public boolean action(Event InEvent, Object SomeObject) {
if (InEvent.target == AButton) {
. . .
} return . .
}
The action void returns, like the handleEvent() method, either true or false depending on whether the method has handled the event. Each different component calls the action method in a different way, using different parameter values in place of the SomeObject parameter. The InEvent parameter in the event is the calling component, and the SomeObject parameter can be anything (since basically everything in Java derives from an Object). When the action method is called for button presses, the argument is the string label of the button. When it is called from an instance of the Choice class, the argument is the selected string.
There are also specialized methods that handle very specific events. You can use them to simplify your other event-handling methods. Table 6.5 is a summary of these specialized event-handling methods.
In summary, user actions in Java can be categorized into those that are handled by the handleEvent() method and those that are handled by the different types of action methods. There is a large amount of overlap. It is easy to use the handleEvent method to deal with many of the component events covered by the action method. To do this, test for the ACTION_EVENT event id and then act based on the value of Event.target.
However, using both the more specific action methods and the handleEvent method ensures that your code is easier to read, design, and maintain. Of course, as with many languages, Java allows you many ways to do things, and with experience you will develop your own style.
Listing 6.7 is a sample applet that deals with handling events using both the action method and the handleEvent method.
Cases and syntax are very important in Java, especially when you're dealing with overriding methods and inheritance. For example, if you declare a HandleEvent() method and execute your Java program expecting it to override the handleEvent() method in the AWT, nothing happens. When in doubt, look for these kinds of errors.
Listing 6.7. The Event applet code listing.
1. import java.awt.*;
2. import java.applet.Applet;
3.
4. public class EventExample extends Applet {
5.
6. // Declare a local variable, AList of the List type.
7. private List AList;
8. // Declare a local variable, AButton of the Button type.
9. private Button AButton;
10.
11. // Override the default init() method.
12. public void init() {
13. /* Allocate space for the AList. Set it to show four
14. items visible at one time and turn off the ability
15. for the user to make multiple selections with the
16. false boolean parameter. */
17. AList = new List(3, false);
18.
19. // Add four items to the List class and initially select item #1..
20. AList.addItem("Blue");
21. AList.addItem("Yellow");
22. AList.addItem("Green");
23. AList.addItem("Red");
24. AList.addItem("Purple");
25. AList.addItem("Orange");
26. AList.select(1);
27. // Add the list class to the applet panel.
28. add(AList);
29.
30. // Allocate space for the AButton.
31. AButton = new Button("Click Me");
32.
33. // Add the button to the applet panel.
34. add(AButton);
35.
36. }
37.
38. /* Overriding the default HandleEvent method. Here we will
39. write code to receive Events from the Java run time. */
40. public boolean handleEvent(Event inEvent) {
41.
42. /* Check to see what kind of Event we're given, if
43. it is an Event where the user selects an item in
44. our list, then we're going to deselect that item
45. and reselect number 1. */
46. if (inEvent.target == AList)
47. {
48. if (inEvent.id == Event.LIST_SELECT)
49. {
50. /* Make the first item in the list the
51. selected item. */
52. AList.select(0);
53. }
54. }
55. return super.handleEvent(inEvent);
56. }
57.
58. public boolean action(Event InEvent, Object SomeObject) {
59. if (InEvent.target == AButton)
60. {
61. // Set the label equal to the selected list item.
62. AButton.setLabel(AList.getSelectedItem());
63. return true;
64. } else
65. return false;
66. }
67. }
** 06wpj09.pcx **
Figure 6.9. Screen shot of the EventExample applet.
If you try out this applet, you'll notice that it is a little frustrating. No matter what you choose, it always selects back to Blue. This is accomplished by the handleEvent() method declared starting in line 34. The method only checks the Event to see if its id is equal to that of having something selected in a list, LIST_SELECT. If it is, line 44 sets the selected member of the list to item number 0. Eventually, Java comes back and repaints the applet window and the list component itself.
Then line 46--return super.handleEvent(Event)--tells the default method to handle the event. This way, it still does tasks like window resizing in stand-alone windows and customizes your own handleEvent() without having to cover all the rudimentary and mundane tasks.
Line 58 is the place where the action method is declared. It is used for a couple of reasons. One, it is an aspect of Java you should know about. Two, it is much easier to use the action method instead of the handleEvent() method to handle events from components like choices. In any case, line 59 checks to see if the Event is generated by the AChoice choice, and if it is, sets the button label to the currently selected item in the list. As is customary, the action method returns a true if it handled the event; it returns false if it did not.
The Shift, Alt, Ctrl, and Meta keys are all supported by Java. For example, if you are testing for the Alt-PgDn combination being pressed, the code is the following:
public boolean handleEvent(Event InEvent)
{
if (InEvent.id == Event.KEY_PRESS)
{
if (InEvent.key == Event.PGDN) &&
(Event.ALT_MASK & InEvent.modifiers) != 0)
{
// Do what you wanna do. .
}
}
}
The only thing new here is dealing with the most interior if statement. This statement checks to see if the key pressed is PgDn and also to see if the modifier contained in the event signifies the Alt key.
The following are basically all the general guidelines to follow when creating an AWT-based program that is event driven.
First, figure out what you want. If you want buttons, declare them. In your applet (preferably in your init() method, although it doesn't have to be there), set up the panel layout. Add your components, and as covered in Chapter 8, manage the applet layout.
Second, design your method to handle the events you receive. In the handleEvent (along with the action method if you want), test the Event.id to see what kind of event you are given. For each id type you want to recognize, check it against the constants such as MOUSE_DOWN and react accordingly. For instance, if the Event.id equals KEY_PRESSED, then you want to check your instance of Event.key to see which key has been pressed.
Third, set the handleEvent() method to return the proper value: true if you have handled the event properly or super.handleEvent(Event) if you want the applet's immediate parent to handle the event.
You need to set up the framework to handle events in the Order Entry system. To do this, write the handleEvent() method and add it to the applet that you have developed so far. Also, add some of the variables that are local to the applet.
First, you handle the events of the Clear button being pressed, as follows:
if (InEvent.target == clearButton)
{
// Reset all of the variables and settings . .
.
.
}
Do the same for each of the other buttons, and put the respective code for their functions in as you go along. You aren't able to code the "Send" button's function completely until later in the book.
You don't need to do anything to make the "Repeat Customer?" checkbox active. When the data is collected and sent under the "Send" button's actions, the checkbox value is checked and sent. You can also accomplish this with the radio button group, checking each checkbox when the send-off time comes.
You may have noticed in the figures of the Order Entry system in Chapter 5, "Writing a Java Applet: The Order Entry System," that the applet keeps a running total and subtotal of the items currently selected to be ordered. You need to declare some internal variables: one for each subtotal being calculated and one for the grand total. Here are the declarations for these variables:
private double SubTotalOne = 0,0; private double Total = 0.0;
For now, you are going to enter only one product and the size for that product. Each product has a base price and a price modifier depending on the size. The subtotal is calculated by multiplying the base price, the number of items desired, and the price modifier together. The total is calculated as the sum of all the subtotals. Here are the local variables used for computing price:
private double multiplierSmall = 0.5; private double multiplierMedium = 1.0; private double multiplierLarge = 1.5;
For now, "hardwire" the prices and the different products into the applet. Later in Chapter 14, "Extending Java," you learn techniques to read files from your server and then use the data in your applets. The following constants are the base prices for each product:
static double ProdOneBaseValue = 1.0; static double ProdTwoBaseValue = 1.33; static double ProdThreeBaseValue = 1.75; static double ProdFourBaseValue = 8.75; static double ProdFiveBaseValue = 0.33;
You have to override the action method in the applet to handle most of the events coming from the components. You don't need to deal with the radio buttons and the checkboxes in the applet because their immediate values don't have an effect on the function of the component. The action method handles the events generated by the choice box. Here is the code section from inside the action method that modifies the subtotal and total variables when a new product choice is made:
public boolean action (Event InEvent, Object SomeObject) {
.
.
if (InEvent.target == SizeChoice) {
updateValues();
}
.
.
The updateValues() function is a void that is declared locally to update all the totals. It is implemented later in the book.
This is all that is taken care of in the action method as opposed to the handleEvent() method, which covers everything. You still have to update the totals and the multipliers when the list selection (for example, what the size chosen is) is made. To do this, add some code to the handleEvent() method to call the updateValues() function when a new list selection is made. It is handled in the same way as the list has been handled in the previous EventExample.
Finally, you need to updateValues() when the slider is changed. The code to do this is placed in the handleEvent() method. Here, you are checking to see if the current event is one of either line up or line down (for example, someone presses the end arrow).
public boolean handleEvent(Event InEvent) {
.
.
if (InEvent.id == Event.SCROLL_LINE_UP ||
InEvent.id == Event.SCROLL_LINE_DOWN)
{
updateValues();
}
And that covers the events in everything that's been added so far. Listing 6.8 is this chapter's final version of the Order Entry system applet.
Listing 6.8. The Order Entry System code listing.
import java.awt.*;
import java.applet.*;
public class OrderEntrySystem extends Applet {
// The subtotal and total variables.
private double SubTotalOne = 0.0;
private double Total = 0.0;
// The price multipliers for each different product size.
private double multiplierSmall = 0.5;
private double multiplierMedium = 1.0;
private double multiplierLarge = 1.5;
// The local constant base prices.
static double ProdOneBaseValue = 1.0;
static double ProdTwoBaseValue = 1.33;
static double ProdThreeBaseValue = 1.75;
static double ProdFourBaseValue = 8.75;
static double ProfFiveBaseValue = 0.33;
// Declare all of the variables we'll use.
private Button SubmitButton;
private Button ClearButton;
private Button QuitButton;
private Checkbox RepeatCustCheckBox;
private Checkbox MailBox;
private Checkbox EmailBox;
private Checkbox PhoneBox;
private List ProductList;
private Choice SizeChoice;
private Scrollbar OrderAmountSlider;
// Initialize the Applet.
public void init()
{
// Add and create three buttons.
SubmitButton = new Button("Submit");
add(SubmitButton);
ClearButton = new Button("Clear");
add(ClearButton);
QuitButton = new Button("Quit");
add(QuitButton);
// Add and create the repeat customer checkbox.
RepeatCustCheckBox = new Checkbox("Repeat Customer?");
add(RepeatCustCheckBox);
// Declare the CheckboxGroup, and allocate space.
CheckboxGroup ContactMethodGroup;
ContactMethodGroup = new CheckboxGroup();
// Create some checkboxes to put in the group.
EmailBox = new Checkbox("Email",ContactMethodGroup,true);
PhoneBox = new Checkbox("Phone",ContactMethodGroup,false);
MailBox = new Checkbox("US Mail",ContactMethodGroup,false);
// Add the checkboxes into the applet panel.
add(EmailBox);
add(PhoneBox);
add(MailBox);
// Create the list, 4 items visible, no multiple selections.
ProductList = new List(4, false);
// AddItems to the List.
ProductList.addItem("Oscar");
ProductList.addItem("Lionhead");
ProductList.addItem("Jack Dempsey");
ProductList.addItem("Angelfish");
// Add the List to the Applet panel.
add(ProductList);
// Create the Choice box.
SizeChoice = new Choice();
// AddItems to the List.
SizeChoice.addItem("Jumbo");
SizeChoice.addItem("Large");
SizeChoice.addItem("Medium");
SizeChoice.addItem("Small");
// Add the Choice to the Applet panel.
add(SizeChoice);
// Create a vertical slider, initial value of 0,
// minimum value of 0, maximum value of 144.
OrderAmountSlider = new Scrollbar(Scrollbar.VERTICAL, 0, 0, 0, 100);
// Insert the slider to the Applet panel.
add(OrderAmountSlider);
}
public boolean handleEvent(Event InEvent) {
if (InEvent.id == Event.SCROLL_LINE_UP ||
InEvent.id == Event.SCROLL_LINE_DOWN) {
updateValues();
} else
if (InEvent.target == ClearButton) {
// Reset all of the variables and settings . .
} else
if (InEvent.target == QuitButton) {
// Quit the applet.
} else
if (InEvent.target == SubmitButton) {
// Submit the order.
}
return super.handleEvent(InEvent);
}
public boolean action (Event InEvent, Object SomeObject) {
if (InEvent.target == SizeChoice) {
updateValues();
return true;
} else return false;
}
private void updateValues() {
// Empty for now. .
}
}
You've made great steps forward with the Order Entry system in this chapter. You started with nothing, and now it has an initial layout along with a framework for functionality. There is, however, still a good amount wrong with the applet. First, the totals, subtotals, and scrollbar values are not placed on the applet panel. You fix this in the next chapter, where the AWT components that deal with text input and output are covered. How to place text on the applet to better organize it is also covered in the next chapter. And finally, where's the text input? Once again, it is coming in the next chapter, "The Order Entry System: Entry Fields, Labels, and Validation."
