giáo trình Java By Example phần 2 pdf

Handling Events DirectlyExample: Overriding handleEvent in an Applet The Event Object In order to understand how to respond to various types of events, you need to know more about Java's

Handling Events Directly

Example: Overriding handleEvent() in an Applet

The Event Object

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

Event class, an object of which is passed to any event-handling method When you want to respond to aJava button control, for example, you override the action() method, whose first argument is an

Event object You then examine the target field of the Event object to determine whether it was the

button control that generated the event The Event class, however, defines many constants and datafields that provide information about the event represented by the object.

First, the Event class defines constants for all of the events to which an event-handling method canrespond In this chapter, you'll learn about some of these constants, which include MOUSE_DOWN,

MOUSE_UP, and KEY_PRESS The class also defines constants for special keys, such as f1, PGUP,

PGDN, HOME, and so on Finally, the Event class defines the data fields shown in Table 25.1 How youuse these data fields depends on the type of event represented by the Event object

Table 25.1 Data Fields of the Event Class.

Field Description

Object arg Event-specific information With a button event, for

example, this field is the button's label

int clickCount The click count for mouse events A value of 1 means a

single click, and 2 means a double-click

int id The event's type, such as MOUSE_DOWN, MOUSE_MOVE,

KEY_PRESS, etc

int key The key for a key-related event For a KEY_PRESS event,

for example, this would be the key that was pressed

int modifiers The key modifiers, including the shift and control keys The

Event class defines constants such as SHIFT_MASK and

CTRL_MASK

Object target The type of object-such as Button, TextField, and so on-that

generated the event

long when The event's time stamp

int x The X coordinate associated with the event, usually used

with mouse events to indicate the mouse's position at thetime of the event

int y The Y coordinate associated with the event, usually used

with mouse events to indicate the mouse's position at thetime of the event

The Mouse

Most people use their computer's mouse darn near as much as its keyboard I can vouch for this fromfirst-hand experience, because my only bout with RSI (repetitive strain injury) came not from typingfuriously all day, but from maneuvering my mouse to mark paragraphs, highlight words, click buttons,make list selections, bring up menus, and any number of other mousely tasks I'm not looking for yoursympathy, though My point is that the mouse is one of the most important input devices attached to yourcomputer To write complete applets, you're going to have to master responding to mouse events in yourJava programs

Luckily, responding to mouse input is a simple matter Because responding to the events generated by themouse are such an important and common task in modern programming, Java's classes already includespecial methods for responding to these events Exactly what events are you expected to handle? A

mouse generates six types of event messages that you can capture in your applets These events are listedbelow, along with their descriptions and the method that handles them:

MOUSE_DOWN-This event, which is handled by the mouseDown() method, is caused when theuser presses the mouse button

In the sections that follow, you'll learn more about the most commonly used of these mouse events

Handling Mouse Clicks

Without a doubt, the most commonly used mouse event in Java programs (and any other program writtenfor a graphical user interface) is the MOUSE_DOWN event, which is generated whenever the user clickswithin an applet It's the MOUSE_DOWN event, for example, that lets Java know when an on-screen buttoncomponent has been clicked You don't have to worry about clicks on on-screen buttons (usually),

because they're handled by Java However, you can respond to MOUSE_DOWN events in your applets inorder to accomplish other input tasks

Java provides a couple of methods by which you can respond to mouse events The easiest way to

capture a MOUSE_DOWN event is to override the applet's mouseDown() method Java automaticallycalls mouseDown() whenever the MOUSE_DOWN event is generated, which makes responding to thisevent easier than melting butter with a blowtorch The mouseDown() method's signature looks likethis:

public boolean mouseDown(Event evt, int x, int y)

The arguments passed to the function are an Event object and the X,Y coordinates of the mouse event.Although Java has already extracted the X,Y mouse coordinates for you, you can also get them from the

Event object by examining the values stored in the x and y data fields, as described in Table 25.1.(Because Java has already extracted the coordinates for you, though, it makes more sense to use the x

and y parameters sent to the function.) What you do with these coordinates depends, of course, on yourapplet In the next section, you'll see how to use the coordinates to display graphics on the screen

Although most of Java's event-handling methods automaticallyreceive as arguments the basic information you need about a specificevent (such as the coordinates of a mouse click), you can extractwhatever additional information you need from the Event object,which is always the first parameter in a message-handling method

Example: Using Mouse Clicks in an Applet

As I was describing the mouseDown() method in the previous section, I felt an example coming on.And, sure enough, here it is The applet in Listing 25.1 responds to mouse clicks by printing the word

"Click!" wherever the user clicks in the applet It does this by storing the coordinates of the mouse click

in the applet's coordX and coordY data fields The paint() method then uses these coordinates todisplay the word Figure 25.1 shows MouseApplet running under Appletviewer

Figure 25.1 : The MouseApplet applet responds to mouse clicks.

Listing 25.1 MouseApplet.java: Using Mouse Clicks in an Applet.

import java.awt.*;

import java.applet.*;

public class MouseApplet extends Applet

{

int coordX, coordY;

public void init()

{

coordX = -1;

coordY = -1;

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

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

Derive the MouseApplet class from Java's Applet class.

Declare the class's data fields

Override the init() method

Initialize the click coordinates

Create and set the font for the applet

Size the applet

Override the paint() method

If the user has selected a coordinate

Draw the word Click! at the selected coordinate

Override the mouseDown() method

Save the mouse click's coordinates

Force Java to repaint the applet

Tell Java that the event was handled

NOTE

When you run MouseApplet, you'll discover that the applet windowgets erased each time the paint() method is called That's whyonly one "Click!" ever appears in the window

Handling Mouse Movement

Although mouse clicks are the most common type of mouse event to which your applet may want torespond, tracking the mouse pointer's movement can also be useful Drawing programs, for example,enable you to draw shapes by tracking the movement of the mouse and displaying the results on the

screen

Unlike mouse clicks, though, which are rare, only occurring when the user presses a mouse button,

MOUSE_MOVE events come flooding into your applet by the hundreds as the user moves the mouse

around the screen Each one of these events can be handled in the mouseMove() method, whose

signature looks like this:

public boolean mouseMove(Event evt, int x, int y)

Yep Except for its name, the mouseMove() method looks exactly like the mouseDown() method,receiving as arguments an Event object and the X,Y coordinates at which the event occurred

Example: Responding to Mouse Movement in an Applet

Responding to mouse movement isn't something you have to do often in your applets Still, it's a handytool to have on your belt You might, for example, need to track mouse movement when writing a gameapplet that uses the mouse as input A more common use is in graphics programs that enable you to draw

on the screen Listing 25.2 is just such an applet

When you run MouseApplet2 with Appletviewer, you see a blank window Click the mouse in the

window to choose a starting point and then move the mouse around the window Wherever the mousepointer goes, it leaves a black line behind (Figure 25.2) Although this is a very simple drawing program,

it gives you some idea of how you might use a mouse to accomplish other similar tasks

Figure 25.2 : This applet draws by tracking the movement of the mouse.

Listing 25.2 MouseApplet2.java: An Applet That Tracks Mouse Movement.

startPoint = new Point(0, 0);

points = new Point[1000];

numPoints = 0;

drawing = false;

resize(400, 300);

}

public void paint(Graphics g)

{

int oldX = startPoint.x;

int oldY = startPoint.y;

for (int x=0; x<numPoints; ++x)

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

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

Derive the MouseApplet2 class from Java's Applet class

Declare the class's data fields

Override the init() method

Initialize the starting point

Create an array for storing the coordinates of line segments

Create and set the font for the applet

Set point count to zero

Set drawing flag off

Size the applet

Override the paint() method

Initialize the drawing's starting point

Cycle through each element in the points[] array

Draw a line segment

Save ending point as the starting point for the next line

Override the mouseDown() method

Set the flag in order to allow drawing to begin

Save the mouse click's coordinates

Tell Java that the event was handled

Override the mouseMove() method

if it's okay to add another line segment

Create a new point and save the mouse's coordinates.

Increment the point counter

Force Java to repaint the applet

Tell Java that the event was handled

The Keyboard

The keyboard has been around even longer than the mouse and has been the primary interface betweenhumans and their computers for decades Given the keyboard's importance, obviously there may be timeswhen you'll want to handle the keyboard events at a lower level than you can with something like a

TextField control Java responds to two basic key events, which are represented by the KEY_PRESS

and KEY_RELEASE constants As you'll soon see, Java defines methods that make it just as easy to

respond to the keyboard as it is to respond to the mouse

Responding to Key Presses

Whenever the user presses a key when an applet is active, Java sends the applet a KEY_PRESS event Inyour applet, you can respond to this event by overriding the keyDown() method, whose signature lookslike this:

public boolean keyDown(Event evt, int key)

As you can see, this method receives two arguments, which are an Event object and an integer

representing the key that was pressed This integer is actually the ASCII representation of the characterrepresented by the key In order to use this value in your programs, however, you must first cast it to a

char value, like this:

char c = (char)key;

Predefined Key Constants

Some of the keys on your keyboard issue commands rather than generate characters These keys includeall the F keys, as well as keys like Shift, Ctrl, Page Up, Page Down, and so on In order to make thesetypes of keys easier to handle in your applets, Java's Event class defines a set of constants that representthese key's values Table 25.2 lists these constants

Table 25.2 Key Constants of the Event Class.

Constant Key

DOWN The down arrow key

LEFT The left arrow key.

PGDN The Page Down key

PGUP The Page Up key

RIGHT The right arrow key

Key Modifiers

The Event class also defines a number of constants for modifier keys that the user might press alongwith the basic key These constants include ALT_MASK, SHIFT_MASK, and CTRL_MASK, which

represent the Alt, Shift, and Ctrl (or Control) keys on your keyboard The SHIFT_MASK and

CTRL_MASK constants are used in the Event class's methods shiftDown() and controlDown(),each which of returns a boolean value indicating whether the modifier key is pressed (There currently

is no altDown() method.) You can also examine the Event object's modifiers field to determinewhether a particular modifier key was pressed For example, if you wanted to check for the Alt key, youmight use a line of Java code like this:

boolean altPressed = (evt.modifiers & Event.ALT_MASK) != 0;

By ANDing the mask with the value in the modifiers field, you end up with a non-zero value if theAlt key was pressed and a 0 if it wasn't You convert this result to a boolean value by comparing theresult with 0

Example: Using Key Presses in an Applet

Although capturing key presses is a fairly simple process, there's nothing like an example applet to putthe theoretical stuff to the test Listing 25.3 is an applet called KeyApplet that displays whatever key theuser presses Figure 25.3 shows the applet running under Appletviewer

Figure 25.3 : This applet displays the last character typed.

NOTE

If you run KeyApplet under a browser like Netscape Navigator, click

on the applet with your mouse before you start typing This ensuresthat the applet has the focus and will receive the key presses

Listing 25.3 KeyApplet.java: An Applet That Captures Key Presses.

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

Derive the KeyApplet class from Java's Applet class

Declare the class's data field

Override the init() method

Initialize keyPressed to indicate no valid key received yet

Create and set the font for the applet

Size the applet

Override the paint() method

Create the empty display string

Draw the character on the screen

Override the keyDown() method

Save the key that was pressed

Force Java to redraw the applet

Tell Java that the event was handled

Handling Events Directly

All of the events received by your applet are processed by the handleEvent() method, which the

Applet class inherits from the Component class When this method is not overridden in your applet,the default implementation is responsible for calling the many methods that respond to events Listing25.4 shows how the handleEvent() method is implemented in the Component class By examiningthis listing, you can easily see why you only have to override methods like mouseDown() to respond toevents In the next section, you see how to customize handleEvent() in your own programs

Listing 25.4 LST25_4.TXT: The Default Implementation of handleEvent( ).

public boolean handleEvent(Event evt) {

return mouseMove(evt, evt.x, evt.y);

Example: Overriding handleEvent() in an Applet

Although the default implementation of handleEvent() calls special methods that you can override

in your applet for each event, you might want to group all your event handling in one method to conserve

on overhead, change the way an applet responds to a particular event, or even create your own events Toaccomplish any of these tasks (or any others you might come up with), you can forget the individualevent-handling methods and override handleEvent() instead

In your version of handleEvent(), you must examine the Event object's id field in order to

determine which event is being processed You can just ignore events in which you're not interested.However, be sure to return false whenever you ignore a message, so that Java knows that it shouldpass the event on up the object hierarchy Listing 25.5 is a rewritten version of the MouseApplet2 applet,called MouseApplet3 This version overrides the handleEvent() method in order to respond to

boolean drawing;

public void init()

{

startPoint = new Point(0, 0);

points = new Point[1000];

int oldX = startPoint.x;

int oldY = startPoint.y;

for (int x=0; x<numPoints; ++x)

public boolean handleEvent(Event evt)

Write an applet that enables the user to type a string of characters on the screen Use a String

object to hold the characters, adding each new character to the string as the user types and

displaying the string in the applet's paint() method

4

Modify MouseApplet2 so that the first MOUSE_DOWN event selects a starting point, after whichthe applet remembers all the MOUSE_MOVE coordinates However, the applet shouldn't draw thelines associated with these moves until the user presses his f2 key Pressing f3 should erase thelines from the applet and signal the applet to start the process over again (You can find the

solution to this exercise, called MouseApplet4, in the CHAP25 folder of this book's CD-ROM.)

5

Chapter 24

Dialog Boxes

CONTNETS

Using a Dialog Box

Creating the Dialog Box

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

Using a Dialog Box

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

Create the dialog box object