ch25.htm

Java_by_Example,初级经典例子哦,珍藏版本

whatever key the user presses. Figure 25.3 shows the applet running

under Appletviewer.

<P>

<A HREF="f25-3.gif"><B> Figure 25.3 : </B><I>This applet displays the last character typed.



</I></A><P>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD><B>NOTE</B></TD></TR>

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

If you run KeyApplet under a browser like Netscape Navigator, click on the applet with your mouse before you start typing. This ensures that the applet has the focus and will receive the key presses.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<HR>

<BLOCKQUOTE>

<B>Listing 25.3&nbsp;&nbsp;KeyApplet.java: An Applet That Captures

Key Presses.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class KeyApplet extends Applet

{

    int keyPressed;



    public void init()

    {

        keyPressed = -1;



        Font font =

            new Font(&quot;TimesRoman&quot;, Font.BOLD, 144);

        setFont(font);



        resize(200, 200);

    }



    public void paint(Graphics g)

    {

        String str = &quot;&quot;;



        if (keyPressed != -1)

        {

            str += (char)keyPressed;

            g.drawString(str, 40, 150);

        }

    }



    public boolean keyDown(Event evt, int key)

    {

        keyPressed = key;

        repaint();

        return true;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<IMG ALIGN=RIGHT SRC="pseudo.gif" HEIGHT=94 WIDTH=94 BORDER=1>

<BLOCKQUOTE>

Tell Java that the applet uses the classes in the <TT>awt</TT>

package.<BR>

Tell Java that the applet uses the classes in the <TT>applet</TT>

package.<BR>

Derive the <TT>KeyApplet</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare the class's data field.<BR>

    Override the <TT>init()</TT> method.<BR>

        Initialize <TT>keyPressed</TT> to indicate no valid key

received yet.<BR>

        Create and set the font for the applet.<BR>

        Size the applet.<BR>

    Override the <TT>paint()</TT> method.<BR>

        Create the empty display string.<BR>

        Draw the character on the screen.<BR>

    Override the <TT>keyDown()</TT> method.<BR>

        Save the key that was pressed.<BR>

        Force Java to redraw the applet.<BR>

        Tell Java that the event was handled.

</BLOCKQUOTE>

<H2><A NAME="HandlingEventsDirectly"><FONT SIZE=5 COLOR=#Ff0000>

Handling Events Directly</FONT></A></H2>

<P>

All of the events received by your applet are processed by the

<TT>handleEvent()</TT> method, which the <TT>Applet</TT> class

inherits from the <TT>Component</TT> class. When this method is

not overridden in your applet, the default implementation is responsible

for calling the many methods that respond to events. Listing 25.4

shows how the <TT>handleEvent()</TT> method is implemented in

the <TT>Component</TT> class. By examining this listing, you can

easily see why you only have to override methods like <TT>mouseDown()</TT>

to respond to events. In the next section, you see how to customize

<TT>handleEvent()</TT> in your own programs.

<HR>

<BLOCKQUOTE>

<B>Listing 25.4&nbsp;&nbsp;LST25_4.TXT: The Default Implementation

of </B><I>handleEvent(&nbsp;)</I><B>.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

public boolean handleEvent(Event evt) {

switch (evt.id) {

  case Event.MOUSE_ENTER:

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



  case Event.MOUSE_EXIT:

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



  case Event.MOUSE_MOVE:

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



  case Event.MOUSE_DOWN:

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



  case Event.MOUSE_DRAG:

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



  case Event.MOUSE_UP:

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



  case Event.KEY_PRESS:

  case Event.KEY_ACTION:

    return keyDown(evt, evt.key);



  case Event.KEY_RELEASE:

  case Event.KEY_ACTION_RELEASE:

    return keyUp(evt, evt.key);

    

  case Event.ACTION_EVENT:

    return action(evt, evt.arg);

  case Event.GOT_FOCUS:

    return gotFocus(evt, evt.arg);

  case Event.LOST_FOCUS:

    return lostFocus(evt, evt.arg);

}

return false;

}

</PRE>

</BLOCKQUOTE>

<HR>

<H3><A NAME="ExampleOverridingIhandleEventIinanApplet">

Example: Overriding <I>handleEvent()</I> in an Applet</A></H3>

<P>

Although the default implementation of <TT>handleEvent()</TT>

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. To accomplish

any of these tasks (or any others you might come up with), you

can forget the individual event-handling methods and override

<TT>handleEvent()</TT> instead.

<P>

In your version of <TT>handleEvent()</TT>, you must examine the

<TT>Event</TT> object's <TT>id</TT> 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 <TT>false</TT>

whenever you ignore a message, so that Java knows that it should

pass the event on up the object hierarchy. Listing 25.5 is a rewritten

version of the MouseApplet2 applet, called MouseApplet3. This

version overrides the <TT>handleEvent()</TT> method in order to

respond to events.

<HR>

<BLOCKQUOTE>

<B>Listing 25.5&nbsp;&nbsp;MouseApplet3.java: Using the </B><I>handleEvent(&nbsp;)</I><B>

Method.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class MouseApplet3 extends Applet

{

    Point startPoint;

    Point points[];

    int numPoints;

    boolean drawing;



    public void init()

    {

        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&lt;numPoints; ++x)

        {

            g.drawLine(oldX, oldY, points[x].x, points[x].y);

            oldX = points[x].x;

            oldY = points[x].y;

        }

    }



    public boolean handleEvent(Event evt)

    {

        switch(evt.id)

        {

            case Event.MOUSE_DOWN:

                drawing = true;

                startPoint.x = evt.x;

                startPoint.y = evt.y;

                return true;

            case Event.MOUSE_MOVE:

                if ((drawing) &amp;&amp; (numPoints &lt; 1000))

                {

                    points[numPoints] = new Point(evt.x, evt.y);

                    ++numPoints;

                    repaint();

                }

                return true;

            default:

                return false;

        }

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<H2><A NAME="Summary"><FONT SIZE=5 COLOR=#Ff0000>

Summary</FONT></A></H2>

<P>

Because the keyboard and the mouse are two of the most important

devices for accepting input from the user, it's important that

you know how to handle these devices in your applets. Maybe most

of your applets will work fine by leaving such details up to Java

or maybe you'll want to have more control over the devices than

the default behavior allows. You can capture most messages received

by a Java applet by overloading the appropriate event handlers,

such as <TT>mouseDown()</TT> and <TT>keyDown()</TT>. However,

if you want to step back even further in your event-handling code,

you can override the <TT>handleEvent()</TT> method, which receives

all events sent to an applet.

<H2><A NAME="ReviewQuestions"><FONT SIZE=5 COLOR=#Ff0000>

Review Questions</FONT></A></H2>

<OL>

<LI>What is the mouse event that's most commonly captured and

responded to in Java applets?

<LI>What is the mouse message an applet receives the most of?

<LI>What are the six different mouse event messages that your

applet may receive?

<LI>What are the two keyboard events your applet is likely to

receive?

<LI>How can you determine what type of object generated an event?

<LI>How can you determine the type of event message that's being

received?

<LI>When your applet receives a mouse-related message, how can

you determine the coordinates of the mouse at the time of the

event?

<LI>How can you tell whether the user single- or double-clicked

his mouse?

<LI>What two methods are associated with the <TT>KEY_PRESS</TT>

and <TT>KEY_RELEASE</TT> event messages?

<LI>What arguments are received by the <TT>keyDown()</TT> method?

<LI>What arguments are received by the <TT>mouseDown()</TT> method?

<LI>Why might you need to use the <TT>SHIFT_MASK</TT> and <TT>CTRL_MASK</TT>

constants?

<LI>If you want to handle all events in a single method, what

method should you override in your applet?

</OL>

<H2><A NAME="ReviewExercises"><FONT SIZE=5 COLOR=#Ff0000>

Review Exercises</FONT></A></H2>

<OL>

<LI>Write an applet that displays a rectangle wherever the user

clicks.

<LI>Write an applet that displays a rectangle, the color which

changes whenever the user presses a key on his keyboard.

<LI>Write an applet that displays the current coordinates of the

mouse as the mouse moves around the applet's window.

<LI>Write an applet that enables the user to type a string of

characters on the screen. Use a <TT>String</TT> object to hold

the characters, adding each new character to the string as the

user types and displaying the string in the applet's <TT>paint()</TT>

method.

<LI>Modify MouseApplet2 so that the first <TT>MOUSE_DOWN</TT>

event selects a starting point, after which the applet remembers

all the <TT>MOUSE_MOVE</TT> coordinates. However, the applet shouldn't

draw the lines associated with these moves until the user presses

his f2 key. Pressing f3 should erase the lines 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.)

</OL>

<HR>











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