ch21.htm

Java_by_Example,初级经典例子哦,珍藏版本

to the "North" position, you get a scrollbar the stretches

across the top of the applet. Failure to place the scrollbar properly

in an appropriate layout manager will result in a useless scrollbar

like the one shown in Figure 21.5.

<P>

<A HREF="f21-5.gif"><B> Figure 21.5 : </B><I>The scrollbar in this applet was not placed properly in an appropriate layout manager.

</I></A><P>



<H3><A NAME="Canvases">

Canvases</A></H3>

<P>

Canvases are nothing more than areas on which you can draw. You

can combine canvases with other types of components, such as buttons,

in order to build layouts that that are attractive, as well as

functional. The first step in creating a canvas is to call the

<TT>Canvas</TT> class's constructor, like this:

<BLOCKQUOTE>

<PRE>

Canvas canvas = new Canvas();

</PRE>

</BLOCKQUOTE>

<P>

The <TT>Canvas</TT> constructor requires no arguments.

<P>

Once you have the canvas created, you add it to your layout just

as you would any other component, by calling the <TT>add()</TT>

method:

<BLOCKQUOTE>

<PRE>

add(canvas);

</PRE>

</BLOCKQUOTE>

<H3><A NAME="ExampleUsingaCanvasinanApplet">

Example: Using a Canvas in an Applet</A></H3>

<P>

To end this chapter, take a look at Listing 21.2, which is an

applet that creates a canvas class and uses the class to display

a colored area on the screen. When you run the applet with Appletviewer,

you see the window shown in Figure 21.6. The applet displays two

components: a button at the top of the applet and a canvas below

the button. When you click the button, the canvas changes color.

<P>

<A HREF="f21-6.gif"><B> Figure 21.6 : </B><I>This is CanvasApplet running under Appletviewer.



</I></A><P>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Often, you'll want to derive your own custom canvas class from Java's <TT>Canvas</TT> class. Then, you can more easily control what's drawn in the canvas, by overriding the canvas's <TT>paint()</TT> method. This is the approach that's used in the CanvasApplet applet.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<HR>

<BLOCKQUOTE>

<B>Listing 21.2&nbsp;&nbsp;CanvasApplet.java: An Applet That Displays

a Canvas.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class CanvasApplet extends Applet

{

    CustomCanvas canvas;



    public void init()

    {

        setLayout(new BorderLayout());



        Button button = new Button(&quot;Color&quot;);

        add(&quot;North&quot;, button);



        canvas = new CustomCanvas();

        add(&quot;Center&quot;, canvas);



        resize(200, 250);

    }



    public boolean action(Event evt, Object arg)

    {

        if (arg == &quot;Color&quot;)

            canvas.swapColor();



        return true;

    }

}



class CustomCanvas extends Canvas

{

    Color color;



    public CustomCanvas()

    {

        color = Color.black;

    }



    public void paint(Graphics g)

    {

        Rectangle r = bounds();

        g.setColor(color);

        g.fillRect(0, 0, r.width, r.height);

        g.setColor(Color.white);

        g.drawString(&quot;CANVAS&quot;, 72, 90);

    }



    public void swapColor()

    {

        if (color == Color.black)

            color = Color.red;

        else if (color == Color.red)

            color = Color.green;

        else

            color = Color.black;



        repaint();

    }

}

</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>CanvasApplet</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare the custom canvas object.<BR>

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

        Create and set the layout.<BR>

        Create and add the button.<BR>

        Create and add the canvas.<BR>

        Size the applet.<BR>

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

        if the &quot;Color&quot; button was pressed...<BR>

            Tell the canvas to change color.<BR>

        Tell Java that the event was handled.<BR>

Define the <TT>CustomCanvas</TT> class.<BR>

    Declare the class's single data field.<BR>

    Declare the class's constructor.<BR>

        Set the initial canvas color.<BR>

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

        Get the canvas's size.<BR>

        Set the currently selected color.<BR>

        Fill the canvas with the selected color.<BR>

        Set the color for the text.<BR>

        Display the text string.<BR>

    Define the <TT>swapColor()</TT> method.<BR>

        Set color to the next color.<BR>

        Repaint the canvas with the new color.

</BLOCKQUOTE>

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

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

<P>

Most often, you see scrollbars being used to manipulate such controls

as scrolling lists and text areas. However, scrollbars also enable

you to create a reasonable facsimile of a Windows 95 slider control,

which enables the user to select a value from a given range. Remember

that you need to use the appropriate layout manager in order for

the scrollbar to be drawn properly. Unlike scrollbars, canvases

aren't usually used to obtain input from the user (although they

do generate some types of event messages). Instead, canvases enable

you to create graphical areas in your applets.

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

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

<OL>

<LI>What are the <TT>Scrollbar</TT> constructor's five arguments?

<LI>What is a canvas?

<LI>What arguments are expected by the <TT>Canvas</TT> class's

constructor?

<LI>With a scrollbar, when would you use a page size of zero?

<LI>What's the easiest way to respond to a scrollbar change?

<LI>What are the five event messages that are generated by a scrollbar?

<LI>How can you create a custom canvas component?

<LI>How do you draw in a custom canvas?

</OL>

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

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

<OL>

<LI>Write an applet that displays a vertical scrollbar.

<LI>Write an applet that displays a scrollbar along with three

buttons. Clicking the buttons should set the scrollbar's values

to its minimum, middle, and maximum values. (Hint: You can set

the scrollbar's value with its <TT>setValue()</TT> method.)

<LI>Modify CanvasApplet so that the display includes, besides

the canvas, three buttons labeled &quot;Black,&quot; &quot;Green,&quot;

and &quot;Red.&quot; Clicking a button should change the canvas

to the appropriate color. You'll probably want to use a <TT>GridLayout</TT>

object for this applet's layout manager. (For more information

on using a <TT>GridLayout</TT> object, see <A HREF="ch22.htm" >Chapter 22</A>, &quot;Panels

and the Layout Manager.&quot; Figure 21.7 shows the resultant

applet. (You can find the solution to this exercise in the CHAP21

folder of this book's CD-ROM. It's called CanvasApplet2.)<BR>

<A HREF="f21-7.gif"><B> Figure 21.7 : </B><I>This is CanvasApplet2 running under Appletviewer.



</I></A><P>

</OL>

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