ch19.htm

Java_by_Example,初级经典例子哦,珍藏版本

add(checkbox1);

add(checkbox2);

add(checkbox3);

</PRE>

</BLOCKQUOTE>

<P>

Now, when the applet appears, the user sees a list something like

that shown in Figure 19.3. In the figure, the first option is

selected. If the user decides to click a different option, the

first option becomes unselected and the new one selected. Notice

that exclusive checkboxes are round rather than square.

<P>

<A HREF="f19-3.gif"><B> Figure 19.3 : </B><I>Only one exclusive checkbox can<r>be selected simultaneously.



</I></A><P>

<H3><A NAME="ICheckboxIMethods">

<I>Checkbox</I> Methods</A></H3>

<P>

Just like other controls supported by Java, the <TT>Checkbox</TT>

class features a number of methods that you can call in order

to manipulate the control or obtain information about it. Table

19.1 lists the public methods for the <TT>Checkbox</TT> class.

<BR>

<P>

<CENTER><B>Table 19.1&nbsp;&nbsp;Public Methods of the </B><I>Checkbox</I><B>

Class.</B></CENTER>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD WIDTH=289><I><B>Method</B></I></TD><TD WIDTH=301><I><B>Description</B></I>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=289><TT>CheckboxGroup getCheckboxGroup()</TT></TD>

<TD WIDTH=301>Returns the checkbox's group object.</TD></TR>

<TR VALIGN=TOP><TD WIDTH=289><TT>String getLabel()</TT></TD><TD WIDTH=301>Returns the checkbox's label.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=289><TT>boolean getState()</TT></TD><TD WIDTH=301>Returns the checkbox's state.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=289><TT>void setCheckboxGroup(CheckboxGroup g)</TT>

</TD><TD WIDTH=301>Sets the checkbox's group object.</TD></TR>

<TR VALIGN=TOP><TD WIDTH=289><TT>void setLabel(String label)</TT></TD><TD WIDTH=301>Sets the checkbox's label.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=289><TT>void setState(boolean state)</TT></TD><TD WIDTH=301>Sets the checkbox's state.

</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

The <TT>get</TT> methods listed in Table 19.1 requires no arguments

and return objects of the appropriate type. The <TT>setCheckboxGroup()</TT>

requires a reference to a <TT>CheckboxGroup</TT> object as its

single argument, whereas <TT>setLabel()</TT> and <TT>setState()</TT>

require a text string and a <TT>boolean</TT> value, respectively,

as their single argument.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Checkboxes that are set to exclusive mode are also known as <I>radio buttons</I> because, like the station-selection buttons on a radio, only one can be selected at a time.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="ExampleHandlingCheckboxesinanApplet">

Example: Handling Checkboxes in an Applet</A></H3>

<P>

Depending on what your applet needs to do, checkboxes can be handled

in a couple of ways. The easiest way to handle checkboxes is to

use their methods to determine the information you need in an

applet. Listing 19.3, for example, is an applet that tracks the

state of a set of checkboxes, displaying their current states

every time there are changes. Listing 19.4 is the applet's HTML

document, and Figure 19.4 shows the applet running under Appletviewer.

<P>

<A HREF="f19-4.gif"><B> Figure 19.4 : </B><I>CheckboxApplet running under Appletviewer.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 19.3&nbsp;&nbsp;CheckboxApplet.java: Handling Checkboxes

in an Applet.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class CheckboxApplet extends Applet

{

    Checkbox checkbox1;

    Checkbox checkbox2;

    Checkbox checkbox3;



    public void init()

    {

        checkbox1 = new Checkbox(&quot;Option 1&quot;, null, true);

        checkbox2 = new Checkbox(&quot;Option 2&quot;, null, false);

        checkbox3 = new Checkbox(&quot;Option 3&quot;, null, false);



        add(checkbox1);

        add(checkbox2);

        add(checkbox3);

    }



    public void paint(Graphics g)

    {

        Font font = g.getFont();

        FontMetrics fontMetrics = g.getFontMetrics(font);

        int height = fontMetrics.getHeight();



        boolean checked = checkbox1.getState();

        if (checked)

            g.drawString(&quot;Option1 selected&quot;, 20, 120);

        else

            g.drawString(&quot;Option1 not selected&quot;, 20, 120);



        checked = checkbox2.getState();

        if (checked)

            g.drawString(&quot;Option2 selected&quot;, 20, 120 + height);

        else

            g.drawString(&quot;Option2 not selected&quot;, 20, 120 + height);



        checked = checkbox3.getState();

        if (checked)

            g.drawString(&quot;Option3 selected&quot;, 20, 120 + 2 * height);

        else

            g.drawString(&quot;Option3 not selected&quot;, 20, 120 + 2 * height);

    }



    public boolean action(Event evt, Object arg)

    {

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

class.<BR>

    Declare three checkbox objects.<BR>

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

        Create the three checkboxes.<BR>

        Add the checkboxes to the applet's display.<BR>

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

        Get the height of the active font.<BR>

        Get the first checkbox's state and display the state.

<BR>

        Get the second checkbox's state and display the state.

<BR>

        Get the third checkbox's state and display the state.

<BR>

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

        Force Java to redraw the applet's display.<BR>

        Tell Java that the event was handled okay.

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 19.4&nbsp;&nbsp;CHECKBOXAPPLET.htmL: The HTML Document

That Runs CheckboxApplet.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

&lt;title&gt;Applet Test Page&lt;/title&gt;

&lt;h1&gt;Applet Test Page&lt;/h1&gt;

&lt;applet

    code=&quot;CheckboxApplet.class&quot;

    width=135

    height=220

    name=&quot;CheckboxApplet&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<H3><A NAME="RespondingtoaCheckboxEvent">

Responding to a Checkbox Event</A></H3>

<P>

Most of Java's user-interface controls generate events when they're

clicked. The checkbox controls are no different. However, unlike

button controls, which send both a reference to the control and

the control's label as parameters to the <TT>action()</TT> method,

checkboxes send only a reference to the control, with the second

<TT>action()</TT> parameter always being <TT>true</TT>. This anomaly

makes it a little more difficult to handle checkbox controls when

you need to respond directly to the event generated by the control.

<P>

To respond to a checkbox event, you must use the <TT>Event</TT>

object's <TT>target</TT> field to call the checkbox's methods

in order to determine which checkbox caused the event. If you

don't remember, the <TT>Event</TT> object is passed as the <TT>action()</TT>

method's first argument.

<P>

First, you obtain a reference to the checkbox, like this:

<BLOCKQUOTE>

<PRE>

Checkbox checkbox = (Checkbox)evt.target;

</PRE>

</BLOCKQUOTE>

<P>

Then, with a reference to the checkbox in hand, you can call whatever

<TT>Checkbox</TT> class members you need in order to determine

which checkbox caused the event and to deal with the event as

appropriately. Probably the best way to determine which checkbox

you're dealing with is to get the object's label, like this:

<BLOCKQUOTE>

<PRE>

String label = checkbox.getLabel();

</PRE>

</BLOCKQUOTE>

<P>

You can then compare the returned string to the labels for each

checkbox object.

<H3><A NAME="ExampleHandlingCheckboxEventsinanApplet">

Example: Handling Checkbox Events in an Applet</A></H3>

<P>

To demonstrate how to use the previously presented technique in

an actual programming situation, you'll now examine the CheckboxApplet2.

The source code for the applet is shown in Listing 19.5. To create

an HTML document for the applet, start with Listing 19.4 and then

change all occurrences of <TT>CheckboxApplet</TT> with <TT>CheckboxApplet2</TT>.

<HR>

<BLOCKQUOTE>

<B>Listing 19.5&nbsp;&nbsp;CheckboxApplet2.java: Responding to

Checkbox Events.</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class CheckboxApplet2 extends Applet

{

    Checkbox checkbox1;

    Checkbox checkbox2;

    Checkbox checkbox3;



    public void init()

    {

        checkbox1 = new Checkbox(&quot;Option 1&quot;, null, true);

        checkbox2 = new Checkbox(&quot;Option 2&quot;, null, false);

        checkbox3 = new Checkbox(&quot;Option 3&quot;, null, false);



     	add(checkbox1);

        add(checkbox2);

        add(checkbox3);

    }



    public boolean action(Event evt, Object arg)

    {

        if (evt.target instanceof Checkbox)

            ChangeLabel(evt);

        repaint();



        return true;

    }



    protected void ChangeLabel(Event evt)

    {

        Checkbox checkbox = (Checkbox)evt.target;

        String label = checkbox.getLabel();



        if (label == &quot;Option 1&quot;)

            checkbox.setLabel(&quot;Changed 1&quot;);

        else if (label == &quot;Option 2&quot;)

            checkbox.setLabel(&quot;Changed 2&quot;);

        else if (label == &quot;Option 3&quot;)

            checkbox.setLabel(&quot;Changed 3&quot;);

        else

        {

            checkbox1.setLabel(&quot;Option 1&quot;);

            checkbox2.setLabel(&quot;Option 2&quot;);

            checkbox3.setLabel(&quot;Option 3&quot;);

        }

    }

}

</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>CheckboxApplet2</TT> class from Java's <TT>Applet</TT>