ch20.htm

Java_by_Example,初级经典例子哦,珍藏版本

        list.addItem("Pearl Jam");

        list.addItem("Dream Theater");

        list.addItem("Joe Satriani");

        list.addItem("Oasis");

        list.addItem("Alanis Morissette");

        list.addItem("Soul Asylum");

        list.addItem("The Rembrandts");

        list.addItem("Smashing Pumpkins");

        list.addItem("Joan Osborne");

        list.addItem("Bjork");



        add(list);

        resize(300, 150);

    }



    public void paint(Graphics g)

    {

        g.drawString("CHOSEN ITEM:", 100, 110);

        String s = list.getSelectedItem();



        if (s == null)

            s = "None";



        g.drawString(s, 100, 130);

    }



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

class.<BR>

    Declare the list object.<BR>

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

        Create the list object.<BR>

        Add items to the list object.<BR>

        Add the list to the applet.<BR>

        Set the applet's size.<BR>

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

        Draw a label in the applet's display.<BR>

        Get the selected item from the list box.<BR>

        If there is no item selected, set the string to &quot;None.&quot;

<BR>

        Display the selected item.<BR>

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

        Force the applet to repaint its display.<BR>

        Tell Java that the event was handled okay.

</BLOCKQUOTE>

<P>

When you run ListApplet with Appletviewer, you see the window

shown in Figure 20.7. When you double-click an item in the list,

Java calls the applet's <TT>action()</TT> method in which the

applet calls the <TT>repaint()</TT> method. This forces Java to

call the <TT>paint()</TT> method, where the applet retrieves the

selected item and displays it.

<P>

<A HREF="f20-7.gif"><B> Figure 20.7 : </B><I>The scrolling list in this applet lets you choose a single musical artist.



</I></A><P>

<P>

Notice the call to <TT>resize()</TT> in the <TT>init()</TT> method.

The <TT>resize()</TT> method enables you to set the applet to

any size you wish. This size overrides any size setting that's

included in the HTML document that ran the applet.

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

The <I>TextArea</I> Control</FONT></A></H2>

<P>

Throughout this book, you've been using the <TT>TextField</TT>

control to retrieve information from the user. In most cases,

the <TT>TextField</TT> control works great, but it does have some

limitations, the most serious being the fact that it can display

only one line of text at a time. There may be situations where

you'd like to display one or more paragraphs of text in your applet,

in a control that enables the user to edit existing text, as well

as enter his or her own text. This is where the <TT>TextArea</TT>

control is useful. The <TT>TextArea</TT> control is a text box

that acts like a simple word processor. When you display a text

box, the user can type and edit multiple lines of text.

<P>

To create a <TT>TextArea</TT> control, you call the class's constructor,

like this:

<BLOCKQUOTE>

<PRE>

TextArea textArea = new TextArea(str, rows, cols);

</PRE>

</BLOCKQUOTE>

<P>

This constructor's three arguments are the string to display in

the control, the number of rows in the control, and the number

of columns. As with the other controls, after you create the <TT>TextField</TT>

object, you add it to the applet by using the <TT>add()</TT> method.

<H3><A NAME="ExampleCreatingaITextAreaIControl">

Example: Creating a <I>TextArea</I> Control</A></H3>

<P>

As an example, suppose that you need to create a <TT>TextArea</TT>

control that'll start off displaying eight lines of text. Listing

20.6 is an applet, called TextAreaApplet, that creates a <TT>TextArea</TT>

control that displays eight lines of text. Figure 20.8 shows what

the applet looks like running under Appletviewer. When you run

the applet, click on the <TT>TextArea</TT> control's box and try

editing the text in the window. As you'll discover, you not only

can edit the existing text, but also add new text.

<P>

<A HREF="f20-8.gif"><B> Figure 20.8 : </B><I>TextAreaApplet applet run-<r>ning under Appletviewer.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 20.6&nbsp;&nbsp;TEXTAREAAPPLET.JAVA: The TextAreaApplet

Applet.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class TextAreaApplet extends Applet

{

    TextArea textArea;



    public void init()

    {

        String s = &quot;This is an example of a\n&quot;;

        s += &quot;textarea control, which is not\n&quot;;

        s += &quot;unlike a textfield control.\n&quot;;

        s += &quot;The big difference is that a\n&quot;;

        s += &quot;textarea control can hold many\n&quot;;

        s += &quot;lines of text, whereas a\n&quot;;

        s += &quot;textfield control deals with\n&quot;;

        s += &quot;only one line of text at a time.\n&quot;;



        textArea = new TextArea(s, 9, 30);

        add(textArea);



        resize(300, 180);

    }

}

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

class.<BR>

    Declare the <TT>TextArea</TT> object.<BR>

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

        Create the string to display in the control.<BR>

        Create the <TT>TextArea</TT> object.<BR>

        Add the control to the applet.<BR>

        Set the applet's size.<BR>

</BLOCKQUOTE>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

If you look at how the <TT>TextArea</TT> control's display string is created in TextAreaApplet, you'll see that you can store multiple lines of text into a single <TT>String</TT> object. You do this by placing the newline character (<TT>\n</TT>) at the end of each line that you add to the string.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

When you run TextAreaApplet, notice how all the text fits within

the text box. Because the text is fully displayed, the control's

scroll bars are inactive. However, if you were to edit the text

such that you added more lines than the control can display, or

made a line longer that the control can display, the control's

scroll bars become active. Figure 20.9 shows TextAreaApplet after

the user has added text that forces the scroll bars to become

active. You can use the scroll bars to view the portions of the

text that are offscreen.

<P>

<A HREF="f20-9.gif"><B> Figure 20.9 : </B><I>When the text contained in the control cannot be fully displayed, a TextArea control activates its scroll bars.



</I></A><P>

<H3><A NAME="MethodsoftheITextAreaIClass">

Methods of the <I>TextArea</I> Class</A></H3>

<P>

To enable you to easily manipulate the text, the <TT>TextArea</TT>

class features a number of public methods. You can use these methods

to modify the text in the control or to obtain information about

the control. Table 20.3 shows the most useful methods and what

they do.<BR>

<P>

<CENTER><B>Table 20.3&nbsp;&nbsp;Useful Methods of the </B><I>TextArea</I><B>

Class.</B></CENTER>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD WIDTH=293><I><B>Method</B></I></TD><TD WIDTH=298><I><B>Description</B></I>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=293><TT>void appendText(String str)</TT></TD><TD WIDTH=298>Appends text to the control.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=293><TT>int getColumns()</TT></TD><TD WIDTH=298>Returns the number of columns in the control.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=293><TT>int getRows()</TT></TD><TD WIDTH=298>Returns the number of rows in the `control.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=293><TT>void insertText(String str,</TT></TD><TD WIDTH=298>Inserts text at the given position.<TT>int pos)</TT>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=293><TT>void replaceText(String str,</TT></TD><TD WIDTH=298>Replaces text specified by the <TT>int start, int end)</TT>starting and ending points.

</TD></TR>

</TABLE>

</CENTER>

<P>

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

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

<P>

Choice menus are a powerful control that enable you to include

a pop-up menu of commands for the user of your applet. By using

such a menu, the user can more easily control the applet, as well

as set options, without the controls' taking up a lot of screen

space. Scrolling lists are a valuable tool for ensuring that the

user always enters a response from a valid set of responses. You

can even set up a list to accept multiple selections. Finally,

the <TT>TextArea</TT> control provides a simple text editor that

you can easily add to your applets.

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

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

<OL>

<LI>How many arguments are accepted by the <TT>Choice</TT> class's

constructor?

<LI>How do you add items to a choice menu?

<LI>What are the two arguments needed by the <TT>List</TT> class's

constructor?

<LI>How do you add items to a scrolling list?

<LI>When would you use a <TT>TextArea</TT> control in place of

a <TT>TextField</TT> control?

<LI>How can you determine which menu command the user selected?

<LI>How do you create a multiple-selection scrolling list?

<LI>How do you retrieve the selected item from a single-selection

scrolling list?

<LI>How do you create a single string containing multiple lines

of text?

<LI>How do you retrieve multiple selections from a scrolling list?

<LI>Can you delete items from a scrolling list?

</OL>

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

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

<OL>

<LI>Write an applet that has a menu containing the commands <TT>On</TT>

and <TT>Off</TT>.

<LI>Write an applet that displays a single-selection scrolling

list containing the titles of five movies.

<LI>Write an applet that displays a <TT>TextArea</TT> control.

The control should display five lines of text at startup.

<LI>Write an applet that changes the size of the applet based

on five selections in a choice menu.

<LI>Revise the applet from exercise 3 such that the user uses

a single-selection list to select the applet's size.

<LI>Write an applet called TextTransferApplet that includes a

list box and a <TT>TextArea</TT> control. The list box should

contain 10 words. When the user clicks a word, the word should

appear in the <TT>TextArea</TT> control on a new line. Figure

20.10 shows what the completed applet should look like, and Figure

20.11 shows the applet after the user has transferred several

words to the <TT>TextArea</TT> control. You can find the solution

for this problem in the CHAP20 folder of this book's CD-ROM.

</OL>

<BR>

<A HREF="f20-10.gif"><B> Figure 20.10 : </B><I>TextTransferApplet should look like this.



</I></A><P>



<P>

<A HREF="f20-11.gif"><B> Figure 20.11 : </B><I>Here's the applet after the user has transferred a few words to the text area.



</I></A><P>

<HR>











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