ch20.htm

Java_by_Example,初级经典例子哦,珍藏版本

        Font font = new Font("TimesRoman", Font.BOLD, 24);

        int height = font.getSize();

        g.setFont(font);



        g.setColor(color);



        g.drawString("This text is drawn in", 32, 75);

        g.drawString("the color selected from", 32, 75+height);

        g.drawString("the above choice menu.", 32, 75+2*height);

    }



    public boolean action(Event evt, Object arg)

    {

        if (evt.target instanceof Choice)

            HandleMenu(arg);



        return true;

    }



    protected void HandleMenu(Object item)

    {

        if (item == "Black")

            color = Color.black;

        else if (item == "Red")

            color = Color.red;

        else if (item == "Green")

            color = Color.green;

        else

            color = Color.blue;



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

class.<BR>

    Declare menu and color objects.<BR>

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

        Create the menu object.<BR>

        Add commands to the menu object.<BR>

        Add the menu to the applet.<BR>

        Initialize the starting color for the text.<BR>

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

        Create and set the display font.<BR>

        Set the selected color.<BR>

        Display text in the selected color.<BR>

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

        If the menu caused the event, call the <TT>HandleMenu()</TT>

method.<BR>

        Tell Java that the event was handled okay.<BR>

    Declare the <TT>HandleMenu()</TT> method.<BR>

        Set the color field based on the menu selection.<BR>

        Tell Java to repaint the applet's display area.

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 20.3&nbsp;&nbsp;CHOICEAPPLET.htmL: ChoiceApplet's HTML

Document.<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;ChoiceApplet.class&quot;

    width=300

    height=150

    name=&quot;ChoiceApplet&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

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

Scrolling Lists</FONT></A></H2>

<P>

Whereas choice menus usually display a set of commands or options,

scrolling lists are best used to display a list of items from

which the user can choose. For example, you might use a scrolling

list to enable the user to choose a state when filling out an

address form. The scrolling list in this case would contain all

50 states. The user would only need to double-click on his or

her state in order to complete that section of the form. Scrolling

lists not only make it easy for users to enter information, but

also ensure that the user makes a choice from a valid set of responses.

<P>

To create a scrolling list, you first call the <TT>List</TT> class's

constructor, like this:

<BLOCKQUOTE>

<PRE>

List list = new List(num, false);

</PRE>

</BLOCKQUOTE>

<P>

The constructor's two arguments are the number of visible lines

in the list and a boolean value indicating whether the list will

support multiple selections. The list created in the preceding

will not allow the user to select more than one item simultaneously.

<P>

When you have the list object created, you can add items to the

list. You do this by calling the <TT>List</TT> object's <TT>addItem()</TT>

method:

<BLOCKQUOTE>

<PRE>

list.addItem(str);

</PRE>

</BLOCKQUOTE>

<P>

Here, <TT>str</TT> is the text string for the item to add to the

list.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

You can add as many items as you like to a list. You are not limited to the number of items given as the <TT>List</TT> constructor's first argument. That value specifies only the size of the list box; that is, the value determines how many items are visible on the screen at one time.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="ExampleCreatingaSingleSelectionList">

Example: Creating a Single-Selection List</A></H3>

<P>

Suppose that you need to create a list that contains musical artists

from which the user must select only one. First, you create the

list object, after which you add the names of the artists you

want to include. Finally, you add the list to the applet. Listing

20.4 shows the Java code that performs these tasks. Figure 20.4

shows the resultant list box.

<P>

<A HREF="f20-4.gif"><B> Figure 20.4 : </B><I>In this list, only one item can be selected at a time.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 20.4&nbsp;&nbsp;LST20_4.TXT: Creating a Single-Selection

List Box.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

List list = new List(10, false);

list.addItem(&quot;Pearl Jam&quot;);

list.addItem(&quot;Dream Theater&quot;);

list.addItem(&quot;Joe Satriani&quot;);

list.addItem(&quot;Oasis&quot;);

list.addItem(&quot;Alanis Morissette&quot;);

list.addItem(&quot;Soul Asylum&quot;);

list.addItem(&quot;The Rembrandts&quot;);

list.addItem(&quot;Smashing Pumpkins&quot;);

list.addItem(&quot;Joan Osborne&quot;);

list.addItem(&quot;Bjork&quot;);

add(list);

</PRE>

</BLOCKQUOTE>

<HR>

<P>

Because the <TT>List</TT> constructor's first argument is <TT>10</TT>

and there are only 10 items in the list, all of the items are

visible on the screen. Moreover, because the <TT>List</TT> constructor's

second parameter is <TT>false</TT>, in the list created by Listing

20.4, the user can select only a single artist at a time.

<H3><A NAME="ExampleCreatingaMultipleSelectionList">

Example: Creating a Multiple-Selection List</A></H3>

<P>

When you display a list of musical artists such as that created

by Listing 20.4, you may want the user to select more than one.

In this case, you can create a multiple-selection list, just by

changing the <TT>List</TT> constructor's second argument to <TT>true</TT>,

like this:

<BLOCKQUOTE>

<PRE>

List list = new List(10, true);

</PRE>

</BLOCKQUOTE>

<P>

As Figure 20.5 shows, the new list enables the user to select

as many artists as he or she likes.

<P>

<A HREF="f20-5.gif"><B> Figure 20.5 : </B><I>Now the user can select more than one artist at a time.



</I></A><P>

<H3><A NAME="ExampleCreatingaScrollingList">

Example: Creating a Scrolling List</A></H3>

<P>

You may have noticed that in this chapter's title, the list controls

are called &quot;scrolling lists.&quot; So far, though, none of

your lists have scrolled. In fact, they haven't even had scroll

bars. Whenever the size of the list box (given as the constructor's

first argument) is greater than or equal to the number of items

in the list, the list doesn't need to scroll, so no scroll bar

appears. However, as soon as the number of items exceeds the size

of the list box, Java enables scrolling.

<P>

As an example, suppose that you were to change the first line

of Listing 20.4 to this:

<BLOCKQUOTE>

<PRE>

List list = new List(5, false);

</PRE>

</BLOCKQUOTE>

<P>

Now, you have a list that can display five items at a time. However,

there are 10 items in your list, which means that, in order for

the user to be able to see all then items, the list must scroll.

Figure 20.6 shows the resultant scrolling list.

<P>

<A HREF="f20-6.gif"><B> Figure 20.6 : </B><I>The user must scroll this list to see all the items.



</I></A><P>

<H3><A NAME="MethodsoftheIListIClass">

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

<P>

Because there's so much you can do with a scrolling list control,

the <TT>List</TT> class has a large set of public methods that

you can call to manipulate a list. The most useful of these methods

are listed in Table 20.2. Although there are a lot of methods

to learn in the table, the most important are <TT>addItem()</TT>,

<TT>getSelectedIndex()</TT>, <TT>getSelectedIndexes()</TT>, <TT>getSelectedItem()</TT>,

and <TT>getSelectedItems()</TT>. Using these five methods, you

can create a basic scrolling list and enable the user to make

selections from it. In the next section, in fact, you'll see how

to determine which item the user selected.<BR>

<P>

<CENTER><B>Table 20.2&nbsp;&nbsp;Most Useful Methods of the </B><I>List</I><B>

Class.</B></CENTER>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD WIDTH=295><I><B>Method</B></I></TD><TD WIDTH=295><I><B>Description</B></I>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void addItem(String item)</TT></TD><TD WIDTH=295>Adds an item to the end of the list.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void addItem(String item, int index) </TT>

</TD><TD WIDTH=295>Adds an item to a specific position in the list.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>boolean allowsMultipleSelections()</TT>

</TD><TD WIDTH=295>Returns a <TT>boolean</TT> value indicating whether the list supports multiple selection.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void clear()</TT></TD><TD WIDTH=295>Clears all items from the list.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>int countItems()</TT></TD><TD WIDTH=295>Returns the number of items in the list.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void delItem(int position)</TT></TD><TD WIDTH=295>Deletes the item at the given position from the list.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void delItems(int start, int end)</TT></TD>

<TD WIDTH=295>Deletes a group of items from the list.</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void deselect(int index)</TT></TD><TD WIDTH=295>Deselects the item at the given index.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>String getItem(int index)</TT></TD><TD WIDTH=295>Returns the item at the given index.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>int getRows()</TT></TD><TD WIDTH=295>Returns the size of the list box.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>int getSelectedIndex()</TT></TD><TD WIDTH=295>Gets the index of the selected item.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>int[] getSelectedIndexes()</TT> </TD><TD WIDTH=295>Gets the indexes of a multiple selection.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>String getSelectedItem()</TT></TD><TD WIDTH=295>Returns the selected item in a list.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>String[] getSelectedItems()</TT></TD><TD WIDTH=295>Returns all the items in a multiple selection.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>int getVisibleIndex()</TT></TD><TD WIDTH=295>Returns the index of the last item that was made visible.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>boolean isSelected(int index)</TT></TD>

<TD WIDTH=295>Returns a <TT>boolean</TT> value indicating whether the item at the given index is selected.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void makeVisible(int index)</TT></TD><TD WIDTH=295>Ensures that the item at the given index is visible.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void replaceItem(String</TT></TD><TD WIDTH=295>Replaces the item at the given <TT>newValue, int index)</TT>index with a new item.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void select(int index)</TT></TD><TD WIDTH=295>Selects the item at the given index.

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=295><TT>void setMultipleSelections(boolean v)</TT>

</TD><TD WIDTH=295>Toggles the multiple-selection mode.</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="ExampleUsingaScrollingListinanApplet">

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

<P>

By now, you've gotten used to working with the list of musical

artists that I've used in the previous few examples. In this example,

you put that list to the test by not only creating and displaying

the list in an applet, but also by displaying the user's selection.

Listing 20.5 is the source code for the ListApplet applet.

<HR>

<BLOCKQUOTE>

<B>Listing 20.5&nbsp;&nbsp;ListApplet.java: An Applet with a Scrolling

List.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class ListApplet extends Applet

{

    List list;



    public void init()

    {

        list = new List(5, false);