chapter13.html

java 是一个很好的网络开发环境。由于它是通过解释的方法

<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">For this example, the simplest
action is to print what button is pressed. Some systems allow you to pop up a
little window with a message in it, but applets discourage this. However, you
can put a message at the bottom of the
<A NAME="Index1595"></A><A NAME="Index1596"></A><A NAME="Index1597"></A><A NAME="Index1598"></A>Web
browser window on its <I>status line</I> by calling the <B>Applet</B> method
<A NAME="Index1599"></A><A NAME="Index1600"></A><B>getAppletContext(&#160;)</B>
to get access to the browser and then
<A NAME="Index1601"></A><A NAME="Index1602"></A><B>showStatus(&#160;)</B> to put
a string on the status
line.</FONT><A NAME="fnB53" HREF="#fn53">[53]</A><FONT FACE="Georgia"> You
can print out a complete description of an event the same way, with
<B>getAppletContext().showStatus(evt + "" ). </B>(The empty <B>String</B> forces
the compiler to convert <B>evt</B> to a <B>String.</B>) Both of these reports
are really useful only for testing and debugging since the browser might
overwrite your message.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Strange as it might seem, you can
also <A NAME="Index1603"></A><A NAME="Index1604"></A>match an event to the
<I>text</I> that&#8217;s on a button through the second argument in
<B>event(&#160;)</B>. Using this technique, the example above
becomes:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: Button3.java</font>
<font color=#009900>// Matching events on button text</font>
<font color=#0000ff>import</font> java.awt.*;
<font color=#0000ff>import</font> java.applet.*;

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Button3 <font color=#0000ff>extends</font> Applet {
  Button 
    b1 = <font color=#0000ff>new</font> Button(<font color=#004488>"Button 1"</font>), 
    b2 = <font color=#0000ff>new</font> Button(<font color=#004488>"Button 2"</font>);
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> init() {
    add(b1);
    add(b2);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>boolean</font> action (Event evt, Object arg) {
    <font color=#0000ff>if</font>(arg.equals(<font color=#004488>"Button 1"</font>))
      getAppletContext().showStatus(<font color=#004488>"Button 1"</font>);
    <font color=#0000ff>else</font> <font color=#0000ff>if</font>(arg.equals(<font color=#004488>"Button 2"</font>))
      getAppletContext().showStatus(<font color=#004488>"Button 2"</font>);
    <font color=#009900>// Let the base class handle it:</font>
    <font color=#0000ff>else</font> 
      <font color=#0000ff>return</font> <font color=#0000ff>super</font>.action(evt, arg);
    <font color=#0000ff>return</font> <font color=#0000ff>true</font>; <font color=#009900>// We've handled it here</font>
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">It&#8217;s difficult to know
exactly what the <B>equals(&#160;)</B> method is doing here. The biggest problem
with this approach is that most new Java programmers who start with this
technique spend at least one frustrating session discovering that they&#8217;ve
gotten the capitalization or spelling wrong when comparing to the text on a
button. (I had this experience.) Also, if you change the text of the button, the
code will no longer work (but you won&#8217;t get any compile-time or run-time
error messages). You should avoid this approach if
possible.</FONT><A NAME="_Toc375545449"></A><A NAME="_Toc408018685"></A><BR></P></DIV>
<A NAME="Heading400"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
Text fields</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">A
<A NAME="Index1605"></A><A NAME="Index1606"></A><B>TextField</B> is a one line
area that allows the user to enter and edit text. <B>TextField </B>is inherited
from <A NAME="Index1607"></A><A NAME="Index1608"></A><B>TextComponent</B>,<B>
</B>which lets you select text, get the selected text as a <B>String</B>, get or
set the text, and set whether the <B>TextField</B> is editable, along with other
associated methods that you can find in your online reference. The following
example demonstrates some of the functionality of a <B>TextField</B>; you can
see that the method names are fairly obvious:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: TextField1.java</font>
<font color=#009900>// Using the text field control</font>
<font color=#0000ff>import</font> java.awt.*;
<font color=#0000ff>import</font> java.applet.*;

<font color=#0000ff>public</font> <font color=#0000ff>class</font> TextField1 <font color=#0000ff>extends</font> Applet {
  Button
    b1 = <font color=#0000ff>new</font> Button(<font color=#004488>"Get Text"</font>),
    b2 = <font color=#0000ff>new</font> Button(<font color=#004488>"Set Text"</font>);
  TextField 
    t = <font color=#0000ff>new</font> TextField(<font color=#004488>"Starting text"</font>, 30);
  String s = <font color=#0000ff>new</font> String();
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> init() {
    add(b1);
    add(b2);
    add(t);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>boolean</font> action (Event evt, Object arg) {
    <font color=#0000ff>if</font>(evt.target.equals(b1)) {
      getAppletContext().showStatus(t.getText());
      s = t.getSelectedText();
      <font color=#0000ff>if</font>(s.length() == 0) s = t.getText();
      t.setEditable(<font color=#0000ff>true</font>);
    }
    <font color=#0000ff>else</font> <font color=#0000ff>if</font>(evt.target.equals(b2)) {
      t.setText(<font color=#004488>"Inserted by Button 2: "</font> + s);
      t.setEditable(<font color=#0000ff>false</font>);
    }
    <font color=#009900>// Let the base class handle it:</font>
    <font color=#0000ff>else</font> 
      <font color=#0000ff>return</font> <font color=#0000ff>super</font>.action(evt, arg);
    <font color=#0000ff>return</font> <font color=#0000ff>true</font>; <font color=#009900>// We've handled it here</font>
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">There are several ways to construct
a <B>TextField</B>; the one shown here provides an initial string and sets the
size of the field in characters.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Pressing button 1 either gets the
text you&#8217;ve selected with the mouse or it gets all the text in the field
and places the result in <B>String s</B>. It also allows the field to be edited.
Pressing button 2 puts a message and <B>s</B> into the text field and prevents
the field from being edited (although you can still select the text). The
editability of the text is controlled by passing
<A NAME="Index1609"></A><A NAME="Index1610"></A><B>setEditable(&#160;)</B> a
<B>true</B> or
<B>false</B>.</FONT><A NAME="_Toc375545450"></A><A NAME="_Toc408018686"></A><BR></P></DIV>
<A NAME="Heading401"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
Text areas</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">A
<A NAME="Index1611"></A><A NAME="Index1612"></A><B>TextArea</B> is like a
<B>TextField</B> except that it can have multiple lines and has significantly
more functionality. In addition to what you can do with a <B>TextField</B>, you
can append text and insert or replace text at a given location. It seems like
this functionality could be useful for <B>TextField</B> as well, so it&#8217;s a
little confusing to try to detect how the distinction is made. You might think
that if you want <B>TextArea</B> functionality everywhere you can simply use a
one line <B>TextArea</B> in places where you would otherwise use a
<B>TextField</B>. In Java 1.0<A NAME="Index1613"></A>, you also got scroll bars
with a <B>TextArea </B>even when they weren&#8217;t appropriate; that is, you
got both vertical and horizontal scroll bars for a one line <B>TextArea</B>. In
Java 1.1<A NAME="Index1614"></A> this was remedied with an extra constructor
that allows you to select which scroll bars (if any) are present. The following
example shows only the Java 1.0<A NAME="Index1615"></A> behavior, in which the
scrollbars are always on. Later in the chapter you&#8217;ll see an example that
demonstrates Java 1.1 <B>TextArea</B>s.</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: TextArea1.java</font>
<font color=#009900>// Using the text area control</font>
<font color=#0000ff>import</font> java.awt.*;
<font color=#0000ff>import</font> java.applet.*;

<font color=#0000ff>public</font> <font color=#0000ff>class</font> TextArea1 <font color=#0000ff>extends</font> Applet {
  Button b1 = <font color=#0000ff>new</font> Button(<font color=#004488>"Text Area 1"</font>);
  Button b2 = <font color=#0000ff>new</font> Button(<font color=#004488>"Text Area 2"</font>);
  Button b3 = <font color=#0000ff>new</font> Button(<font color=#004488>"Replace Text"</font>);
  Button b4 = <font color=#0000ff>new</font> Button(<font color=#004488>"Insert Text"</font>);
  TextArea t1 = <font color=#0000ff>new</font> TextArea(<font color=#004488>"t1"</font>, 1, 30);
  TextArea t2 = <font color=#0000ff>new</font> TextArea(<font color=#004488>"t2"</font>, 4, 30);
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> init() {
    add(b1);
    add(t1);
    add(b2);
    add(t2);
    add(b3);
    add(b4);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>boolean</font> action (Event evt, Object arg) {
    <font color=#0000ff>if</font>(evt.target.equals(b1))
      getAppletContext().showStatus(t1.getText());
    <font color=#0000ff>else</font> <font color=#0000ff>if</font>(evt.target.equals(b2)) {
      t2.setText(<font color=#004488>"Inserted by Button 2"</font>);
      t2.appendText(<font color=#004488>": "</font> + t1.getText());
      getAppletContext().showStatus(t2.getText());
    }
    <font color=#0000ff>else</font> <font color=#0000ff>if</font>(evt.target.equals(b3)) {
      String s = <font color=#004488>" Replacement "</font>;
      t2.replaceText(s, 3, 3 + s.length());
    }
    <font color=#0000ff>else</font> <font color=#0000ff>if</font>(evt.target.equals(b4))
      t2.insertText(<font color=#004488>" Inserted "</font>, 10);
    <font color=#009900>// Let the base class handle it:</font>
    <font color=#0000ff>else</font> 
      <font color=#0000ff>return</font> <font color=#0000ff>super</font>.action(evt, arg);
    <font color=#0000ff>return</font> <font color=#0000ff>true</font>; <font color=#009900>// We've handled it here</font>
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">There are several different
<B>TextArea</B> constructors, but the one shown here gives a starting string and
the number of rows and columns. The different buttons show getting, appending,
replacing, and inserting
text.</FONT><A NAME="_Toc375545451"></A><A NAME="_Toc408018687"></A><BR></P></DIV>
<A NAME="Heading402"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
Labels</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">A
<A NAME="Index1616"></A><A NAME="Index1617"></A><B>Label</B> does exactly what
it sounds like it should: places a label on the form. This is particularly
important for text fields and text areas that don&#8217;t have labels of their
own, and can also be useful if you simply want to place textual information on a
form. You can, as shown in the first example in this chapter, use
<B>drawString(&#160;)</B> inside <B>paint(&#160;)</B> to place text in an exact
location. When you use a <B>Label</B> it allows you to (approximately) associate
the text with some other component via the layout manager (which will be
discussed later in this chapter).</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">With the constructor you can create
a blank label, a label with initial text in it (which is what you&#8217;ll
typically do), and a label with an alignment of <B>CENTER</B>, <B>LEFT</B>, or
<B>RIGHT</B> (<B>static final int</B>s defined in class <B>Label</B>). You can
also change the label and its alignment with
<A NAME="Index1618"></A><A NAME="Index1619"></A><B>setText(&#160;)</B> and
<A NAME="Index1620"></A><A NAME="Index1621"></A><B>setAlignment(&#160;)</B>, and
if you&#8217;ve forgotten what you&#8217;ve set these to you can read the values
with <A NAME="Index1622"></A><A NAME="Index1623"></A><B>getText(&#160;)</B> and
<A NAME="Index1624"></A><A NAME="Index1625"></A><B>getAlignment(&#160;)</B>.
This example shows what you can do with labels:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: Label1.java</font>
<font color=#009900>// Using labels</font>
<font color=#0000ff>import</font> java.awt.*;
<font color=#0000ff>import</font> java.applet.*;

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Label1 <font color=#0000ff>extends</font> Applet {