ch4.htm

JAVA Developing Professional JavaApplets

<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dispose();&nbsp;&nbsp;//
Erase frame<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return
true;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;case e.ACTION_EVENT:
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Process menu selection...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(e.target instanceof MenuItem) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Get the name of the menu selection..<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String
menuName = e.arg.toString();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Dispose of frame if quit is chosen...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(menuName.equals(&quot;Quit&quot;))<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dispose();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//
Otherwise, set the cursor...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(menuName.equals(&quot;Default&quot;))<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;setCursor(Frame.DEFAULT_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(menuName.equals(&quot;Wait&quot;))<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;setCursor(Frame.WAIT_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(menuName.equals(&quot;Hand&quot;))<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;setCursor(Frame.HAND_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(menuName.equals(&quot;Move&quot;))<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;setCursor(Frame.MOVE_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(menuName.equals(&quot;Text&quot;))<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;setCursor(Frame.TEXT_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if
(menuName.equals(&quot;SE Resize&quot;))<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;setCursor(Frame.SE_RESIZE_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return
true;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
// end if<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return
true;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;default:
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return
false;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;}<BR>
}</TT>
</BLOCKQUOTE>
<HR>
<P>
A MenuItem can be removed from a Menu with the <TT>remove()</TT>
method, much as it is in the MenuBar class. A separator, dividing
menu items, can be added with the <TT>addSeparator()</TT>
method of the Menu class. This is useful for menus that have different
categories of options.
<P>
The current example also illustrates how to process menu selections.
When a menu item is selected, an <TT>ACTION_EVENT</TT>
is issued. The <TT>instanceof</TT>
operator can be used in the handler to verify that the target
of the action is a menu. By taking the Event argument (the <TT>arg</TT>
variable) and converting it to a string, the program can get the
name of the menu item. Through the name, the appropriate course
of action can then be taken. In this example, the Quit menu item
forces the frame to shut down by using the <TT>dispose()</TT>
method. The other menu choices result in a new state for the cursor.
<P>
It should be mentioned that the CheckboxMenuItem class can be
implemented for menus that need to use checkmarks to indicate
whether the item has been selected. This is handy for menu items
that toggle.
<H3><A NAME="Dialogs">Dialogs</A></H3>
<P>
Like the Frame class, the Dialog class is a subclass of Window.
Dialogs differ from Frames in a couple of subtle ways, however.
The most important of these differences is that Dialogs can be
<I>modal</I>. When a modal Dialog is displayed, input to other
windows in the Applet is blocked until the Dialog is disposed.
This feature points to the general purpose of Dialogs, which is
to give the user a warning or a decision to be made before the
program can continue. Although non-modal, or <I>modeless,</I>
Dialogs are supported, most Dialogs are modal.
<P>
There are two constructors for the Dialog class. Both take a Frame
object as a parameter; they also take a boolean flag, which indicates
whether the dialog should be modal. If the flag is set to true,
the dialog is modal. The constructors differ only in a parameter
that specifies whether the dialog should have a title caption.
It is this constructor that is used in the example that follows.
<P>
Figure 4.2 shows a variation of the previous applet, except that
a Dialog is used to change the cursor state. Listing 4.3 shows
the code for the Dialog class, called ChangeCursorDialog. The
Frame class (still called FrameMenuCursor from the previous example)
declares the dialog and instantiates it as follows:
<P>
<A HREF="f4-2.gif" ><B>Figure 4.2 </B>: <I>Using Dialog to change the state of the cursor</I></A>
<BLOCKQUOTE>
<TT>ChangeCursorDialog dlg;<BR>
dlg = new ChangeCursorDialog(this,true,&quot;Change the cursor&quot;);</TT>
</BLOCKQUOTE>
<P>
The menu for this application is different from the previous example.
The frame constructs the menu as follows:
<BLOCKQUOTE>
<TT>// First create the menu bar<BR>
MenuBar mbar = new MenuBar();<BR>
setMenuBar(mbar); // Attach to the frame...<BR>
<BR>
// Add the File submenu...<BR>
Menu m = new Menu(&quot;File&quot;);<BR>
mbar.add(m);&nbsp;&nbsp;// Add to menu bar<BR>
// Add Dialog to the submenu...<BR>
m.add(new MenuItem(&quot;Cursor Dialog&quot;));<BR>
// Add a separator<BR>
m.addSeparator();<BR>
// Add Quit to the submenu...<BR>
m.add(new MenuItem(&quot;Quit&quot;));<BR>
</TT>Note that a separator is added to divide the two menu
items.<BR>
When the Cursor Dialog menu item is chosen, the dialog box is
presented with the following code:<BR>
<TT>if (menuName.equals(&quot;Cursor Dialog&quot;))
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;dlg.show(); // Make the dialog visible...
<BR>
</TT>Listing 4.3. Code for a Dialog to change the cursor
state.<BR>
<TT>import java.awt.*;<BR>
import java.lang.*;<BR>
import java.applet.*;<BR>
<BR>
// Dialog that presents a grid of buttons<BR>
// for choosing the Frame cursor. A Cancel<BR>
// button exits the dialog...<BR>
class ChangeCursorDialog extends Dialog {<BR>
FrameMenuCursor fr;<BR>
// Create the dialog and store the title string...<BR>
public ChangeCursorDialog(Frame parent,boolean modal,String title)
{<BR>
 // Create dialog with title<BR>
 super(parent,title,modal);<BR>
 fr = (FrameMenuCursor)parent;<BR>
 // The layout is Grid layout...<BR>
 setLayout(new GridLayout(3,2));<BR>
 // Add the button options<BR>
 add(new Button(&quot;Default&quot;));<BR>
 add(new Button(&quot;Wait&quot;));<BR>
 add(new Button(&quot;Hand&quot;));<BR>
 add(new Button(&quot;Move&quot;));<BR>
 add(new Button(&quot;Text&quot;));<BR>
 add(new Button(&quot;Cancel&quot;));<BR>
 // Pack and size for display...<BR>
 pack();<BR>
 resize(300,200);<BR>
}<BR>
// Look for button selections to<BR>
// change the cursor...<BR>
public boolean action(Event e,Object arg) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp; // If button was selected then exit dialog..
<BR>
if (e.target instanceof Button) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// And possibly
change the cursor...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (arg.equals(&quot;Default&quot;))
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fr.setCursor(Frame.DEFAULT_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (arg.equals(&quot;Wait&quot;))
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fr.setCursor(Frame.WAIT_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (arg.equals(&quot;Hand&quot;))
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fr.setCursor(Frame.HAND_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (arg.equals(&quot;Move&quot;))
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fr.setCursor(Frame.MOVE_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (arg.equals(&quot;Text&quot;))
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fr.setCursor(Frame.TEXT_CURSOR);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dispose();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;return false;<BR>
}<BR>
}</TT>
</BLOCKQUOTE>
<P>
Another AWT class called FileDialog is a subclass of Dialog. It
is used for creating stock Load and Save dialog boxes. The upcoming
tutorial has examples of the FileDialog class.
<H3><A NAME="Colors">Colors</A></H3>
<P>
The Color class is used to set an object's colors. This class
represents a color as a combination of RGB values. RGB is a 24-bit
representation of a color composed of red, green, and blue byte
values. This 24-bit representation is virtual since many platforms
do not support 24-bit color. In these situations, Java maps the
color to the appropriate value; this will usually be an index
into a palette.
<P>
The Color class defines a set of stock color values, implemented
as static variables. For example, Color.red is available with
an RGB value of 255,0,0. Other colors supported are white, black,
gray, green, blue, yellow, magenta, and cyan. Various Color constructors
can also be used to define other colors. For example, the statement
<BLOCKQUOTE>
<TT>Color col = new Color(0,0,120);</TT>
</BLOCKQUOTE>
<P>
produces a dark blue color. Other methods, such as <TT>darker()</TT>
and <TT>brighter()</TT>, can also
be used to create a new Color object from an existing one.
<P>
There are generally two approaches to tying a color to an object.
The first approach is to use methods provided by the Component
class. The <TT>setBackground()</TT>
and <TT>setForeground()</TT> methods
are used to set the colors of an object in the two respective
positions. For example, the following code creates a Panel object
and sets its background to white:
<BLOCKQUOTE>
<TT>Panel p = new Panel();<BR>
p.setBackground(Color.white);</TT>
</BLOCKQUOTE>
<P>
You can use the respective methods prefixed by &quot;get&quot;
to return the background and foreground colors of a component.
<P>
The other approach to setting a color occurs in the <TT>paint()</TT>
method. Recall that a Graphics object is passed to this method.
The <TT>setColor()</TT> method can
then be used to set the color of the next items to be painted.
For example, the following code paints a blue framed rectangle:
<BLOCKQUOTE>
<TT>public synchronized void paint (Graphics
g)<BR>
g.setColor(Color.blue);<BR>
g.drawRect(0,0,100,40);</TT>
</BLOCKQUOTE>
<P>
The tutorial in this section gives more examples of using color.
A dialog box presents a choice of colors through the use of colored
components. The spreadsheet is modified to use these chosen colors
when it is painted.
<H3><A NAME="Fonts">Fonts</A></H3>
<P>
Fonts are a critical aspect of graphics-based programming. Not
only are they important for making an application attractive,
they are a seminal part of programming a visual interface. Often
how a component is sized or placed turns on what font is being
used. The spreadsheet tutorial in this chapter, for example, uses
the current dimensions of the font to determine how the spreadsheet
cell should be sized and located. Therefore, there are two aspects
of font programming: tying fonts to display components and getting
font information to program how graphical objects are positioned.
The former aspect generally falls in the domain of the Font class;
the latter is tied to the FontMetrics class, although Font is
involved here also.
<P>
To construct a Font object, you need a font name, style, and size.
The font name represents a family of fonts, such as Courier. The
font families available to an applet depend on where the program
is running, and so decisions about what font is to be used should
be made dynamically. The java.awt.Toolkit has a method called
<TT>getFontList()</TT> that returns
a string array of font names on the host system. An example of
how this method is used is found in the next section and in the
font dialog in the tutorial. The family of a Font can be retrieved
through the <TT>getFamily()</TT> method.
<P>
The size of a font is its point size, such as 8 for a small font
and 72 for a large one. There are three styles defined as constants
in the Font class: <TT>PLAIN</TT>,
<TT>BOLD</TT>, or <TT>ITALIC</TT>.
The latter two can be combined to produce a font with both features.
For example, you could create a Helvetica font that has a 36-point
size and is both bold and italicized as follows: