practical.html

一些JAVA的小程序

<center><p><IMG SRC="../../figures/java/concepts-messageParts.gif" WIDTH="420" HEIGHT="64" ALIGN="BOTTOM" ALT="g2d.setColor(Color.white), with the receiving object (g2d), method name (setColor), and parameters (Color.white) called out."></p><p class="FigureCaption">The components of a message sent by the <code>ClickMe</code> object.</p></center>
</blockquote>

<h3>Inheritance in the Example</h3>
<blockquote>
To paint itself onscreen, an object must be a component. 
This means that the object must be an instance of a class that 
derives from the <code>Component</code> class provided by the Java platform. 
<p>
The <code>ClickMe</code> object is an instance
of the <code>ClickMe</code> class, which is declared this way:
<blockquote><pre>
public class ClickMe<font color="#6b7402"><B> extends JComponent</B></font>
                     implements MouseListener {
   ...
}
</pre></blockquote>

The <code>extends JComponent</code> clause makes <code>ClickMe</code> 
a subclass of <code>JComponent</code> &#151; a class that derives from 
the <code>Component</code> class. <code>ClickMe</code> inherits 
a lot of functionality from <code>Component</code> and 
<code>JComponent</code>, including the ability to paint 
itself, to have a standard border around its edges, and to 
register listeners to be notified of mouse events. Along with 
these benefits, the <code>ClickMe</code> class has certain 
obligations: Its painting code must be in a standard place,
such as the <code>paintComponent</code> method; it must
specify its preferred and minimum sizes; and so on.
<blockquote><pre>
public ClickMe() {
    ...
    setPreferredSize(<I>aDimensionObject</I>);
    setMinimumSize(<I>anotherDimensionObject</I>);
    ...
}

public void paintComponent(Graphics g) {
    ... //<I>ClickMe's painting code is here.</I>
}
</pre></blockquote>
</blockquote>

<h3>Interfaces in the Example</h3>
<blockquote>
The <code>ClickMe</code> component responds to mouse clicks by
displaying a spot at the click location.
Any object can detect mouse clicks on a component. It just needs 
to implement the <code>MouseListener</code> interface and register with the 
component as a mouse listener.
<P>
The <code>MouseListener</code> interface declares five methods, each of 
which is called for a different kind of mouse event: when 
the mouse is clicked, when the mouse moves outside of the 
component, and so on. Even though the <code>ClickMe</code> component 
responds only to mouse clicks, its mouse listener implementation 
must have all five methods. The methods for the events that 
the object isn't interested in are empty.
<P>
The complete code for the <code>ClickMe</code> component follows. 
The code that participates in mouse event handling 
is in 
<font color="#6b7402" ><b>colored boldface</b></font>.

<blockquote><pre>
import javax.swing.BorderFactory;
import javax.swing.JComponent;
import java.awt.*;
import java.awt.event.*;

public class ClickMe extends JComponent
                    <font color="#6b7402"><B> implements MouseListener</B></font> {
    private Spot spot = null;
    private static final int RADIUS = 7;
    private Color spotColor = new Color(107, 116, 2); //olive

    /** Creates and initializes the ClickMe component. */
    public ClickMe() {
       <font color="#6b7402"><B> addMouseListener(this);</B></font>
        
        //Hint at good sizes for this component.
        setPreferredSize(new Dimension(RADIUS*30,
                                       RADIUS*15));
        setMinimumSize(new Dimension(RADIUS*4, RADIUS*4));
        
        //Request a black line around this component.
        setBorder(
            BorderFactory.createLineBorder(Color.BLACK));
    }
    
    /**
     * Paints the ClickMe component.  This method is
     * invoked by the Swing component-painting system.
     */
    public void paintComponent(Graphics g) {
        /**
         * Copy the graphics context so we can change it.
         * Cast it to Graphics2D so we can use antialiasing.
         */
        Graphics2D g2d = (Graphics2D)g.create();
        
        //Turn on antialiasing so that painting is smooth.
        g2d.setRenderingHint(
                RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        
        //Paint the background.
        g2d.setColor(Color.WHITE);
        g2d.fillRect(0, 0, getWidth() - 1, getHeight() - 1);

        //Paint the spot.
        if (spot != null) {
            int radius = spot.getSize();
            g2d.setColor(spotColor);
            g2d.fillOval(spot.x - radius, spot.y - radius,
                         radius * 2, radius * 2);
        }
    }

    <font color="#6b7402"><B>//Methods required by the MouseListener interface.
    public void mousePressed(MouseEvent event) {
        if (spot == null) {
            spot = new Spot();
            spot.setSize(RADIUS);
        }
        spot.x = event.getX();
        spot.y = event.getY();
        repaint();
    }
    public void mouseClicked(MouseEvent event) {}
    public void mouseReleased(MouseEvent event) {}
    public void mouseEntered(MouseEvent event) {}
    public void mouseExited(MouseEvent event) {}</B></font>
}
</pre></blockquote>
</blockquote>

<h3>API Documentation</h3>
<blockquote>

To learn more about how <code>ClickMe</code> works, you 
need to learn about its superclasses &#151; <code>JComponent</code> 
and <code>Component</code>. How do you find that information? 
The API documentation, which constitutes the specification for 
the classes that make up the Java platform,
contains detailed descriptions of every class.
<P>
The API documentation for the Java 2 Platform is online at 
<code>java.sun.com</code>. 



It's helpful to have the API documentation for all releases 
you use bookmarked in your browser. 
You can find the 5.0 API documentation here:

<blockquote><pre>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/"><font color="#8800bb">http://java.sun.com/j2se/5.0/docs/api/</font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a></pre></blockquote>

<P>
To learn more about all the classes and interfaces from the
Java platform used by the <code>ClickMe</code> class,
you can look at the API documentation for the following classes.
<UL>
<LI>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/javax/swing/JComponent.html"><font color="#8800bb"><code>javax.swing.JComponent</code></font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/javax/swing/JComponent.html"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a><LI>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/Graphics.html"><font color="#8800bb"><code>java.awt.Graphics</code></font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/Graphics.html"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a><LI>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/Graphics2D.html"><font color="#8800bb"><code>java.awt.Graphics2D</code></font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/Graphics2D.html"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a><LI>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/Dimension.html"><font color="#8800bb"><code>java.awt.Dimension</code></font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/Dimension.html"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a>
<LI>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/Color.html"><font color="#8800bb"><code>java.awt.Color</code></font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/Color.html"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a><LI>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/javax/swing/BorderFactory.html"><font color="#8800bb"><code>javax.swing.BorderFactory</code></font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/javax/swing/BorderFactory.html"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a><LI>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/event/MouseListener.html"><font color="#8800bb"><code>java.awt.event.MouseListener</code></font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/event/MouseListener.html"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a>
<LI>
<a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/event/MouseEvent.html"><font color="#8800bb"><code>java.awt.event.MouseEvent</code></font></a><a target="_blank" href="http://java.sun.com/j2se/5.0/docs/api/java/awt/event/MouseEvent.html"><img src="../../images/apiIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the API reference documentation)"></a></UL>


A complete discussion of using
these classes
them
to create GUIs is in
<a target="_top" href="../../uiswing/index.html">Creating a GUI with JFC/Swing</a><a target="_top" href="../../uiswing/index.html"><img src="../../images/tutorialIcon.gif" width=11 height=11 border=0 align="MIDDLE" alt=" (in the Learning the Java Language trail)"></a>.


</blockquote>
<h3>Summary</h3>
<blockquote>
The discussion in this chapter glossed over many details and left some things 
unexplained, but now you should have some understanding of what 
object-oriented concepts look like in code. 
Specifically, you should have 
a general understanding of the following:
<ul>
<li>
That a class is a blueprint for objects
<li>
That objects are created from classes
<li>
How to create an object from a class
<li>
What constructors are
<li>
How to initialize objects
<li>
What the code for a class looks like
<LI>
What class variables and methods are
<LI>
What instance variables and methods are
<LI>
How to find out what a class's superclass is
<li>
That an interface is a protocol of behavior
<LI>
What it means to implement an interface
</ul>
</blockquote>

</blockquote>

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