_chapter 7.htm

Core Java 2(中文名称：JAVA 2 核心技术 卷二：高级特性）这是英文版的。

  <li>
  <p class="docList"><tt>void draw(Shape s)</tt></p>
  <p class="docList">draws the outline of the given shape with the current 
  stroke.</li>
  <li>
  <p class="docList"><tt>void fill(Shape s)</tt></p>
  <p class="docList">fills the interior of the given shape with the current 
  paint.</li>
</ul>
<h3 class="docSection1Title" id="c7s2">Shapes</h3>
<p class="docText">Here are some of the methods in the <tt>Graphics</tt> class 
to draw shapes:</p>
<pre>drawLine
drawRectangle
drawRoundRect
draw3DRect
drawPolygon
drawPolyline
drawOval
drawArc
</pre>
<p class="docText">There are also corresponding <tt>fill</tt> methods. These 
methods have been in the <tt>Graphics</tt> class ever since JDK 1.0. The Java 2D 
API uses a completely different, object-oriented approach. Instead of methods, 
there are classes:</p>
<pre>Line2D
Rectangle2D
RoundRectangle2D
Ellipse2D
Arc2D
QuadCurve2D
CubicCurve2D
GeneralPath
</pre>
<p class="docText">These classes all implement the <tt>Shape</tt> interface.</p>
<p class="docText">Finally, there is a <tt>Point2D</tt> class that describes a 
point with an <span class="docEmphasis">x-</span> and a
<span class="docEmphasis">y-</span>coordinate. Points are useful to define 
shapes, but they aren't themselves shapes.</p>
<p class="docText">To draw a shape, you first create an object of a class that 
implements the <tt>Shape</tt> interface and then call the <tt>draw</tt> method 
of the <tt>Graphics2D</tt> class.</p>
<p class="docText">The <tt>Line2D</tt>, <tt>Rectangle2D</tt>, <tt>
RoundRectangle2D</tt>, <tt>Ellipse2D</tt>, <tt>and Arc2D</tt> classes correspond 
to the <tt>drawLine</tt>, <tt>drawRectangle</tt>, <tt>drawRoundRect</tt>, <tt>
drawOval</tt>, and <tt>drawArc</tt> methods. (The concept of a &quot;3D rectangle&quot; 
has died the death that it so richly deserved梩here is no analog to the <tt>
draw3DRect</tt> method.) The Java 2D API supplies two additional classes: 
quadratic and cubic curves. We discuss these shapes later in this section. There 
is no <tt>Polygon2D</tt> class. Instead, the <tt>GeneralPath</tt> class 
describes paths that are made up from lines, quadratic and cubic curves. You can 
use a <tt>GeneralPath</tt> to describe a polygon; we show you how later in this 
section.</p>
<p class="docText">The classes</p>
<pre>Rectangle2D
RoundRectangle2D
Ellipse2D
Arc2D
</pre>
<p class="docText">all inherit from a common superclass <tt>RectangularShape</tt>. 
Admittedly, ellipses and arcs are not rectangular, but they have a
<span class="docEmphasis">bounding rectangle</span> (see
<a class="docLink" href="#ch07fig02">Figure 7-2</a>).</p>
<center>
<h5 id="ch07fig02" class="docFigureTitle">Figure 7-2. The bounding rectangle of an ellipse and 
an arc</h5>
<p>
<img alt="graphics/07fig02.gif" src="07fig02.gif" border="0" width="500" height="137"></p>
</center>
<p class="docText">Each of the classes whose name ends in &quot;2D&quot; has two 
subclasses for specifying coordinates as <tt>float</tt> or <tt>double</tt> 
quantities. In Volume 1, you already encountered</p>
<pre>Rectangle2D.Float
Rectangle2D.Double
</pre>
<p class="docText">The same scheme is used for the other classes, such as</p>
<pre>Arc2D.Float
Arc2D.Double
</pre>
<p class="docText">Internally, all graphics classes use <tt>float</tt> 
coordinates since <tt>float</tt> numbers use less storage space, and they have 
sufficient precision for geometric computations. However, the Java programming 
language makes it a bit more tedious to manipulate <tt>float</tt> numbers. For 
that reason, most methods of the graphics classes use <tt>double</tt> parameters 
and return values. Only when constructing a 2D object, you need to choose 
between a constructor with <tt>float</tt> or <tt>double</tt> coordinates. For 
example,</p>
<pre>Rectangle2D floatRect
   = new Rectangle2D.Float(5F, 10F, 7.5F, 15F);
Rectangle2D doubleRect
   = new Rectangle2D.Double(5, 10, 7.5, 15);
</pre>
<p class="docText">The <span class="docEmphasis"><tt>Xxx</tt></span><tt>2D.Float</tt> 
and <span class="docEmphasis"><tt>Xxx</tt></span><tt>2D.Double</tt> classes are 
subclasses of the <span class="docEmphasis"><tt>Xxx</tt></span><tt>2D</tt> 
classes. After object construction, there is essentially no benefit in 
remembering the subclass, and you can just store the constructed object in a 
superclass variable, just like in the code example.</p>
<p class="docText">As you can see from the curious names, the
<span class="docEmphasis"><tt>Xxx</tt></span><tt>2D.Float</tt> and
<span class="docEmphasis"><tt>Xxx</tt></span><tt>2D.Double</tt> classes are also 
inner classes of the <span class="docEmphasis"><tt>Xxx</tt></span><tt>2D</tt> 
classes. That is just a minor syntactical convenience, to avoid an inflation of 
outer class names.</p>
<p class="docText"><a class="docLink" href="#ch07fig03">Figure 7-3</a> shows the 
relationships between the shape classes. However, the <tt>Double</tt> and <tt>
Float</tt> subclasses are omitted. Legacy classes from the pre-2D library are 
marked with a gray fill.</p>
<center>
<h5 id="ch07fig03" class="docFigureTitle">Figure 7-3. Relationships between the shape classes</h5>
<p>
<img alt="graphics/07fig03.gif" src="07fig03.gif" border="0" width="500" height="366"></p>
</center>
<h4 class="docSection2Title" id="ch07lev2sec2">Using the Shape Classes</h4>
<p class="docText">You already saw how to use the <tt>Rectangle2D</tt>, <tt>
Ellipse2D</tt>, and <tt>Line2D</tt> classes in Chapter 7 of Volume 1. In this 
section, you will learn how to work with the remaining 2D shapes.</p>
<p class="docText">For the <tt>RoundRectangle2D</tt> shape, you specify the top 
left corner, width and height, and the x- and y-dimension of the corner area 
that should be rounded (see <a class="docLink" href="#ch07fig04">Figure 7-4</a>). 
For example, the call</p>
<center>
<h5 id="ch07fig04" class="docFigureTitle">Figure 7-4. Constructing a <tt>RoundRectangle2D</tt></h5>
<p>
<img alt="graphics/07fig04.gif" src="07fig04.gif" border="0" width="500" height="312"></p>
</center>
<pre>RoundRectangle2D r = new RoundRectangle2D.Double(150, 200,
   100, 50, 20, 20);
</pre>
<p class="docText">produces a rounded rectangle with circles of radius 20 at 
each of the corners.</p>
<p class="docText">To construct an arc, you specify the bounding box, followed 
by the start angle and the angle swept out by the arc (see
<a class="docLink" href="#ch07fig05">Figure 7-5</a>) and the closure type, one 
of <tt>Arc2D.OPEN</tt>, <tt>Arc2D.PIE</tt>, or <tt>Arc2D.CHORD</tt>.</p>
<center>
<h5 id="ch07fig05" class="docFigureTitle">Figure 7-5. Constructing an elliptical arc</h5>
<p>
<img alt="graphics/07fig05.gif" src="07fig05.gif" border="0" width="500" height="226"></p>
</center>
<pre>Arc2D a = new Arc2D(x, y, width, height,
   startAngle, arcAngle, closureType);
</pre>
<p class="docText"><a class="docLink" href="#ch07fig06">Figure 7-6</a> 
illustrates the arc types.</p>
<center>
<h5 id="ch07fig06" class="docFigureTitle">Figure 7-6. Arc types</h5>
<p>
<img alt="graphics/07fig06.gif" src="07fig06.gif" border="0" width="400" height="557"></p>
</center>
<p class="docText">However, the angles are not simply given in degrees, but they 
are distorted such that a 45-degree angle denotes the diagonal position,
<span class="docEmphasis">even if width and height are not the same.</span> If 
you draw circular arcs (for example in a pie chart), then you don't need to 
worry about this. However, for elliptical arcs, be prepared for an adventure in 
trigonometry梥ee the sidebar for details.</p>
<p class="docText">The Java 2D package supplies <span class="docEmphasis">
quadratic</span> and <span class="docEmphasis">cubic</span> curves. In this 
chapter, we do not want to get into the mathematics of these curves. We suggest 
you get a feel for how the curves look by running the program in
<a class="docLink" href="#ch07list01">Example 7-1</a>. As you can see in
<a class="docLink" href="#ch07fig07">Figures 7-7</a> and
<a class="docLink" href="#ch07fig08">7-8</a>, quadratic and cubic curves are 
specified by two <span class="docEmphasis">end points</span> and one or two
<span class="docEmphasis">control points.</span> Moving the control points 
changes the shape of the curves.</p>
<center>
<h5 id="ch07fig07" class="docFigureTitle">Figure 7-7. A quadratic curve</h5>
<p>
<img alt="graphics/07fig07.gif" src="07fig07.gif" border="0" width="290" height="245"></p>
</center><center>
<h5 id="ch07fig08" class="docFigureTitle">Figure 7-8. A cubic curve</h5>
<p>
<img alt="graphics/07fig08.gif" src="07fig08.gif" border="0" width="291" height="247"></p>
</center>
<table cellSpacing="0" width="90%" border="1">
  <tr>
    <td>
    <h2 class="docSidebarTitle">Specifying Angles for Elliptical Arcs</h2>
    <p class="docText">The algorithm for drawing elliptical arcs uses distorted 
    angles, which the caller must precompute. This sidebar tells you how. If you 
    belong to the large majority of programmers who never draw elliptical arcs, 
    just skip the sidebar. However, since the official documentation completely 
    glosses over this topic, we thought it is worth recording it to save those 
    who need this information a few hours of trigonometric agony.</p>
    <p class="docText">You convert actual angles to distorted angles with the 
    following formula.</p>
    <pre>distortedAngle = Math.atan2(Math.sin(angle) * width,
   Math.cos(angle) * height);
</pre>
    <p class="docText">Sometimes (such as in the example program at the end of 
    this section), you know an end point of the arc, or another point on the 
    line joining the center of the ellipse and that end point. In that case, 
    first compute</p>
    <pre>dx = p.getX() - center.getX();
dy = p.getY() - center.getY();
</pre>
    <p class="docText">Then, the distorted angle is</p>
    <pre>distortedAngle = Math.atan2(-dy * width, dx * height);
</pre>
    <p class="docText">(The minus sign in front of <tt>dy</tt> is necessary 
    because in the pixel coordinate system, the y-axis points downwards, which 
    leads to angle measurements that are clockwise, but you need to supply an 
    angle that is measured counterclockwise.)</p>
    <p class="docText">Convert the result from radians to degrees:</p>
    <pre>distortedAngle = Math.toDegrees(distortedAngle);
</pre>
    <p class="docText">The result is a value between