ch16.htm

Java_by_Example,初级经典例子哦,珍藏版本

</PRE>

</BLOCKQUOTE>

<P>

The <TT>g</TT> in the preceding code line is the <TT>Graphics</TT>

object passed to the <TT>paint()</TT> method. As you can see,

the <TT>drawLine()</TT> method takes four arguments, which are

X,Y coordinate pairs that specify the starting and ending points

of the line.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

There may be times when you need to retrieve information about the system's currently set graphical attributes. Java's <TT>Graphics</TT> class supplies methods like <TT>getColor()</TT>, <TT>getFont()</TT>, and <TT>getFontMetrics()</TT> to enable you to obtain this information.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

Example: Drawing a Rectangle</FONT></A></H2>

<P>

Most of the shape-drawing methods are as easy to use as the <TT>drawLine()</TT>

method is. Suppose that you want to write an applet that draws

a filled rounded rectangle inside a hollow rectangle. You'd then

add calls to the <TT>Graphics</TT> class's <TT>fillRoundRect()</TT>

and <TT>drawRect()</TT> to the applet's <TT>paint()</TT> method.

Listing 16.1 is just such an applet, whereas Listing 16.2 is the

HTML document that displays the applet. Figure 16.4 shows the

applet running under Appletviewer.

<P>

<A HREF="f16-4.gif"><B> Figure 16.4 : </B><I>This is RectApplet running under Appletviewer.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 16.1&nbsp;&nbsp;RECTAPPLET.JAVA: Drawing Rectangles.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class RectApplet extends Applet

{

    public void paint(Graphics g)

    {

        g.drawRect(35, 15, 125, 200);

        g.fillRoundRect(50, 30, 95, 170, 15, 15);

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<HR>

<BLOCKQUOTE>

<B>Listing 16.2&nbsp;&nbsp;RECTAPPLET.htmL: HTML Document for

RectApplet.<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;RectApplet.class&quot;

    width=200

    height=250

    name=&quot;RectApplet&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

In RectApplet's <TT>paint()</TT> method, you can see the method

calls that produce the graphical display. The first line creates

the outside rectangle. That method call looks like this:

<BLOCKQUOTE>

<PRE>

g.drawRect(35, 15, 125, 200);

</PRE>

</BLOCKQUOTE>

<P>

The <TT>drawRect()</TT> method's four arguments are the X,Y coordinates

of the rectangle's upper-left corner and the width and height

of the rectangle. The rounded filled rectangle is almost as easy

to draw:

<BLOCKQUOTE>

<PRE>

g.fillRoundRect(50, 30, 95, 170, 15, 15);

</PRE>

</BLOCKQUOTE>

<P>

The first four arguments of the <TT>fillRoundRect()</TT> method

are the same as those for the <TT>drawRect()</TT> method. The

fifth and sixth arguments are the size of the rectangle that represents

the rounded corners. Think of this rectangle as being placed on

each corner of the main rectangle and a curved line drawn between

its corners, as shown in Figure 16.5.

<P>

<A HREF="f16-5.gif"><B> Figure 16.5 : </B><I>The coordinates for the rounded corners are given as the width and height of the rectangle that encloses the rounded corner.



</I></A><P>

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

Example: Drawing Other Shapes</FONT></A></H2>

<P>

Some shapes you can draw with the <TT>Graphics</TT> class are

more complex than others. For example, the <TT>drawArc()</TT>

method requires six arguments in order to draw a simple curved

line. To see how drawing other shapes works, you'll now create

the ShapeApplet applet, which enables you to switch from one shape

to another in the applet's display. Listing 16.3 is ShapeApplet's

source code. Figures 16.6 and 16.7 show what the applet looks

like running under the Appletviewer application.

<P>

<A HREF="f16-6.gif"><B> Figure 16.6 : </B><I>This is what ShapeApplet looks like when it first runs.



</I></A><P>

<P>

<A HREF="f16-7.gif"><B> Figure 16.7 : </B><I>This is ShapeApplet displaying an oval.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 16.3&nbsp;&nbsp;ShapeApplet.java: An Applet That Draws

Various Shapes.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class ShapeApplet extends Applet

{

    int shape;

    Button button;



    public void init()

    {

        shape = 0;

        button = new Button(&quot;Next Shape&quot;);

        add(button);

    }



    public void paint(Graphics g)

    {

        int x[] = {35, 150, 60, 140, 60, 150, 35};

        int y[] = {50, 80, 110, 140, 170, 200, 230};

        int numPts = 7;



        switch(shape)

        {

            case 0:

                g.drawLine(35, 50, 160, 230);

                break;

            case 1:

                g.drawRect(35, 50, 125, 180);

                break;

            case 2:

                g.drawRoundRect(35, 50, 125, 180, 15, 15);

                break;

            case 3:

                g.drawOval(35, 50, 125, 180);

                break;

            case 4:

                g.drawArc(35, 50, 125, 180, 90, 180);

                break;

            case 5:

                g.drawPolygon(x, y, numPts);

                break;

            case 6:

                g.fillPolygon(x, y, numPts);

                break;

        }

    }



    public boolean action(Event event, Object arg)

    {

        ++shape;

        if (shape == 7)

            shape = 0;

        repaint();

        return true;

    }

}

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

class.<BR>

    Declare the class's shape data field.<BR>

    Declare the class's button data field.<BR>

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

        Initialize the shape counter.<BR>

        Create the applet's <TT>Button</TT> object.<BR>

        Add the <TT>Button</TT> object to the applet.<BR>

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

        Initialize the X and Y coordinates for the polygons.<BR>

        Initialize the polygon point count.<BR>

        Display a shape based on the value of the shape counter.

<BR>

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

        Increment the shape counter.<BR>

        Reset the shape counter if it has reached its maximum

value.<BR>

        Force the applet to repaint its canvas with the next shape.

<BR>

        Tell Java that the method executed okay.

</BLOCKQUOTE>

<P>

To run ShapeApplet, use the HTML document shown in Listing 16.2,

except change all occurrences of RectApplet to ShapeApplet. When

you run the applet with Appletviewer, you see the window shown

in Figure 16.6. To change the shape displayed in the applet's

canvas, click the Next Shape button.

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

Understanding the ShapeApplet Applet</FONT></A></H2>

<P>

You don't need to concern yourself at this point with the button

control that ShapeApplet uses to switch shapes, except to know

that just like the <TT>TextField</TT> controls you've been using,

clicking the button causes Java to call the applet's <TT>action()</TT>

method. The <TT>action()</TT> method increments the shape counter,

<TT>shape</TT>, and tells the applet to redraw itself. In the

<TT>paint()</TT> method, the value of <TT>shape</TT> is used in

a <TT>switch</TT> statement to determine which shape gets drawn.

You learned about <TT>switch</TT> statements back in <A HREF="ch9.htm" >Chapter 9</A>

&quot;The <TT>if</TT> and <TT>switch</TT> Statements.&quot;

<H3><A NAME="DrawingOvals">

Drawing Ovals</A></H3>

<P>

The real meat of this program are the calls to the <TT>Graphics</TT>

object's various shape-drawing methods. You already know about

the first three: <TT>drawLine()</TT>, <TT>drawRect()</TT>, and

<TT>drawRoundRect()</TT>. The call to <TT>drawOval()</TT>, however,

is new and looks like this:

<BLOCKQUOTE>

<PRE>

g.drawOval(35, 50, 125, 180);

</PRE>

</BLOCKQUOTE>

<P>

As you can see, this method, which draws ovals and circles, takes

four arguments. These arguments are the X,Y coordinates, width,

and height of a rectangle that can enclose the oval. Figure 16.8

shows how the resultant oval relates to its enclosing rectangle.

<P>

<A HREF="f16-8.gif"><B> Figure 16.8 : </B><I>An oval's coordinates are actually the coordinates of <r>an enclosing rectangle.



</I></A><P>

<H3><A NAME="DrawingArcs">

Drawing Arcs</A></H3>

<P>

Next in <TT>paint()</TT> is the <TT>drawArc()</TT> method, which

is the most complicated (at least, from an understanding point

of view) of the shape-drawing methods. The call to <TT>drawArc()</TT>

looks like this:

<BLOCKQUOTE>

<PRE>

g.drawArc(35, 50, 125, 180, 90, 180);

</PRE>

</BLOCKQUOTE>

<P>