ch9.htm

Java_by_Example,初级经典例子哦,珍藏版本

statement. This time, the program compares the value of <TT>choice</TT>

with the number 2. Because <TT>choice</TT> doesn't equal 2, the

program ignores the following part of the statement and drops

down to the next <TT>if</TT> statement. The variable <TT>choice</TT>

doesn't equal 3 either, so the code portion of the third <TT>if</TT>

statement is also ignored.

<P>

Suppose <TT>choice</TT> equals 2 when Java executes the code in

Listing 9.2. When the program gets to the first <TT>if</TT> statement,

it discovers that <TT>choice</TT> is not equal to 1, so it ignores

the <TT>num = 1</TT> statement and drops down to the next program

line, which is the second <TT>if</TT> statement. Again, the program

checks the value of <TT>choice</TT>. Because <TT>choice</TT> equals

2, the program can execute the second portion of the statement;

that is, <TT>num</TT> gets set to 2. Program execution drops down

to the third <TT>if</TT> statement, which does nothing because

<TT>choice</TT> doesn't equal 3.

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

The <TT>if</TT> statement, no matter how complex it becomes, always evaluates to either <TT>true</TT> or <TT>false</TT>. If the statement evaluates to <TT>true</TT>, the second portion of the statement is executed. If the statement evaluates to <TT>false</TT>, the second portion of the statement is not executed.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="MultipleLineIifIStatements">

Multiple-Line <I>if</I> Statements</A></H3>

<P>

Listings 9.1 and 9.2 demonstrate the simplest <TT>if</TT> statement.

This simple statement usually fits your program's decision-making

needs just fine. Sometimes, however, you want to perform more

than one command as part of an <TT>if</TT> statement. To perform

more than one command, enclose the commands within curly braces.

Listing 9.3 is a revised version of Listing 9.2 that uses this

technique.

<HR>

<BLOCKQUOTE>

<B>Listing 9.3&nbsp;&nbsp;LST9_3.LST: Multiple-Line </B><I>if</I><B>

Statements.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

if (choice == 1)

{

    num = 1;

    num2 = 10;

}



if (choice == 2)

{

    num = 2;

    num2 = 20;

}



if (choice == 3)

{

    num = 3;

    num2 = 30;

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

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

<TR VALIGN=TOP><TD>

<BLOCKQUOTE>

Notice that some program lines in Listing 9.3 are indented. By indenting the lines that go with each <TT>if</TT> block, you can more easily see the structure of your program. Listing 9.3 also uses blank lines to separate blocks of code that go together. The compiler doesn't care about the indenting or the blank lines, but these features make your programs easier for you (or another programmer) to read.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

What's happening in Listing 9.3? Suppose <TT>choice</TT> equals

2. When Java gets to the first <TT>if</TT> statement, it compares

the value of <TT>choice</TT> with the number 1. Because these

values don't match (or, as programmers say, the statement doesn't

evaluate to <TT>true</TT>), Java skips over every line until it

finds the next <TT>if</TT> statement.

<P>

This brings Java to the second <TT>if</TT> statement. When Java

evaluates the expression, it finds that <TT>choice</TT> equals

2, and it executes the second portion of the <TT>if</TT> statement.

This time the second portion of the statement is not just one

command, but two. The program sets the values of both <TT>num</TT>

and <TT>num2</TT>.

<P>

This brings the program to the last <TT>if</TT> statement, which

Java skips over because <TT>choice</TT> doesn't equal 3. Notice

that, when you want to set up an <TT>if</TT> statement that executes

multiple lines of code, you must use the curly braces-<TT>{</TT>

and <TT>}</TT>-to denote the block of instructions that should

be executed.

<H3><A NAME="TheIelseIClause">

The <I>else</I> Clause</A></H3>

<P>

You might think it's a waste of time for Listing 9.3 to evaluate

other <TT>if</TT> statements after it finds a match for the value

of <TT>choice</TT>. You'd be right, too. When you write programs,

you should always look for ways to make them run faster; one way

to make a program run faster is to avoid all unnecessary processing.

But how, you may ask, do you avoid unnecessary processing when

you have to compare a variable with more than one value?

<P>

One way to keep processing to a minimum is to use Java's <TT>else</TT>

clause. The <TT>else</TT> keyword enables you to use a single

<TT>if</TT> statement to choose between two outcomes. When the

<TT>if</TT> statement evaluates to <TT>true</TT>, the second part

of the statement is executed. When the <TT>if</TT> statement evaluates

to <TT>false</TT>, the <TT>else</TT> portion is executed. (When

the <TT>if</TT> statement evaluates to neither <TT>true</TT> nor

<TT>false</TT>, it's time to get a new computer!) Listing 9.4

demonstrates how <TT>else</TT> works.

<HR>

<BLOCKQUOTE>

<B>Listing 9.4&nbsp;&nbsp;LST9_4.LST: Using the </B><I>else</I><B>

Clause.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

if (choice == 1)

{

    num = 1;

    num2 = 10;

}

else

{

    num = 2;

    num2 = 20;

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

In Listing 9.4, if <TT>choice</TT> equals 1, Java sets <TT>num</TT>

to 1 and <TT>num2</TT> to 10. If <TT>choice</TT> is any other

value, Java executes the <TT>else</TT> portion, setting <TT>num</TT>

to 2 and <TT>num2</TT> to 20. As you can see, the <TT>else</TT>

clause provides a default outcome for an <TT>if</TT> statement.

A default outcome doesn't help much, however, if an <TT>if</TT>

statement has to deal with more than two possible outcomes (as

in the original Listing 9.3). Suppose you want to rewrite Listing

9.3 so that it works the same but doesn't force Java to evaluate

all three <TT>if</TT> statements unless it really has to. No problem.

Listing 9.5 shows you how to use the <TT>else if</TT> clause:

<HR>

<BLOCKQUOTE>

<B>Listing 9.5&nbsp;&nbsp;LST9_5.LST: Using </B><I>if</I><B> and

</B><I>else</I><B> Efficiently.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

if (choice == 1)

{

    num = 1;

    num2 = 10;

}

else if (choice == 2)

{

    num = 2;

    num2 = 20;

}

else if (choice == 3)

{

    num = 3;

    num2 = 30;

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

When Java executes the program code in Listing 9.5, if <TT>choice</TT>

is 1, Java will look at only the first <TT>if</TT> section and

skip over both of the <TT>else if</TT> clauses. That is, Java

will set <TT>num</TT> to 1 and <TT>num2</TT> to 10 and then continue

on its way to whatever part of the program followed the final

<TT>else if</TT> clause. Note that, if <TT>choice</TT> doesn't

equal 1, 2, or 3, Java must evaluate all three clauses in the

listing but will not do anything with <TT>num</TT> or <TT>num2</TT>.

<H3><A NAME="ExampleUsingtheIifIStatementinaProgram">

Example: Using the <I>if</I> Statement in a Program</A></H3>

<P>

Now that you've studied what an <TT>if</TT> statement looks like

and how it works, you probably want to see it at work in a real

program. Listing 9.6 is a Java program that uses the menu example

you studied earlier in this chapter, whereas Listing 9.7 is the

HTML document that runs the applet. Figure 9.1 shows the applet

running in the Appletviewer application.

<P>

<A HREF="f9-1.gif"><B> Figure 9.1 : </B><I>The Applet6 applet enables you to choose colors.



</I></A><P>

<HR>

<BLOCKQUOTE>

<B>Listing 9.6&nbsp;&nbsp;Applet6.java: Using an </B><I>if</I><B>

Statement in a Program.<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

import java.awt.*;

import java.applet.*;



public class Applet6 extends Applet

{

    TextField textField1;



    public void init()

    {

        textField1 = new TextField(5);

        add(textField1);

g.drawString(&quot;3. Green&quot;, 40, 115);

        String s = textField1.getText();

        int choice = Integer.parseInt(s);



        if (choice == 1)

            g.setColor(Color.red);

        else if (choice == 2)

            g.setColor(Color.blue);

        else if (choice == 3)

            g.setColor(Color.green);

        else

            g.setColor(Color.black);



        if ((choice &gt;= 1) &amp;&amp; (choice &lt;= 3))

            g.drawString(&quot;This is the color you chose.&quot;, 60, 140);

        else

            g.drawString(&quot;Invalid menu selection.&quot;, 60, 140);

    }



    public boolean action(Event event, Object arg)

    {

        repaint();

        return true;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

<IMG ALIGN=RIGHT SRC="pseudo.gif" HEIGHT=94 WIDTH=94 BORDER=1>

<BLOCKQUOTE>

Tell Java that the program uses classes in the <TT>awt</TT> package.

<BR>

Tell Java that the program uses classes in the <TT>applet</TT>

package.<BR>

Derive the <TT>Applet6</TT> class from Java's <TT>Applet</TT>

class.<BR>

    Declare a <TT>TextField</TT> object called <TT>textField1</TT>.

<BR>

    Override the <TT>Applet</TT> class's<TT> init()</TT> method.

<BR>

        Create the <TT>TextField</TT> object.<BR>

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

        Initialize the text in the <TT>TextField</TT> object to

&quot;1&quot;.<BR>

    Override the <TT>Applet</TT> class's <TT>paint()</TT> method.

<BR>

        Display user instructions in the applet's display area.

<BR>

        Display the color menu on three lines.<BR>

        Get the text from the <TT>TextField</TT> object.<BR>

        Convert the text to an integer.<BR>

        Select a color based on the value the user entered.<BR>

        Display a message in the appropriate color.<BR>

    Override the <TT>Applet</TT> class's <TT>action()</TT> method.

<BR>

        Tell Java to redraw the applet's display area.<BR>

        Tell Java that the <TT>action()</TT> method finished successfully.

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 9.7&nbsp;&nbsp;APPLET6.htmL: The HTML Document That

Runs Applet6.<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;Applet6.class&quot;

    width=250

    height=150

    name=&quot;Applet6&quot;&gt;

&lt;/applet&gt;

</PRE>

</BLOCKQUOTE>

<HR>

<P>

If you were awake at all during <A HREF="ch7.htm" >Chapter 7</A> you should already

know how most of the Applet6 applet works. But, there's some new

stuff in the <TT>paint()</TT> method that warrants a close look.

First, after converting the user's typed selection from text to

an integer, the program uses the integer in an <TT>if</TT> statement

to match up each menu selection with the appropriate color. The

<TT>setColor()</TT> method is part of the <TT>Graphics</TT> object;

you call it to set the color of graphical elements, such as text,

that the program draws in the applet's display area. The <TT>setColor()</TT>

method's single argument is the color to set. As you can see,

Java has a <TT>Color</TT> object that you can use to select colors.

You'll learn more about the <TT>Color</TT> object in <A HREF="ch36.htm" >Chapter 36</A>,

&quot;The Java Class Libraries.&quot; Notice that, if the user's

selection does not match a menu selection, the color is set to

black.

<P>

After the program sets the color, a second <TT>if</TT> statement

determines which text to display, based on whether the user entered

a valid selection. If the user's selection is valid, the program

displays &quot;This is the color you chose.&quot; in the selected

color. Otherwise, the program displays &quot;Invalid menu selection&quot;

in black.

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

The <I>switch</I> Statement</FONT></A></H2>

<P>

Another way you can add decision-making code to your programs

is with the <TT>switch</TT> statement. The <TT>switch</TT> statement

gets its name from the fact that it enables a computer program

to switch between different outcomes based on a given value. The

truth is, a <TT>switch</TT> statement does much the same job as

an <TT>if</TT> statement, but it is more appropriate for situations

where you have many choices, rather than only a few. Look at the

<TT>if</TT> statement in Listing 9.8:

<HR>

<BLOCKQUOTE>

<B>Listing 9.8&nbsp;&nbsp;LST9_8.TXT: A Typical </B><I>if</I><B>

Statement.<BR>

</B>

</BLOCKQUOTE>