ch5.htm

Java_by_Example,初级经典例子哦,珍藏版本

<P>

Although the special characters in Table 5.1 are represented by

two symbols, the first of which is always a backslash, you still

use them as single characters. For example, to define a <TT>char</TT>

variable as a backspace character, you might write something like

the following in your Java program:

<BLOCKQUOTE>

<PRE>

char backspace = '\b';

</PRE>

</BLOCKQUOTE>

<P>

When Java's compiler sees the backslash, it knows that it's about

to encounter a special character of some type. The symbol following

the backslash tells the compiler which special character to use.

Because the backslash is used to signify a special character,

when you want to specify the backslash character yourself, you

must use two backslashes, which keeps the compiler from getting

confused. Other special characters that might confuse the compiler

because they are used as part of the Java language are single

and double quotes. When you want to use these characters in your

program's data, you must also precede them with a backslash.

<H3><A NAME="BooleanValues">

Boolean Values</A></H3>

<P>

Many times in a program, you need a way to determine if a specific

condition has been met. For example, you might need to know whether

a part of your program executed properly. In such cases, you can

use Boolean values, which are represented in Java by the <TT>boolean</TT>

data type. Boolean values are unique in that they can be only

one of two possible values: true or false. You declare a <TT>boolean</TT>

value like this:

<BLOCKQUOTE>

<PRE>

boolean identifier;

</PRE>

</BLOCKQUOTE>

<P>

or

<BLOCKQUOTE>

<PRE>

boolean identifier = value;

</PRE>

</BLOCKQUOTE>

<P>

In the second example, value must be <TT>true</TT> or <TT>false</TT>.

In an actual program, you might write something like this:

<BLOCKQUOTE>

<PRE>

boolean file_okay = true;

</PRE>

</BLOCKQUOTE>

<P>

Boolean values are often used in <TT>if</TT> statements, which

enable you to do different things depending on the value of a

variable. You'll learn about <TT>if</TT> statements in Chapter

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

<P>

Table 5.2 summarizes Java's various data types. Take some time

now to look over the table and make sure you understand how the

data types differ from each other. You might also want to think

of ways you might use each data type in an actual program.<BR>

<P>

<CENTER><B>Table 5.2&nbsp;&nbsp;Summary of Java's Data Types.</B></CENTER>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD WIDTH=109><I><B>Type</B></I></TD><TD WIDTH=378><I><B>Value</B></I>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=109>byte</TD><TD WIDTH=378>-128 to 127</TD></TR>

<TR VALIGN=TOP><TD WIDTH=109>short</TD><TD WIDTH=378>-32,768 to 32,767</TD>

</TR>

<TR VALIGN=TOP><TD WIDTH=109>int</TD><TD WIDTH=378>-2,147,483,648 to 2,147,483,647

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=109>long</TD><TD WIDTH=378>Huge</TD></TR>

<TR VALIGN=TOP><TD WIDTH=109>float</TD><TD WIDTH=378>-3.402823e38 to 3.402823e38

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=109>double</TD><TD WIDTH=378>-1.79769313486232e308 to 1.79769313486232e308

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=109>char</TD><TD WIDTH=378>Symbols used in text

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=109>boolean</TD><TD WIDTH=378>True or false</TD>

</TR>

</TABLE>

</CENTER>

<P>

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

Variable Scope</FONT></A></H2>

<P>

When you write your Java programs, you can't just declare your

variables willy-nilly all over the place. You first have to consider

how and where you need to use the variables. This is because variables

have an attribute known as scope, which determines where in your

program variables can be accessed. In Java, a variable's scope

is determined by the program block in which the variable first

appears. The variable is &quot;visible&quot; to the program only

from the beginning of its program block to the end of the program

block. When a program's execution leaves a block, all the variables

in the block disappear, a phenomenon that programmers call &quot;going

out of scope.&quot;

<P>

Now you're probably wondering, &quot;What the devil is a program

block?&quot; Generally, a program block is a section of program

code that starts with an opening curly brace ({) and ends with

a closing curly brace (}). (Sometimes, the beginning and ending

of a block are not explicitly defined, but you don't have to worry

about that just yet.) Specifically, program blocks include things

like classes, functions, and loops, all of which you'll learn

about later in this book.

<P>

Of course, things aren't quite as simple as all that (you're dealing

with computers, after all). The truth is that you can have program

blocks within other program blocks. When you have one block inside

another, the inner block is considered to be nested. Figure 5.1

illustrates the concept of nested program blocks.

<P>

<A HREF="f5-1.gif"><B> Figure 5.1 : </B><I>Program blocks can be nested inside other program blocks.



</I></A><P>

<P>

In the figure, Block 1 encloses both Block 2 and Block 3. That

is, Block 2 and Block 3 are nested within Block 1, because these

blocks occur after Block 1's opening brace but before Block 1's

closing brace. If you wanted, you could also create a Block 4

and nest it within Block 2 or Block 3, and thus create even another

level of nesting. As you'll see when you start writing full-length

Java programming, all programs have a lot of nesting going on.

<P>

The ability to nest program blocks adds a wrinkle to the idea

of variable scope. Because a variable remains in scope from the

beginning of its block to the end of its block, such a variable

is also in scope in any blocks that are nested in the variable's

block. For example, looking back at Figure 5.1, a variable that's

defined in Block 1 is accessible in not just Block 1, but also

in Block 2 and Block 3. However, a variable defined inside Block

2 is accessible only in Block 2, because such a variable goes

into scope at the start of Block 2 and goes out of scope at the

end of Block 2. If you're a little confused, the following example

ought to clear things up.

<H3><A NAME="ExampleDeterminingaVariablesScope">

Example: Determining a Variable's Scope</A></H3>

<P>

Suppose you've written the small Java program shown in Listing

5.3. (Nevermind, at this point, that you don't know much about

writing Java programs. Such minor details will be remedied by

the time you complete this book.) The program shown in the listing

follows the same program structure as that shown in Figure 5.1.

That is, there is one large main block that contains two nested

blocks. The main block begins with the opening brace on the second

line and ends with the closing brace at the end of the program.

The first inner block begins with the opening brace after the

line labeling <TT>Function1</TT> and ends with the closing brace

three lines below the opening brace. The second inner block is

defined similarly, with its own opening and closing braces.

<HR>

<BLOCKQUOTE>

<B>Listing 5.3&nbsp;&nbsp;LST5_3.TXT: Determining Variable Scope.

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

public class Block1 extends Applet

{

    int value1 = 32;



    void Block2()

    {

        float value2 = 4.5f;

        value1 = 45;

    }



    void Block3()

    {

        value1 = 100;



        // The following line causes an error.

        value2 = 55.46f;

    }

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

Now look at the variables being used in this program. The first

variable defined in the program is <TT>value1</TT>, which is found

right after the main block's opening brace. This means that <TT>value1</TT>

is accessible in all three blocks, as you can see by looking at

the <TT>Block2</TT> and <TT>Block3</TT> blocks, both of which

assign new values to <TT>value1</TT>.

<P>

The second variable in the program, <TT>value2</TT>, is defined

inside the <TT>Block2</TT> block, where it's both declared and

assigned the value 4.5f. In the <TT>Block3</TT> block, the program

tries to assign a value to <TT>value2</TT>. If you tried to compile

this program, you'd see that this line creates an error message,

as shown in Figure 5.2. In the figure, the compiler is insisting

that <TT>value2</TT> in the <TT>Block3</TT> block is undefined,

which, of course, is true as far as the <TT>Block3</TT> block

is concerned. You'd get a similar message if you tried to access

<TT>value2</TT> anywhere but within the scope of the <TT>Block2</TT>

block.

<P>

<A HREF="f5-2.gif"><B> Figure 5.2 : </B><I>When you try to access a variable that's out of scope, Java's compiler thinks that the variable is undefined.



</I></A><P>

<P>

You can use variable scope to simplify the access of variables

in a program. For example, you will usually declare variables

that you need in many places, so that they are in scope in the

entire class. That way, you can access the variables without having

to pass them as arguments to functions. (If you don't know about

argument passing just yet, you will after you read <A HREF="ch12.htm" >Chapter 12</A>,

&quot;Functions.&quot;) On the other hand, you'll have lots of

variables that you use only inside one particular program block.

You can keep your program uncluttered by being sure to declare

these types of variables only in the blocks in which they're used.

You'll learn more about setting up variables with the proper scope

as you write Java programs later in this book.

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

Summary</FONT></A></H2>

<P>

All computers must manipulate data in order to produce output.

Java, like all programming languages, features many data types

that you can use for constants and variables in your programs.

These data types enable you to store everything from simple integers

like 23 and -10 to strings and complex floating-point numbers.

There's a lot to know about variables, so your head may be spinning

a bit at this point. Rest assured, however, that once you start

writing programs and using variables, all the theoretical stuff

will make sense.

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

Review Questions</FONT></A></H2>

<OL>

<LI>What is a constant?

<LI>What is a variable?

<LI>How do constants and variables make writing programs easier?

<LI>Name the eight data types used in Java.

<LI>What is variable scope?

</OL>

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

Review Exercises</FONT></A></H2>

<OL>

<LI>Suppose you need to write a Java program that calculates an

employee's paycheck for a given number of hours of work. Write

declarations for the variables you'll need.

<LI>Using the variables you declared in exercise 1, write the

program lines needed to perform the paycheck calculations.

<LI>Using Figure 5.1 as a guide, create a new figure that adds

a program block to the class such that any variables declared

in the new block cannot be accessed in any other program block.

<LI>Using the modified figure, add yet another program block,

this time adding an additional level of block nesting.

</OL>

<HR>











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