ch8.htm

Java_by_Example,初级经典例子哦,珍藏版本

<TR VALIGN=TOP><TD WIDTH=169>3 + 4 &lt;= 10</TD><TD WIDTH=126><CENTER>true</CENTER>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=169>3 + 4 == 4 + 4</TD><TD WIDTH=126><CENTER>false</CENTER>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=169>3 + 4 &gt; 10</TD><TD WIDTH=126><CENTER>false</CENTER>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=169>3 + 4 &gt;= 7</TD><TD WIDTH=126><CENTER>true</CENTER>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=169>3 + 4 &gt;= 8</TD><TD WIDTH=126><CENTER>false</CENTER>

</TD></TR>

</TABLE>

</CENTER>

<P>

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

Logical Operators</FONT></A></H2>

<P>

The comparison operators enable you to compare two expressions.

But another type of operator-logical operators-supercharges comparison

operators so that you can combine two or more logical expressions

into a more complex logical expression. Even if you've never programmed

a computer before, you're already familiar with logical operators

because you use them in everyday speech. For example, when you

say, &quot;Do you have a credit card or ten dollars in cash?&quot;

you're using the logical operator OR. Similarly, when you say,

&quot;I have a dog and a cat,&quot; you're using the AND operator.

Table 8.3 lists Java's logical operators and what they mean.<BR>

<P>

<CENTER><B>Table 8.3&nbsp;&nbsp;Java's Logical Operators.</B></CENTER>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD WIDTH=157><CENTER><I><B>Operator</B></I></CENTER></TD>

<TD WIDTH=144><I><B>Description</B></I></TD></TR>

<TR VALIGN=TOP><TD WIDTH=157><CENTER>&amp;&amp;</CENTER></TD><TD WIDTH=144>AND

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=157><CENTER>||</CENTER></TD><TD WIDTH=144>OR</TD>

</TR>

<TR VALIGN=TOP><TD WIDTH=157><CENTER>^</CENTER></TD><TD WIDTH=144>Exclusive OR

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=157><CENTER>!</CENTER></TD><TD WIDTH=144>NOT</TD>

</TR>

</TABLE>

</CENTER>

<P>

<P>

The AND (<TT>&amp;&amp;</TT>) operator requires all expressions

to be true for the entire expression to be true. For example,

the expression

<BLOCKQUOTE>

<PRE>

(3 + 2 == 5) &amp;&amp; (6 + 2 == 8)

</PRE>

</BLOCKQUOTE>

<P>

is true because the expressions on both sides of the <TT>&amp;&amp;</TT>

are true. However, the expression

<BLOCKQUOTE>

<PRE>

(4 + 3 == 9) &amp;&amp; (3 + 3 == 6)

</PRE>

</BLOCKQUOTE>

<P>

is false because the expression on the left of the <TT>&amp;&amp;</TT>

is not true. Remember this when combining expressions with AND:

If any expression is false, the entire expression is false.

<P>

The OR operator (<TT>||</TT>) requires only one expression to

be true for the entire expression to be true. For example, the

expressions

<BLOCKQUOTE>

<PRE>

(3 + 6 == 2) || (4 + 4 == 8)

</PRE>

</BLOCKQUOTE>

<P>

and

<BLOCKQUOTE>

<PRE>

(4 + 1 == 5) || (7 + 2 == 9)

</PRE>

</BLOCKQUOTE>

<P>

are both true because at least one of the expressions being compared

is true. Notice that in the second case both expressions being

compared are true, which also makes an OR expression true.

<P>

The exclusive OR operator (^) is used to determine if one and

only one of the expressions being compared is true. Unlike a regular

OR, with an exclusive OR, if both expressions are true, the result

is false (weird, huh?). For example, the expression

<BLOCKQUOTE>

<PRE>

(5 + 7 == 12) ^ (4 + 3 == 8)

</PRE>

</BLOCKQUOTE>

<P>

evaluates to true, whereas these expressions evaluate to false:

<BLOCKQUOTE>

<PRE>

(5 + 7 == 12) ^ (4 + 3 == 7)

(5 + 7 == 10) ^ (4 + 3 == 6)

</PRE>

</BLOCKQUOTE>

<P>

The NOT (<TT>!</TT>) operator switches the value of (or negates)

a logical expression. For example, the expression

<BLOCKQUOTE>

<PRE>

(4 + 3 == 5)

</PRE>

</BLOCKQUOTE>

<P>

is false; however, the expression

<BLOCKQUOTE>

<PRE>

!(4 + 3 == 5)

</PRE>

</BLOCKQUOTE>

<P>

is true.

<H3><A NAME="ExampleUsingLogicalOperators">

Example: Using Logical Operators</A></H3>

<P>

Take a look at the following expression:

<BLOCKQUOTE>

<PRE>

(4 + 5 == 9) &amp;&amp; !(3 + 1 = 3)

</PRE>

</BLOCKQUOTE>

<P>

Is this expression true or false? If you said true, you understand

the way the logical operators work. The expressions on either

side of the &amp;&amp; are both true, so the entire expression

is true. If you said false, you must go to bed without any dinner.

<H3><A NAME="ExampleUsingMultipleLogicalOperators">

Example: Using Multiple Logical Operators</A></H3>

<P>

Just as with mathematical operators, you can use multiple logical

operators to compare several logical expressions. For example,

look at this expression:

<BLOCKQUOTE>

<PRE>

(4 == 4) &amp;&amp; (5 == 5) &amp;&amp; (6 == 6)

</PRE>

</BLOCKQUOTE>

<P>

This expression gives a result of <TT>true</TT> because each expression

to the left and right of each AND operator is true. However, this

expression yields a value of false:

<BLOCKQUOTE>

<PRE>

(4 == 4) &amp;&amp; (5 == 6) &amp;&amp; (6 == 6)

</PRE>

</BLOCKQUOTE>

<P>

Remember that, when using AND, if any sub-expression is false,

the entire expression is false. This is kind of like testifying

in court. To be true, it's got to be the truth, the whole truth,

and nothing but the truth.

<H3><A NAME="ExampleCombiningDifferentComparisonandLogicalOperators">

Example: Combining Different Comparison and Logical Operators

</A></H3>

<P>

Again, just like mathematical operators, there's no restriction

on how you can combine the different comparison and logical operators,

although if you build a very complex expression, you may have

trouble evaluating it yourself. Check out this expression:

<BLOCKQUOTE>

<PRE>

(3 &lt; 5) &amp;&amp; (2 == 2) &amp;&amp; (9 &gt; 6)

</PRE>

</BLOCKQUOTE>

<P>

Here you've used four different comparison and logical operators

in the same complex expression. But because you're comparing the

sub-expressions with the AND operator, and because each of the

sub-expressions is true, the result of the above expression is

true.

<P>

Now, look at this expression:

<BLOCKQUOTE>

<PRE>

((3 &lt; 5) &amp;&amp; (2 == 1)) || (7 == 7)

</PRE>

</BLOCKQUOTE>

<P>

Yep, things are getting tricky. Is the above expression true or

false? (Hey, give it a shot. You've got a fifty-fifty chance.)

Ready for the answer? The above expression is true. First, look

at the parentheses. The outermost parentheses, on the left, group

the two expressions being compared by the AND operator into a

single expression, so evaluate it first. The value 3 is less than

5, but 2 does not equal 1, so the entire expression on the left

of the OR operator is false. On the right of the OR operator,

however, 7 does indeed equal 7, so this sub-expression is true.

Because one of the expressions in the OR comparison is true, the

entire expression is true. Here's how the expression breaks down,

step-by-step:

<BLOCKQUOTE>

<PRE>

((3 &lt; 5) &amp;&amp; (2 == 1)) || (7 == 7)

((true) &amp;&amp; (false)) || (7 == 7)

false || (7 == 7)

false || true

true

</PRE>

</BLOCKQUOTE>

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

Writing Logical Expressions</FONT></A></H2>

<P>

You wouldn't write expressions such as

<BLOCKQUOTE>

<PRE>

(4 + 5 == 9) &amp;&amp; !(3 + 1 == 3)

</PRE>

</BLOCKQUOTE>

<P>

in your programs. They would serve no purpose because you already

know how the expressions evaluate. However, when you use variables,

you have no way of knowing in advance how an expression may evaluate.

For example, is the expression

<BLOCKQUOTE>

<PRE>

(num &lt; 9) &amp;&amp; (num &gt; 15)

</PRE>

</BLOCKQUOTE>

<P>

true or false? You don't know without being told the value of

the numerical variable <TT>num</TT>. By using logical operators,

though, your program can do the evaluation, and, based on the

result-<TT>true</TT> or <TT>false</TT>-take the appropriate action.

In the next chapter, which is about <TT>if</TT> and <TT>switch</TT>

statements, you'll see how your programs can use logical expressions

to make decisions.

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

Order of Operations</FONT></A></H2>

<P>

Like all operators, comparison and logical operators have an order

of operations, or operator precedence. When you evaluate a complex

expression, you must be sure to evaluate any sub-expressions in

the correct order. As you learned in the previous example, however,

you can use parentheses to group expressions so that they're easier

to understand or to change the order of operations. Table 8.4

lists the comparison and logical operators in order of precedence.

<BR>

<P>

<CENTER><B>Table 8.4&nbsp;&nbsp;Comparison and Logical Operators'

Order of Operations.</B></CENTER>

<P>

<CENTER>

<TABLE BORDER=1 WIDTH=80%>

<TR VALIGN=TOP><TD WIDTH=139><I><B>Operators</B></I></TD><TD WIDTH=210><I><B>Description</B></I>

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=139>!</TD><TD WIDTH=210>NOT</TD></TR>

<TR VALIGN=TOP><TD WIDTH=139>&lt; &gt; &lt;= &gt;=</TD><TD WIDTH=210>Relational

</TD></TR>

<TR VALIGN=TOP><TD WIDTH=139>== !=</TD><TD WIDTH=210>Equality</TD></TR>

<TR VALIGN=TOP><TD WIDTH=139>^</TD><TD WIDTH=210>Exclusive OR</TD></TR>

<TR VALIGN=TOP><TD WIDTH=139>&amp;&amp;</TD><TD WIDTH=210>Logical AND</TD>

</TR>

<TR VALIGN=TOP><TD WIDTH=139>||</TD><TD WIDTH=210>Logical OR</TD></TR>

</TABLE>

</CENTER>

<P>

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

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

<P>

Expressions, which are lines of Java code that can be reduced

to a value or that assign a value, come in several types. In this

chapter, you not only experimented with numerical and assignment

expressions, but you also learned about logical expressions, which

you create using the comparison and logical operators. Now that

you know how to use comparison and logical operators to build

logical expressions, you're ready to discover how computers make

decisions. You'll make that discovery in the next chapter, where

you'll also start using the operators you learned in order to

write actual applets. Before you turn to that chapter, however,

test your knowledge of expressions, comparison operators, and

logical operators by answering the following review questions

and by completing the review exercises.

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

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

<OL>

<LI>What is an expression?

<LI>What are the three types of expressions?

<LI>What is the result of the logical expression <TT>(3 &lt; 5)</TT>?

<LI>What is the result of the logical expression <TT>(3 &lt; 5)

&amp;&amp; (5 == 4 + 1)</TT>?

<LI>Explain why expressions are recursive in nature.

<LI>What are the six comparison operators?

<LI>What are the four logical operators?

<LI>What is the result of the logical expression <TT>(3 &lt; 5)

|| (6 == 5) || (3 != 3)</TT>?

<LI>What's the result of the logical expression <TT>(5 != 10)

&amp;&amp; ((3 == 2 + 1) ||(4 &lt; 2 + 5))</TT>?

<LI>What's the result of the logical expression <TT>!(5 == 2 +

3) &amp;&amp; !(5 + 2 !=7 - 5)</TT>?

</OL>

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

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

<OL>

<LI>Write an expression that compares three numbers for equality.

<LI>Write an expression that determines whether one number is

less than or equal to another.

<LI>Write an expression that uses three different types of comparison

operators and two different types of logical operators. The expression

must be false.

<LI>Suppose you have three variables, called <TT>num1</TT>, <TT>num2</TT>,

and <TT>num3</TT>, in a program. Write an expression that compares

the variables for equality.

<LI>If the variable <TT>num1</TT> is equal to 5 and the variable

<TT>num2</TT> is equal to 10, how would you evaluate the logical

expression <TT>((num1 != 5) || (num2 == 10)) &amp;&amp; !(num1

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