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SHAPE ="RECT" HREF ="/reference/author.html" COORDS ="361,0,440,29"><AREA SHAPE ="RECT" HREF ="/reference/basic.html" COORDS ="254,0,360,29"><AREA SHAPE ="RECT" HREF ="/reference/help.html" COORDS ="183,0,251,29"><AREA SHAPE ="RECT" HREF ="/reference/cool.html" COORDS ="100,0,181,29"><AREA SHAPE ="RECT" HREF ="/reference/mybook.html" COORDS ="0,0,100,29"></MAP><MAP NAME ="header_r1.gif.map"><AREA SHAPE ="RECT" HREF ="/reference/personalize.html" COORDS ="339,0,439,29"></MAP><p><font face="Arial,Helvetica" size="-1" color="#006666"><A HREF="/reference/r_library.html"><B>All Categories</A> :</B><b><A HREF="/reference/r_java.html">Java</A></b></font><p><META><H1><FONT SIZE=6 COLOR=#Ff0000>Chapter 8</FONT></H1><H1><FONT SIZE=6 COLOR=#Ff0000>Expressions</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#TypesofExpressions">Types of Expressions</A><LI><A HREF="#ExpressionsWithinExpressions">Expressions Within Expressions</A><LI><A HREF="#ComparisonOperators">Comparison Operators</A><UL><LI><A HREF="#ExampleUsingComparisonOperators">Example: Using Comparison Operators</A></UL><LI><A HREF="#LogicalOperators">Logical Operators</A><UL><LI><A HREF="#ExampleUsingLogicalOperators">Example: Using Logical Operators</A><LI><A HREF="#ExampleUsingMultipleLogicalOperators">Example: Using Multiple Logical Operators</A><LI><A HREF="#ExampleCombiningDifferentComparisonandLogicalOperators">Example: Combining Different Comparison and Logical Operators</A></UL><LI><A HREF="#WritingLogicalExpressions">Writing Logical Expressions</A><LI><A HREF="#OrderofOperations">Order of Operations</A><LI><A HREF="#Summary">Summary</A><LI><A HREF="#ReviewQuestions">Review Questions</A><LI><A HREF="#ReviewExercises">Review Exercises</A></UL><HR><P>Without a doubt, expressions are the main building blocks of aprogram. This is because there are so many different kinds ofexpressions that a majority of the source-code lines in a programend up being-you guessed it-expressions. There are expressionsthat result in numerical values. There are expressions that resultin strings. There are simple expressions, complex expressions,and all manner of expressions in between.<P>In the previous chapter, you got a quick look at some kinds ofexpressions as you put the mathematical operators to work. Nowyou'll learn not only more about those types of expressions, butalso about logical expressions, which help a computer seem tobe able to think and make choices. Along the way, you'll discovercomparison and logical operators, which make logical expressionspossible.<H2><A NAME="TypesofExpressions"><FONT SIZE=5 COLOR=#Ff0000>Types of Expressions</FONT></A></H2><P>To put it simply, an expression is a line of code that can bereduced to a value or that assigns a value. For example, you knowthat the addition operator adds one expression to another, likethis:<BLOCKQUOTE><PRE>sum = expr1 + expr2;</PRE></BLOCKQUOTE><P>In the preceding line, <TT>expr1</TT> can be something as simpleas the variable <TT>x</TT> or as complex as <TT>(4 + 5) * 2 *5 / 7 + x / y</TT>. The same goes for <TT>expr2</TT>, of course.And, in fact, the first example containing <TT>expr1</TT> and<TT>expr2</TT> is an expression itself!<P>But no matter how complicated, all expressions can be classifiedinto one of three main categories:<UL><LI>Numerical expressions combine numbers, variables, or constantsusing mathematical operators. An example is <TT>2 + 3 / x</TT>.<LI>Assignment expressions assign a value to a variable. An exampleis <TT>num = 3</TT>.<LI>Logical expressions are unique in that they result in a valueof <TT>true</TT> or <TT>false</TT>. An example is x < 3 (whichreads "x is less than 3").</UL><H2><A NAME="ExpressionsWithinExpressions"><FONT SIZE=5 COLOR=#Ff0000>Expressions Within Expressions</FONT></A></H2><P>In the previous chapter, whether you were aware of it or not,you used lots of numerical and assignment expressions as you learnedabout mathematical operators. And if you look closely at someof those expressions, you'll make a neat discovery: Like a bunchof boxes that fit into each other, expressions often contain othersimpler expressions. For example, look at the following assignmentexpression:<BLOCKQUOTE><PRE>num = (5 - x) * (2 + y);</PRE></BLOCKQUOTE><P>This is an assignment expression because it assigns a value tothe variable <TT>num</TT>. However, the stuff on either side ofthe equals sign contains these other expressions:<BLOCKQUOTE><PRE>num(5 - x) * (2 + y)</PRE></BLOCKQUOTE><P>Both of the above lines are numerical expressions because theycan be reduced to a numerical value (assuming that you know thevalues of <TT>num</TT>, <TT>x</TT>, and <TT>y</TT>.<P>But, wait a second-you're not done yet. You can still find moresub-expressions. Look at the multiplication operation. Can yousee that it's multiplying two expressions together? Those twoexpressions look like this:<BLOCKQUOTE><PRE>(5 - x)(2 + y)</PRE></BLOCKQUOTE><P>And the above simplified expressions contain yet more sub-expressions.Those expressions are:<BLOCKQUOTE><PRE>5x2y</PRE></BLOCKQUOTE><P>Expressions are what programmers like to call recursive, meaningthat the definition of an expression keeps coming back on itself.An expression contains expressions that contain other expressions,which themselves contain other expressions. How deep you can digdepends on the complexity of the original expression. But, asyou saw demonstrated, even the relatively simple expression <TT>num= (5 - x) * (2 + y)</TT> has four levels of depth.<H2><A NAME="ComparisonOperators"><FONT SIZE=5 COLOR=#Ff0000>Comparison Operators</FONT></A></H2><P>Now that you've dug into the secrets of expressions, it's timeto learn about a new type of operator. So far, you've gotten somepractice with mathematical operators, which enable you to buildvarious types of numerical and assignment expressions. Anothertype of operator you can use to build expressions is the comparisonoperator. Comparison operators are used to create logical expressions,which, if you recall, result in a value of <TT>true</TT> or <TT>false</TT>.Table 8.1 lists the logical expressions used in Java programming.C and C++ programmers will find these operators very familiar.<BR><P><CENTER><B>Table 8.1 Java's Logical Operators.</B></CENTER><P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD WIDTH=151><CENTER><I><B>Operators</B></I></CENTER></TD><TD WIDTH=198><I><B>Description</B></I></TD></TR><TR VALIGN=TOP><TD WIDTH=151><CENTER>==</CENTER></TD><TD WIDTH=198>Equal to</TD></TR><TR VALIGN=TOP><TD WIDTH=151><CENTER><</CENTER></TD><TD WIDTH=198>Less than</TD></TR><TR VALIGN=TOP><TD WIDTH=151><CENTER>></CENTER></TD><TD WIDTH=198>Greater than</TD></TR><TR VALIGN=TOP><TD WIDTH=151><CENTER><=</CENTER></TD><TD WIDTH=198>Less than or equal to</TD></TR><TR VALIGN=TOP><TD WIDTH=151><CENTER>>=</CENTER></TD><TD WIDTH=198>Greater than or equal to</TD></TR><TR VALIGN=TOP><TD WIDTH=151><CENTER>!=</CENTER></TD><TD WIDTH=198>Not equal to</TD></TR></TABLE></CENTER><P><H3><A NAME="ExampleUsingComparisonOperators">Example: Using Comparison Operators</A></H3><P>Just how do you use comparison operators? As their name suggests,you use them to compare two expressions, with the result of thecomparison being either <TT>true</TT> or <TT>false</TT>. For example,look at this logical expression:<BLOCKQUOTE><PRE>3 == 2 + 1</PRE></BLOCKQUOTE><P>The result of the above expression is <TT>true</TT> because the<TT>==</TT> operator determines whether the expressions on eitherside are equal to each other. If you were to change the expressionto<BLOCKQUOTE><PRE>3 == 2 + 2</PRE></BLOCKQUOTE><P>the result would be <TT>false</TT>. That is, 3 does not equal4. However, the previous sentence suggests a way to rewrite theexpression, like this:<BLOCKQUOTE><PRE>3 != 2 + 2</PRE></BLOCKQUOTE><P>This expression results in a value of <TT>true</TT>, because 3does not equal 4.<P>The other logical expressions work similarly. Table 8.2 listsa number of logical expressions and the results they produce.<BR><P><CENTER><B>Table 8.2 Examples of Logical Expressions.</B></CENTER><P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD WIDTH=169><I><B>Expression</B></I></TD><TD WIDTH=126><CENTER><I><B>Result</B></I></CENTER></TD></TR><TR VALIGN=TOP><TD WIDTH=169>3 + 4 == 7</TD><TD WIDTH=126><CENTER>true</CENTER></TD></TR><TR VALIGN=TOP><TD WIDTH=169>3 + 4 != 7</TD><TD WIDTH=126><CENTER>false</CENTER></TD></TR><TR VALIGN=TOP><TD WIDTH=169>3 + 4 != 2 + 6</TD><TD WIDTH=126><CENTER>true</CENTER></TD></TR><TR VALIGN=TOP><TD WIDTH=169>3 + 4 < 10</TD><TD WIDTH=126><CENTER>true</CENTER></TD></TR>⌨️ 快捷键说明
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