基本数据类型与表达式.ppt

Java程序基本结构解析,程序解析：1、注释2、类声明3、标识符4、关键字5、字符串6、标准输出对象7、语句8、方法与参数,第二章基本数据类型与表达式,数据类型常量与变量输入与输出表达式,1.数据类型,基本数据类型,数据类型转换,自动数据类型转换低级别向高级别转换：byte-short-int-long-float-double例：3*a+6.85强制数据类型转换格式：(数据类型名称）表达式可能导致精度损失,2.常量与变量,标识符标识符只能由字母、数字、美元符和下划线组成，如root-1,my=you,100%等不符合要求第一个字符不能是数字，如3abc标识符可以是任意长的字符，但只有前32个字符有效关键字：Java保留使用的标识符判断标识符定义对错：char0ax_lifLu1Cy%ty$123aa命名习惯：变量与方法：radiusareashowInputDialog类名：ComputeArea常量：PIMAX_VALUE,Java标识符是大小写敏感的,保留字,常量,定义：程序运行过程中不能改变其值的量整型常量不允许包含逗号、小数点和特殊符号（包括美元符号）默认是int类型浮点型常量具有小数点的数字默认是double类型字符型常量（Unicode编码集）布尔型常量:true,false,常量,finaldatatypeCONSTANTNAME=VALUE;finaldoublePI=3.14159;finalintSIZE=3;,变量,DeclaringVariablesintx;/Declarextobeanintegervariable;doubleradius;/Declareradiustobeadoublevariable;chara;/Declareatobeacharactervariable;AssignmentStatementsx=1;/Assign1tox;radius=1.0;/Assign1.0toradius;a=A;/AssignAtoa;DeclaringandInitializinginOneStepintx=1;doubled=1.4;,TraceaProgramExecution,publicclassComputeArea/*Mainmethod*/publicstaticvoidmain(Stringargs)finaldoublePI=3.14159;doubleradius;doublearea;/Assignaradiusradius=20;/Computeareaarea=radius*radius*PI;/DisplayresultsSystem.out.println(Theareaforthecircleofradius+radius+is+area);,20,radius,memory,1256.636,area,printamessagetotheconsole,charletter=A;(ASCII)charnumChar=4;(ASCII)charletter=u0041;(Unicode)charnumChar=u0034;(Unicode),Fourhexadecimaldigits.,NOTE:TheincrementanddecrementoperatorscanalsobeusedoncharvariablestogetthenextorprecedingUnicodecharacter.Forexample,thefollowingstatementsdisplaycharacterb.charch=a;System.out.println(+ch);,字符类型,UnicodeFormat,JavacharactersuseUnicode,a16-bitencodingschemeestablishedbytheUnicodeConsortiumtosupporttheinterchange,processing,anddisplayofwrittentextsintheworldsdiverselanguages.Unicodetakestwobytes,precededbyu,expressedinfourhexadecimalnumbersthatrunfromu0000touFFFF.So,Unicodecanrepresent65535+1characters.,Unicodeu03b1u03b2u03b3forthreeGreekletters,Example:DisplayingUnicodes,WriteaprogramthatdisplaystwoChinesecharactersandthreeGreekletters.,importjavax.swing.JOptionPane;publicclassDisplayUnicodepublicstaticvoidmain(Stringargs)JOptionPane.showMessageDialog(null,u6B22u8FCEu03b1u03b2u03b3,u6B22u8FCEWelcome,JOptionPane.INFORMATION_MESSAGE);,EscapeSequencesforSpecialCharacters,DescriptionEscapeSequenceUnicodeBackspacebu0008Tabtu0009Linefeednu000ACarriagereturnru000DBackslashu005CSingleQuoteu0027DoubleQuoteu0022,AppendixB:ASCIICharacterSet,ASCIICharacterSetisasubsetoftheUnicodefromu0000tou007f,ASCIICharacterSet,cont.,ASCIICharacterSetisasubsetoftheUnicodefromu0000tou007f,Stringmessage=WelcometoJava;StringisactuallyapredefinedclassintheJavalibraryjustliketheSystemclassandJOptionPaneclass.TheStringtypeisnotaprimitivetype.Itisknownasareferencetype.AnyJavaclasscanbeusedasareferencetypeforavariable.,字符串类型,StringConcatenation,/ThreestringsareconcatenatedStringmessage=Welcome+to+Java;/StringChapterisconcatenatedwithnumber2Strings=Chapter+2;/sbecomesChapter2/StringSupplementisconcatenatedwithcharacterBStrings1=Supplement+B;/sbecomesSupplementB,ObtainingInput,UsingJOptionPaneinputdialogsUsingtheJDK1.5Scannerclass,GettingInputfromInputDialogBoxes,Stringstring=JOptionPane.showInputDialog(null,“PromptingMessage”,“DialogTitle”,JOptionPane.QUESTION_MESSAGE);,TwoWaystoInvoketheMethod,ThereareseveralwaystousetheshowInputDialogmethod.Forthetimebeing,youonlyneedtoknowtwowaystoinvokeit.Oneistouseastatementasshownintheexample:Stringstring=JOptionPane.showInputDialog(null,x,y,JOptionPane.QUESTION_MESSAGE);wherexisastringforthepromptingmessage,andyisastringforthetitleoftheinputdialogbox.Theotheristouseastatementlikethis:JOptionPane.showInputDialog(x);wherexisastringforthepromptingmessage.,ConvertingStringstoIntegers,Theinputreturnedfromtheinputdialogboxisastring.Ifyouenteranumericvaluesuchas123,itreturns“123”.Toobtaintheinputasanumber,youhavetoconvertastringintoanumber.Toconvertastringintoanintvalue,youcanusethestaticparseIntmethodintheIntegerclassasfollows:intintValue=Integer.parseInt(intString);whereintStringisanumericstringsuchas“123”.,ConvertingStringstoDoubles,Toconvertastringintoadoublevalue,youcanusethestaticparseDoublemethodintheDoubleclassasfollows:doubledoubleValue=Double.parseDouble(doubleString);wheredoubleStringisanumericstringsuchas“123.45”.,importjavax.swing.JOptionPane;/importjava.lang.String;publicclassAdditionpublicstaticvoidmain(Stringargs)StringinputX=JOptionPane.showInputDialog(请输入第一个整数：);intx=Integer.parseInt(inputX);StringinputY=JOptionPane.showInputDialog(请输入第二个整数：);inty=Integer.parseInt(inputY);intsum;sum=x+y;charch=A;System.out.printf(sum=%d,sum);JOptionPane.showMessageDialog(null,sum+sum);,Example:,GettingInputUsingScanner,1.CreateaScannerobjectScannerscanner=newScanner(System.in);2.Usethemethodsnext(),nextByte(),nextShort(),nextInt(),nextLong(),nextFloat(),nextDouble(),ornextBoolean()toobtaintoastring,byte,short,int,long,float,double,orbooleanvalue.Forexample,System.out.print(Enteradoublevalue:);Scannerscanner=newScanner(System.in);doubled=scanner.nextDouble();,Example:,importjava.util.Scanner;/importjava.lang.String;publicclassAdditionpublicstaticvoidmain(Stringargs)Scannersc=newScanner(System.in);System.out.print(请输入第一个整数：);intx=sc.nextInt();System.out.print(请输入第二个整数：);inty=sc.nextInt();intsum;sum=x+y;System.out.printf(sum=%d,sum),Output,JOptionPane.showMessageDialog(null,x);System.out.print方法输出后光标不换行println方法输出后光标换行printf格式化输出(占位符%s）注意：printf方法在jdk1.5之后,3.表达式,赋值表达式算术表达式关系表达式逻辑表达式条件表达式位运算表达式,赋值表达式,赋值运算符：=赋值表达式：变量=表达式数据类型转换数据类型兼容：