thinking injava习题答案4.docx
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thinkinginjava习题答案4
这是第5章的答案：
Chapter5
Notethatmanyoftheseexercisessimplytakeyouthroughthestepsofdiscoveringissueswithpackages.Thisiswhysomanyofthemarelefttothereader.
Exercise1
//:
c05:
E01_ImplicitImports.java
//+MjavaE01_ImplicitImports
/******************Exercise1******************
*WriteaprogramthatcreatesanArrayList
*objectwithoutexplicitlyimporting
*java.util.*.
***********************************************/
publicclassE01_ImplicitImports{
publicstaticvoidmain（Stringargs[]）{
java.util.ArrayListal=
newjava.util.ArrayList（）;
}
}///:
~
Todothis,anyreferencestotheobjectmustbemadeusingitsfullpackagename.
Exercise2
//:
c05:
E02_LeftToReader.java
//+MjavaE02_LeftToReader
/******************Exercise2******************
*Inthesectionlabeled"package:
thelibrary
*unit,"turnthecodefragmentsconcerning
*mypackageintoacompilingandrunningsetof
*Javafiles.
***********************************************/
publicclassE02_LeftToReader{
publicstaticvoidmain（Stringargs[]）{
System.out.println（"Exerciselefttoreader"）;
}
}///:
~
Exercise3
//:
c05:
E03_LeftToReader.java
//+MjavaE03_LeftToReader
/******************Exercise3******************
*Inthesectionlabeled"Collisions,"takethe
*codefragmentsandturnthemintoaprogram,
*andverifythatcollisionsdoinfactoccur.
***********************************************/
publicclassE03_LeftToReader{
publicstaticvoidmain（Stringargs[]）{
System.out.println（"Exerciselefttoreader"）;
}
}///:
~
Exercise4
//:
c05:
E04_LeftToReader.java
//+MjavaE04_LeftToReader
/******************Exercise4******************
*GeneralizetheclassPdefinedinthischapter
*byaddingalltheoverloadedversionsof
*rint（）andrintln（）necessarytohandleall
*thedifferentbasicJavatypes.
***********************************************/
publicclassE04_LeftToReader{
publicstaticvoidmain（Stringargs[]）{
System.out.println（"Exerciselefttoreader"）;
}
}///:
~
Exercise5
//:
c05:
E05_LeftToReader.java
//+MjavaE05_LeftToReader
/******************Exercise5******************
*ChangetheimportstatementinTestAssert.java
*toenableanddisabletheassertionmechanism.
***********************************************/
publicclassE05_LeftToReader{
publicstaticvoidmain（Stringargs[]）{
System.out.println（"Exerciselefttoreader"）;
}
}///:
~
Exercise6
//:
c05:
E06_AccessControl.java
//+Mjavac05.E06_AccessControl
/******************Exercise6******************
*Createaclasswithpublic,private,
*protected,and"friendly"datamembersand
*methodmembers.Createanobjectofthisclass
*andseewhatkindofcompilermessagesyouget
*whenyoutrytoaccessalltheclassmembers.
*Beawarethatclassesinthesamedirectory
*arepartofthe"default"package.
***********************************************/
packagec05;
publicclassE06_AccessControl{
publicinta;
privateintb;
protectedintc;
intd;//"Friendly"
publicvoidf1（）{}
privatevoidf2（）{}
protectedvoidf3（）{}
voidf4（）{}//"Friendly"
publicstaticvoidmain（Stringargs[]）{
E06_AccessControltest=
newE06_AccessControl（）;
//Noproblemaccessingeverythinginside
//ofmain（）forthisclass,sincemain（）
//isamemberandthereforehasaccess:
test.a=1;
test.b=2;
test.c=3;
test.d=4;
test.f1（）;
test.f2（）;
test.f3（）;
test.f4（）;
}
}///:
~
Youcanseethatmain（）hasaccesstoeverythingbecauseit’samember.Ifyoucreateaseparateclasswithinthesamepackage,thatclasscannotaccesstheprivatemembers:
//:
c05:
E06_Other.java
//Aseparateclassinthesamepackagecannot
//accessprivateelements:
packagec05;
publicclassE06_Other{
publicE06_Other（）{
E06_AccessControltest=
newE06_AccessControl（）;
test.a=1;
//!
test.b=2;//Can'taccess:
private
test.c=3;
test.d=4;
test.f1（）;
//!
test.f2（）;//Can'taccess:
private
test.f3（）;
test.f4（）;
}
}///:
~
Ifyoucreateaclassinaseparatepackage（whichyoucandoeitherbyexplicitlygivingapackagestatement,orbysimplyputtingitinadifferentdirectory）thenitisunabletoaccessanythingbutpublicmembers:
//:
c05:
other2:
E06_Other2.java
//Aseparateclassinthesamepackagecannot
//accessprivateelements:
packagec05.other2;
publicclassE06_Other2{
publicE06_Other2（）{
c05.E06_AccessControltest=
newc05.E06_AccessControl（）;
test.a=1;
//!
test.b=2;//Can'taccess:
private
//!
test.c=3;//Can'taccess:
protected
//!
test.d=4;//Can'taccess:
friendly
test.f1（）;
//!
test.f2（）;//Can'taccess:
private
//!
test.f3（）;//Can'taccess:
protected
//!
test.f4（）;//Can'taccess:
friendly
}
}///:
~
Exercise7
//:
c05:
E07_ProtectedManipulation.java
//+MjavaE07_ProtectedManipulation
/******************Exercise7******************
*Createaclasswithprotecteddata.Createa
*secondclassinthesamefilewithamethod
*thatmanipulatestheprotecteddatainthe
*firstclass.
***********************************************/
classWithProtected{
protectedinti;
}
publicclassE07_ProtectedManipulation{
publicstaticvoidmain（Stringargs[]）{
WithProtectedwp=newWithProtected（）;
wp.i=47;
System.out.println（"wp.i="+wp.i）;
}
}///:
~
Thepointofthisexerciseistoshowthatprotectedalsomeans“packageaccess”（aka“friendly”）.Thatis,youcanaccessprotectedfieldswithinthesamepackage.Justtobesure,tryaddingaprotectedmethodtoWithProtectedandaccessingitfromwithinE07_ProtectedManipulation.
Exercise8
//:
c05:
E08_LeftToReader.java
//+MjavaE08_LeftToReader
/******************Exercise8******************
*ChangetheclassCookieasspecifiedinthe
*sectionlabeled"protected:
'sortof
*friendly.'"Verifythatbite（）isnotpublic.
***********************************************/
publicclassE08_LeftToReader{
publicstaticvoidmain（Stringargs[]）{
System.out.println（"Exerciselefttoreader"）;
}
}///:
~
Justfollowtheinstructionsinthebook.
Exercise9
//:
c05:
E09_Widget.java
//+MjavaE09_Widget
/******************Exercise9******************
*Inthesectiontitled"Classaccess"you'll
*findcodefragmentsdescribingmyliband
*Widget.Createthislibrary,thencreatea
*Widgetinaclassthatisnotpartofthe
*mylibpackage.
***********************************************/
importmylib.Widget;
publicclassE09_Widget{
publicstaticvoidmain（Stringargs[]）{
newWidget（）;
}
}///:
~
//:
c05:
mylib:
Widget.java
packagemylib;
publicclassWidget{
publicWidget（）{
System.out.println（"MakingaWidget"）;
}
}///:
~
Exercise10
//:
c05:
E10_LeftToReader.java
//+MjavaE10_LeftToReader
/******************Exercise10*****************
*CreateanewdirectoryandedityourCLASSPATH
*toincludethatnewdirectory.Copythe
*P.classfile（producedbycompiling
*com.bruceeckel.tools.P.java）toyournew
*directoryandthenchangethenamesofthe
*file,thePclassinsideandthemethodnames.
*（Youmightalsowanttoaddadditionaloutput
*towatchhowitworks.）Createanotherprogram
*inadifferentdirectorythatusesyournew
*class.
***********************************************/
publicclassE10_LeftToReader{
publicstaticvoidmain（Stringargs[]）{
System.out.println（"Exerciselefttoreader"）;
}
}///:
~
Exercise11
//:
c05:
E11_ConnectionManager.java
//+MjavaE11_ConnectionManager
/******************Exercise11*****************
*FollowingtheformoftheexampleLunch.java,
*createaclasscalledConnectionManagerthat
*managesafixedarrayofConnectionobjects.
*Theclientprogrammermustnotbeableto
*explicitlycreateConnectionobjects,butcan
*onlygetthemviaastaticmethodin
*ConnectionManager.WhentheConnectionManager
*runsoutofobjects,itreturnsanull
*reference.Testtheclassesinmain（）.
***********************************************/
importc05.connection.*;
publicclassE11_ConnectionManager{
publicstaticvoidmain（Stringargs[]）{
Connectionc=
ConnectionManager.getConnection（）;
while（c!
=null）{
System.out.println（c）;
c.doSomething（）;
c=ConnectionManager.getConnection（）;
}
}
}///:
~
//:
c05:
connection:
Connection.java
packagec05.connection;
publicclassConnection{
privatestaticintcounter=0;
privateintid=counter++;
Connection（）{}
publicStringtoString（）{
return"Connection"+id;
}
publicvoiddoSomething（）{}
}///:
~
//:
c05:
connection:
ConnectionManager.java
packagec05.connection;
publicclassConnectionManager{
privatestaticConnection[]pool=
newConnection[10];
privatestaticintcounter=0;
static{
for（inti=0;ipool[i]=newConnection（）;
}
//Verysimple--justhandsouteachoneonce:
publicstaticConnectiongetConnection（）{
if（counterreturnpool[counter++];
returnnull;
}
}///:
~
TheConnectionclassusesastaticintcalledcountertoautomaticallygiveeachConnectionobjectanidentifier,whichitwillproduceaspartofit’stoString（）representation.ConnectionalsohasadoSomething（）method,whichispresumablytheusefulworkthatmotivatesyoutousetheConnectionobject.
NotethattheconstructorforConnectionis“friendly,”soitisunavailableoutsideofthispackage.Thus,theclientprogrammerisunabletoaccessthatconstructorandcannotmakeinstancesofConnectiondirectly.TheonlywaytogetConnectionobjectsisthroughtheConnectionManager.
IntheConnectionManager,astaticarrayofobjectsisinitiali
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