（（Gerbil）gerbils.get（i））.hop（）;
}
}///:
~
Theoutputis:
gerbil1ishopping
gerbil2ishopping
gerbil3ishopping
gerbil4ishopping
gerbil5ishopping
gerbil6ishopping
gerbil7ishopping
gerbil8ishopping
gerbil9ishopping
gerbil10ishopping
Exercise3
//:
c09:
E03_GerbilIterator.java
//+MjavaE03_GerbilIterator
/******************Exercise3******************
*ModifyExercise2soyouuseanIteratorto
*movethroughtheListwhilecallinghop（）.
***********************************************/
importjava.util.*;
publicclassE03_GerbilIterator{
publicstaticvoidmain（Stringargs[]）{
ArrayListgerbils=newArrayList（）;
for（inti=0;i<10;i++）
gerbils.add（newGerbil（））;
for（Iteratorit=gerbils.iterator（）;
it.hasNext（）;）
（（Gerbil）it.next（））.hop（）;
}
}///:
~
Thisproducesthesameoutputasbefore.
Exercise4
//:
c09:
E04_GerbilMap.java
//+MjavaE04_GerbilMap
/******************Exercise4******************
*TaketheGerbilclassinExercise2andputit
*intoaMapinstead,associatingthenameof
*theGerbilasaString（thekey）foreach
*Gerbil（thevalue）youputinthetable.Get
*anIteratorforthekeySet（）anduseitto
*movethroughtheMap,lookinguptheGerbil
*foreachkeyandprintingoutthekeyand
*tellingthegerbiltohop（）.
***********************************************/
importjava.util.*;
publicclassE04_GerbilMap{
publicstaticvoidmain（Stringargs[]）{
HashMapmap=newHashMap（）;
map.put（"Bob",newGerbil（））;
map.put（"Frank",newGerbil（））;
map.put（"Tiffany",newGerbil（））;
map.put（"Ted",newGerbil（））;
map.put（"Wallace",newGerbil（））;
map.put（"Heather",newGerbil（））;
Iteratorit=map.keySet（）.iterator（）;
while（it.hasNext（））{
Stringkey=（String）it.next（）;
Gerbilvalue=（Gerbil）map.get（key）;
System.out.println（"name="+key
+",value="+value）;
}
}
}///:
~
Theoutputis:
name=Frank,value=gerbil2
name=Bob,value=gerbil1
name=Wallace,value=gerbil5
name=Ted,value=gerbil4
name=Tiffany,value=gerbil3
name=Heather,value=gerbil6
Notethattheobjectsarenotstoredintheorderthattheywereentered,becauseofthenatureofthehashingfunction（describedinthebook）.IfyouuseaTreeMapinstead,you’llseethattheorderismaintained.
Exercise5
//:
c09:
E05_CountryList.java
//+MjavaE05_CountryList
/******************Exercise5******************
*CreateaList（trybothArrayListand
*LinkedList）andfillitusing
*Collections2.countries.Sortthelistand
*printit,thenapplyCollections.shuffle（）to
*thelistrepeatedly,printingiteachtimeso
*thatyoucanseehowtheshuffle（）method
*randomizesthelistdifferentlyeachtime.
***********************************************/
importcom.bruceeckel.util.*;
importjava.util.*;
publicclassE05_CountryList{
publicstaticvoidmain（Stringargs[]）{
Listlst=newArrayList（）;
Collections2.fill（lst,
Collections2.countries,8）;
Collections.sort（lst）;
System.out.println（lst）;
for（inti=0;i<5;i++）{
Collections.shuffle（lst）;
System.out.println（lst）;
}
}
}///:
~
IonlydiditforArrayList.Youcaneasilychangeit（inonlyoneplace）forLinkedList.
Here’stheoutputfromonerun:
[ALGERIA,ANGOLA,BENIN,BOTSWANA,BURKINAFASO,BURUNDI,CAMEROON,CAPEVERDE]
[BENIN,ANGOLA,ALGERIA,CAPEVERDE,BURUNDI,CAMEROON,BOTSWANA,BURKINAFASO]
[ALGERIA,BURKINAFASO,CAPEVERDE,BURUNDI,BOTSWANA,BENIN,ANGOLA,CAMEROON]
[BENIN,ALGERIA,CAPEVERDE,CAMEROON,ANGOLA,BOTSWANA,BURUNDI,BURKINAFASO]
[BOTSWANA,BURKINAFASO,ALGERIA,CAMEROON,CAPEVERDE,BURUNDI,ANGOLA,BENIN]
[ALGERIA,BENIN,BOTSWANA,BURKINAFASO,BURUNDI,CAPEVERDE,CAMEROON,ANGOLA]
Exercise6
//:
c09:
E06_MouseListRestriction.java
//+MjavaE06_MouseListRestriction
/******************Exercise6******************
*Demonstratethatyoucan'taddanythingbuta
*MousetoaMouseList.
***********************************************/
importjava.util.*;
classMouse{
privateintmouseNumber;
Mouse（inti）{mouseNumber=i;}
//OverrideObject.toString（）:
publicStringtoString（）{
return"ThisisMouse#"+mouseNumber;
}
publicintgetNumber（）{
returnmouseNumber;
}
}
classMouseList{
privateArrayListlist=newArrayList（）;
publicvoidadd（Mousem）{
list.add（m）;
}
publicMouseget（intindex）{
return（Mouse）list.get（index）;
}
publicintsize（）{returnlist.size（）;}
}
publicclassE06_MouseListRestriction{
publicstaticvoidmain（Stringargs[]）{
MouseListml=newMouseList（）;
//Compile-timeerror:
notaMouse
//!
ml.add（newObject（））;
}
}///:
~
Exercise7
//:
c09:
E07_MouseListProblem.java
//+MjavaE07_MouseListProblem
/******************Exercise7******************
*ModifyMouseList.javasothatitinheritsfrom
*ArrayListinsteadofusingcomposition.
*Demonstratetheproblemwiththisapproach.
***********************************************/
importjava.util.*;
classMouseList2extendsArrayList{
publicvoidadd（Mousem）{
System.out.println（"Addingamouse"）;
super.add（m）;
}
//Can'toverridewithdifferentreturntype:
//!
publicMouseget（inti）{
//!
return（Mouse）super.get（i）;
//!
}
//Sidestep:
publicMousegetMouse（inti）{
return（Mouse）super.get（i）;
}
}
publicclassE07_MouseListProblem{
publicstaticvoidmain（Stringargs[]）{
MouseList2mice=newMouseList2（）;
for（inti=0;i<3;i++）
mice.add（newMouse（i））;
for（inti=0;iSystem.out.println（mice.getMouse（i））;
//Oops!
Canaddanon-mouse:
mice.add（newObject（））;
}
}///:
~
Thecompilerdetectsaproblemwhentryingtooverrideget（）becausetheonlythingthat’sdifferentisthereturnvalue.Wecan“cleverly”sidestepthisbymakingagetMouse（）method.However,notethattheArrayList.get（）methodisstillaliveandwell,soit’sstillpossibletopullobjectsoutthroughthatchannel.
Here’stheoutput:
Addingamouse
Addingamouse
Addingamouse
ThisisMouse#0
ThisisMouse#1
ThisisMouse#2
NotethattheObjectdoesnotcausean“Adding...”messagetobeprinted,becausetheadd（）methodisactuallyoverloadingratherthanoverriding,whichmeansthattheadd（Object）methodstillexists,andiscalledinthelastcase.InheritingfromArrayListthusdoesnotlimitthetypeofobjectsthatcanbeadded,andsoitdefinitelyisn’twhatwewant.Andwithanon-Mouseinthelist,whenyoucallgetMouse（）withthatobjectanexceptionwillbethrownduringthecast.
Exercise8
//:
c09:
E08_RepairCatsAndDogs.java
//+MjavaE08_RepairCatsAndDogs
/******************Exercise8******************
*RepairCatsAndDogs.javabycreatingaCats
*container（utilizingArrayList）thatwillonly
*acceptandretrieveCatobjects.
***********************************************/
importjava.util.*;
classCat{
privateintcatNumber;
Cat（inti）{catNumber=i;}
voidprint（）{
System.out.println（"Cat#"+catNumber）;
}
}
classDog{
privateintdogNumber;
Dog（inti）{dogNumber=i;}
voidprint（）{
System.out.println（"Dog#"+dogNumber）;
}
}
classCatContainer{
privateArrayListcats=newArrayList（）;
publicvoidadd（Catc）{cats.add（c）;}
publicCatget（inti）{
return（Cat）cats.get（i）;
}
publicintsize（）{returncats.size（）;}
}
publicclassE08_RepairCatsAndDogs{
publicstaticvoidmain（String[]args）{
CatContainercats=newCatContainer（）;
for（inti=0;i<7;i++）
cats.add（newCat（i））;
//WillnotacceptaDog:
//!
cats.add（newDog（7））;
for（inti=0;i
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