Queues are used in many kinds of software such as Operating systems Queues keep track of processes waiting for a turn in the CPU Simulations In a Laundromat simulation a queue might keep track of loads of laundry waiting for a dryer Graphical user i ID: 31372
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QUEUES1 TheIdeaofaQueueAqueueislikealineupinabank:objectsenterattheback,andleavefromthefront.Queuesareusedinmanykindsofsoftware,suchas:Operatingsystems.QueueskeeptrackofprocesseswaitingforaturnintheCPU.Simulations.InaLaundromatsimulation,aqueuemightkeeptrackofloadsoflaundrywaitingforadryer.Graphicaluserinterfaces(GUIs).Queueskeeptrackofeventswaitingtobehandled,likemultiplebuttonclicks.IntheseslidesweshowJavacodeforqueueswithoutthe\generics"providedinthenewerJava1.5.That'sbecausewe'dliketoworkwitharrays,whicharenotsoeasytousewithgenerics.Ingeneral,thatmightnotbetheidealchoice.2 De nitionofaQueueData:Asequenceofobjects.Theobjectsareremovedintheordertheywereinserted;thisisreferredtoas\ rstin, rstout",orFIFO.The rstelement,atthe\front"ofthequeue,iscalledthe\head".Thelastelementiscalledthe\tail".Operations:enqueue(o):Appendotothequeue.head():Returnfrontelementofthequeue.Precondition:Thequeueisnotempty.dequeue():Removeandreturnthefrontelementofthequeue.Precondition:Thequeueisnotempty.size():Returnthenumberofelementsinthequeue.3 AJavaInterfaceRememberthataninterfaceislikeaclass,butwithoutanyinstancevariablesormethodbodies.Itde neswhataclassmustdo,butisnotaclassitself.Weneedtode neaclasslaterthat\implements"theinterface.publicinterfaceQueue{/**Appendotome.*/voidenqueue(Objecto);/**Returnmyfrontelement.Precondition:size()!=0.*/Objecthead();/**Removeandreturnmyfrontelement.Precondition:size()!=0.*/Objectdequeue();/**Returnthenumberofelementsinme.*/intsize();}4 ImplementingQueue/**AQueuewithfixedcapacity.*/publicclassArrayQueueimplementsQueue{/**Thenumberofelementsinme.*/privateintsize;/**contents[0..size-1]containsmyelements.*/privateObject[]contents;//Representationinvariant://size�=0.//Ifsizeis0,Iamempty.//Ifsize�0://contents[0]isthehead//contents[size-1]isthetail//contents[0..size-1]containsthe//Objectsintheordertheywereinserted./**AnArrayQueuewithcapacityfornelements.*/publicArrayQueue(intn){contents=newObject[n];}(Continuedonnextslide)5 /**Appendotome.*/publicvoidenqueue(Objecto){contents[size++]=o;}/**Removeandreturnmyfrontelement.ThisisO(size()).*Precondition:size()!=0.*/publicObjectdequeue(){Objecthead=contents[0];//Movealltheotherelementsuponespot.for(inti=0;i!=size-1;++i){contents[i]=contents[i+1];}--size;returnhead;}/**Returnmyfrontelement.*Precondition:size()!=0.*/publicObjecthead(){returncontents[0];}/**Returnthenumberofelementsinme.*/publicintsize(){returnsize;}}6 QuestionsWhyistheconstructorO(capacity)?Wouldthefollowinghaveworkedindequeue()?for(inti=size-1;i!=0;--i){contents[i-1]=contents[i];}UsingArrayQueueWecouldn'tconstructaQueue,butwecanconstructanArrayQueue.Example(compilesandruns):publicclassAlsoOkay{publicstaticvoidfill(Queueq,intn){for(inti=0;i!=n;i++){q.enqueue(newInteger(i));}}publicstaticvoidmain(String[]args){//CouldalsobedeclaredasArrayQueue.Queueq=newArrayQueue(15);fill(q,15);}}7 QueueimprovementsAJavaArrayListhasoperationsthatletitbeusedlikeaQueue(Exercise:lookupwhichones).WhynotjustuseArrayListallthetime?BecausewecanoptimizeourQueueimplementation(s)forthespeci cQueueoperations.Question:WhatoperationtakesthemosttimeinArrayQueue?8 SomealternativetechniquesDon'tbothershiftingKeepavariablethatsayswheretheheadis(sinceit'sgoingtomovedown).Whatifthequeueeventuallyslidesdowntotheendofthearray?UseacirculararrayAsabove,butwhenthetail(orhead)getstotheend,wraparoundtothebeginning.Thatis,treatthearrayasifitwerecircular.9 Herearetwowaysofdrawingaqueuecontainingabcd:01234567abcdQuestion:Deviseasequenceofoperationsthatwouldleadtothissituation.10 ClassCircularQueue/**AQueuewithfixedcapacityand(exceptforcreation)*constant-timeoperations.*/publicclassCircularQueueimplementsQueue{/**Thenumberofelementsinme.*/privateintsize;/**Theindexoftheheadandtailofthequeue.*/privateinthead,tail;/**Theitemsinme,storedincontents[head..tail],withwraparound.*/privateObject[]contents;//Representationinvariant://Let"capacity"becontents.length.//size�=0.//Ifsizeis0,Iamempty.//Ifsize�0://contents[head]isthehead//contents[tail]isthetail//ifhead=tail,contents[head..tail]contains//theObjectsintheordertheywereinserted;size=tail-head+1.//ifhead.23;础tail,contents[head..capacity-1,0..tail]//containstheObjectsintheordertheywereinserted;//size=tail-head+1+capacity.11 /**ACircularQueuewithcapacityfornelements.*/publicCircularQueue(intn){contents=newObject[n];tail=contents.length-1;}/**Appendotome.*/publicvoidenqueue(Objecto){tail=(tail+1)%contents.length;contents[tail]=o;++size;}/**Removeandreturnmyfrontelement.Precondition:size()!=0.*/publicObjectdequeue(){Objectresult=contents[head];head=(head+1)%contents.length;--size;returnresult;}/**Returnmyfrontelement.Precondition:size()!=0.*/publicObjecthead(){returncontents[head];}/**Returnthenumberofelementsinme.*/publicintsize(){returnsize;}}Question:Whynotcalculatesizeinsteadofstoringit?12