JAVA認(rèn)證歷年真題-SCJP考試真題和解析1

iiiitheprogrammerhasamechanismthatexplicityandimmediatelyfreesthememoryusedbyjavaobjects.thegarbagecollectionmechanismcanfreethememoryusedbyjavaobjectatexplectiontime.thegarbagecollectionsystemneverreclaimsmemoryfromobjectswhilearestillaccessibletorunninguserthreads.1。b、ejava的垃圾回收機(jī)制是通過一個(gè)后臺(tái)系統(tǒng)級(jí)線程對(duì)內(nèi)存分配情況進(jìn)行跟蹤實(shí)現(xiàn)的，對(duì)程序員來說是透明的，程序員沒有任何方式使無用內(nèi)存顯示的、立即的被釋放。而且它是在程序運(yùn)行期間發(fā)生的。答案b告訴我們程序員可以使一個(gè)本地變量失去任何意義，例如給本地變量賦值為null;答案e告訴我們在程序運(yùn)行期間不可能完全釋放內(nèi)存。givethefollowingmethod:publicvoidmethod(){stringa,b;a=newstring(helloworld);b=newstring(gameover);system.out.println(a+b+ok);a=null;a=b;system.out.println(a);}intheabsenceofcompileroptimization,whichistheearliestpointtheobject areferedisdefinitelyelibiletobegarbagecollection.a.beforeline3b.beforeline5beforeline6d.beforeline7beforeline92。d第6行將null賦值給a以后，a以前保存的引用所指向的內(nèi)存空間就失去了作用，它可能被釋放。所以對(duì)象a可能最早被垃圾回收是在第7行以前，故選擇d選項(xiàng)。intheclassjava.awt.awtevent,whichistheparentclassuponwhichjdkl.1awteventsarebasedthereisamethodcalledgetidwhichphraseaccuratelydescribesthereturnvalueofthismethod?hecaseofamouseclick,itisanindicationofthetextunderthemouseatthetimeoftheevent.ittellsthestateofcertainkeysonthekeybordatthetimeoftheevent.itisanindicationofthetimeatwhichtheeventoccurred.3。b請(qǐng)查閱java類庫。getid方法的返回值是eventtype。在認(rèn)證考試中，總會(huì)有類似的書本以外的知識(shí)，這只能靠多實(shí)踐來增長知識(shí)了。whichstatementaboutlisteneristrue?mostcomponentallowmultiplelistenerstobeadded.ifmultiplelistenerbeaddtoasinglecomponent,theeventonlyaffectedoneponentdon?tallowmultiplelistenerstobeadd.thelistenermechanismallowsyoutocallanaddxxxxlistenermethodasmanytimesasisneeded,specifyingasmanydifferentlistenersasyourdesignrequire.4。a、d控件可以同時(shí)使用多個(gè)addxxxxlistener方法加入多個(gè)監(jiān)聽器。并且當(dāng)多個(gè)監(jiān)聽器加入到同一控件中時(shí)，事件可以響應(yīng)多個(gè)監(jiān)聽器，響應(yīng)是沒有固定順序的。5.givethefollowingcode:publicclassexample{publicstaticvoidmain(stringargs口){intl=0;do{system.out.println(doingitforlis:+l);}while(--l0)system.out.println(finish);}}whichwellbeoutput:doingitforlis3doingitforlis1doingitforlis2doingitforlis0doingitforlis?c1finish5。d、f本題主要考察考生對(duì)流程控制的掌握情況。這是當(dāng)型循環(huán),條件為真執(zhí)行，條件為假則退出。循環(huán)體至少執(zhí)行一次，故會(huì)輸出de循環(huán)體以外的語句總會(huì)被執(zhí)行，故輸出f。givethecodefragment:switch(x){case1:system.out.println(test1);break;case2:case3:system.out.println(test2);break;default:system.out.println(end);}whichvalueofxwouldcausetest2totheoutput:123default6。b.c在開關(guān)語句中，標(biāo)號(hào)總是不被當(dāng)做語句的一部分，標(biāo)號(hào)的作用就是做為條件判斷而已，一旦匹配成功，就執(zhí)行其后的語句，一直遭遇break語句為止。(包括default語句在內(nèi))giveincompletedmethod:{if(unsafe()){//dosomething}elseif(safe()){//dotheother}}themethodunsafe()wellthroeanioexception,whichcompletesthemethodofdeclarationwhenaddedatlineone?publicioexceptionmethodname()publicvoidmethodname()publicvoidmethodname()throwioexceptionpublicvoidmethodname()throwsioexceptionpublicvoidmethodname()throwsexception7。d、fioexception異常類是exception的子類。根據(jù)多態(tài)性的定義，ioexception對(duì)象也可以被認(rèn)為是exception類型。還要注意在方法聲明中拋出異常應(yīng)用關(guān)鍵字throws。givethecodefragment:if(x4){system.out.println(test1);}elseif(x9){system.out.println(test2);}else{system.out.println(test3);}whichrangeofvaluexwouldproduceofoutputtest2?x4x4x9none8。d只有兩種情況：大于4時(shí)輸出test1，小于等于4時(shí)輸出test3。givethefollowingmethod:publicvoidexample(){try{unsafe();system.out.println(test1);}catch(safeexceptione){system.out.println(test2);}finally{system.out.println(test3);}system.out.println(test4);whichwilldisplayifmethodunsafe()runnormally?test1test2test3test49。a、c、d在正常情況下，打印testl、test3、test4;在產(chǎn)生可捕獲異常時(shí)打印test2、test3、test4;在產(chǎn)生不可捕獲異常時(shí)，打印test3,然后終止程序。注意finally后面的語句總是被執(zhí)行。whichmethodyoudefineasthestartingpointofnewthreadinaclassfromwhichnewthethreadcanbeexcution?publicvoidstart()publicvoidrun()publicvoidint()publicstaticvoidmain(stringargs[])publicvoidrunnable()10。b線程的執(zhí)行是從方法run()開始的，該方法是由系統(tǒng)調(diào)用的。程序員手工調(diào)用方法start(),使線程變?yōu)榭蛇\(yùn)行狀態(tài)。giventhefollowingclassdefinition:classa{protectedinti;a(inti){this.i=i;}whichofthefollowingwouldbeavalidinnerclassforthisclass?selectallvalidanswers:classb{}classbextendsa{}classbextendsa{b(){system.out.println(i=+i);}}classb{classa{}}classa{}11。a此題考查內(nèi)部類及關(guān)鍵字super的用法。內(nèi)部類不能與外部類同名。另外，當(dāng)b繼承a時(shí)，a中的構(gòu)造函數(shù)是帶參數(shù)的，b中缺省構(gòu)造函數(shù)的函數(shù)體為空；而Java編譯器會(huì)為空構(gòu)造函數(shù)體自動(dòng)添加語句super();調(diào)用父類構(gòu)造函數(shù)，更進(jìn)一步是調(diào)用父類的參數(shù)為空的構(gòu)造函數(shù)。而父類中沒有參數(shù)為空的構(gòu)造函數(shù)。whichmodifiershouldbeappliedtoamethodforthelockofobjectthistobeobtainedpriortoexcutionanyofthemethodbody?synchronizedabstractfinalstaticpublic12。a此關(guān)鍵字可以在兩個(gè)線程同時(shí)試圖訪問某一數(shù)據(jù)時(shí)避免數(shù)據(jù)毀損。thefollowingcodeisentirecontentsofafilecalledexample.java,causespreciselyoneerrorduringcompilation:classsubclassextendsbaseclass{}classbaseclass(){stringstr;publicbaseclass(){system.out.println(ok);}publicbaseclass(strings){str=s;}}publicclassexample{publicvoidmethod(){subclasss=newsubclass(hello);baseclassb=newbaseclass(world);}}whichlinewouldbecausetheerror?a.9b.10c.11d.1213。c當(dāng)一個(gè)類中未顯式定義構(gòu)造函數(shù)時(shí)做省的構(gòu)造函數(shù)是以類名為函數(shù)名，參數(shù)為空，函數(shù)體為空。雖然父類中的某一構(gòu)造函數(shù)有字符串參數(shù)s,但是子類繼承父類時(shí)并不繼承構(gòu)造函數(shù)，所以它只能使用缺省構(gòu)造函數(shù)。故在第11行出錯(cuò)。whichstatementiscorrectlydeclareavariableawhichissuitableforreferingtoanarrayof50stringemptyobject?string口astringa[]chara□口stringa[50]objecta[50]14。a、b注意，題中問的是如何正確聲明一個(gè)一維數(shù)組，并非實(shí)例化或者初始化數(shù)組givethefollowingjavasourcefragement://pointxpublicclassinteresting{//dosomething}whichstatementiscorrectlyjavasyntaxatpointx?importjava.awt.*;b.packagemypackagestaticintpi=3.14publicclassmyclass{//dootherthing}e.classmyclass{//dosomething}15。a、ex處可以是一個(gè)輸入，包的定義，類的定義。由于常量或變量的聲明只能在類中或方法中，故不能選擇 c;由于在一個(gè)文件中只能有一個(gè)public類，故不能選擇dogivethisclassoutline:classexample{privateintx;//restofclassbody}assumingthatxinvokedbythecodejavaexample,whichstatementcanmadexbedirectlyaccessibleinmain()methodofexample.java?changeprivateintxtopublicintxchangeprivateintxtostaticintxchangeprivateintxtoprotectedintxchangeprivateintxtofinalintx16。b靜態(tài)方法除了自己的參數(shù)外只能顛訪問靜態(tài)成員訪問非靜態(tài)成員，必須先實(shí)例化本類的一個(gè)實(shí)例，再用實(shí)例名點(diǎn)取。thepieceofpreliminaryanalsisworkdescribesaclassthatwillbeusedfrequentlyinmanyunrelatedpartsofaprojectthepolygonobjectisadrawable,apolygonhasvertexinformationstoredinavector,acolor,lengthandwidth.whichdatatypewouldbeused?vectorintstringcolordatea、b、dpolygon的頂點(diǎn)信息存放在vector類型的對(duì)象內(nèi)部，color定義為color,length和width定義為int。注意，這是考試中常見的題型。aclassdesignrequiresthatamembervariableshouldbeaccessibleonlybysamepackage,whichmodiferwordshouldbeused?protectedpublicnomodiferprivate18。c此題考點(diǎn)是高級(jí)訪問控制。請(qǐng)考生查閱高級(jí)訪問控制說明表格。whichdeclaresfornativemethodinajavaclasscorrected?publicnativevoidmethod(){}publicnativevoidmethod();publicnativemethod();publicvoidmethod(){native;}publicvoidnativemethod();19。bnative關(guān)鍵字指明是對(duì)本地方法的調(diào)用，在java中是只能訪問但不能寫的方法，它的位置在訪問權(quán)限修飾語的后面及返回值的前面。whichmodifershouldbeappliedtoadeclarationofaclassmembervariableforthevalueofvariabletoremain

constantafterthecreationoftheobject?20。final定義常量的方法是在變量定義前加final關(guān)鍵字whichisthemain()methodreturnofaapplication?a.stringb.bytec.chard.void21。dmain()方法沒有返回值，所以必須用 void修飾。main()方法的返回值不能任意修改。whichiscorrectedargumentofmain()methodofapplication?stringargsstringar[]charargs□口stringbufferarg[]22。bmain()方法的參數(shù)是字符串?dāng)?shù)組，參數(shù)名可以任意定義。appointmentstoreinatheemployeeobjectisaperson,anemployeehasappointmentstoreinavector,ahiredateandanumber

ofdependentshortanswer:useshorteststatementdeclareaclassofemployee.23。publicclassemployeeextendsperson這也是真實(shí)考試中常見的一種題型.這也是真實(shí)考試中常見的一種題型.要