SCJP模拟考试一及答案.docx
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SCJP模拟考试一及答案
SCJP模拟考试一及答案
1.Whatwillhappenwhenyouattempttocompileandrunthefollowingcode?
（Assumethatthecodeiscompiledandrunwithassertionsenabled.）
publicclassAssertTest{
publicvoidmethodA（inti）{
asserti>=0:
methodB（）;
System.out.println（i）;
}
publicvoidmethodB（）{//无返回值
System.out.println（"Thevaluemustnotbenegative"）;
}
publicstaticvoidmain（Stringargs[]）{
AssertTesttest=newAssertTest（）;
test.methodA（-10）;
}
}
A.itwillprint-10
B.itwillresultinAssertionErrorshowingthemessage-“thevaluemustnotbenegative”.
C.thecodewillnotcompile.
D.Noneofthese.
Ciscorrect.Anassertstatementcantakeanyoneofthesetwoforms-
assertExpression1;
assertExpression1:
Expression2;
Notethat,inthesecondform;thesecondpartofthestatementmustbeanexpression-Expression2.Inthiscode,themethodB（）returnsvoid,whichisnotanexpressionandhenceitresultsinacompiletimeerror.ThecodewillcompileifmethodB（）returnsanyvaluesuchasint,Stringetc.
Also,inbothformsoftheassertstatement,Expression1musthavetypebooleanoracompile-timeerroroccurs.
2.Whatwillhappenwhenyouattempttocompileandrunthefollowingcode?
publicclassStatic{
static{
intx=5;//在static内有效
}
staticintx,y;//初始化为0
publicstaticvoidmain（Stringargs[]）{
x--;//-1
myMethod（）;
System.out.println（x+y+++x）;
}
publicstaticvoidmyMethod（）{
y=x+++++x;//y=-1+1x=1
}
}
A.compiletimeerror
B.prints:
1
C.prints:
2
D.prints:
3
E.prints:
7
F.prints:
8
Disthecorrectchoice.Theabovecodewillnotgiveanycompilationerror.Notethat"Static"isavalidclassname.ThuschoiceAisincorrect.
Intheabovecode,onexecution,firstthestaticvariables（xandy）willbeinitializedto0.Thenstaticblockwillbecalledandfinallymain（）methodwillbecalled.Theexecutionofstaticblockwillhavenoeffectontheoutputasitdeclaresanewvariable（intx）.
Thefirststatementinsidemain（x--）willresultinxtobe-1.AfterthatmyMethod（）willbeexecuted.Thestatement"y=x+++++x;"willbeevaluatedtoy=-1+1andxwillbecome1.Incasethestatementbe"y=++x+++x",itwouldbeevaluatedtoy=0+1andxwouldbecome1.FinallywhenSystem.outisexecuted"x+y+++x"willbeevaluatedto"1+0+2"whichresultin3astheoutput.ThuschoiceDiscorrect.
3.Giventhefollowingcode,whatwillbetheoutput?
classValue{
publicinti=15;
}
publicclassTest{
publicstaticvoidmain（Stringargv[]）{
Testt=newTest（）;
t.first（）;
}
publicvoidfirst（）{
inti=5;
Valuev=newValue（）;
v.i=25;
second（v,i）;
System.out.println（v.i）;
}
publicvoidsecond（Valuev,inti）{
i=0;
v.i=20;
Valueval=newValue（）;
v=val;
System.out.println（v.i+""+i）;
}
}
A.15020
B.15015
C.20020
D.01520
Aiscorrect.WhenwepassreferencesinJavawhatactuallygetspassedisthevalueofthatreference（i.e.memoryaddressoftheobjectbeingreferencedandnottheactualobjectreferencedbythatreference）anditgetspassedasvalue（i.eacopyofthereferenceismade）.Nowwhenwemakechangestotheobjectreferencedbythatreferenceitreflectsonthatobjectevenoutsideofthemethodbeingcalledbutanychangesmadetothereferenceitselfisnotreflectedonthatreferenceoutsideofthemethodwhichiscalled.Intheexampleabovewhenthereferencevispassedfrommethodfirst（）tosecond（）thevalueofvispassed.Whenweassignthevaluevaltovitisvalidonlyinsidethemethodsecond（）andthusinsidethemethodsecond（）whatgetsprintedis15（initialvalueofiintheobjectreferencedbyval）,thenablankspaceandthen0（valueoflocalvariablei）.Afterthiswhenwereturntothemethodfirst（）vactuallyreferstothesameobjecttowhichitwasreferringbeforethemethodsecond（）wascalled,butonethingshouldbenotedherethatthevalueofiinthatobject（referredbyvinsidethemethodfirst（））waschangedto20inthemethodsecond（）andthischangedoesreflectevenoutsidethemethodsecond（）,hence20getsprintedinthemethodfirst（）.Thusoveralloutputofthecodeinconsiderationis
150
20
4.Whatwillhappenwhenyouattempttocompileandrunthefollowingcode?
classMyParent{
intx,y;
MyParent（intx,inty）{
this.x=x;
this.y=y;
}
publicintaddMe（intx,inty）{
returnthis.x+x+y+this.y;
}
publicintaddMe（MyParentmyPar）{
returnaddMe（myPar.x,myPar.y）;
}
}
classMyChildextendsMyParent{
intz;
MyChild（intx,inty,intz）{
super（x,y）;
this.z=z;
}
publicintaddMe（intx,inty,intz）{
returnthis.x+x+this.y+y+this.z+z;
}
publicintaddMe（MyChildmyChi）{
returnaddMe（myChi.x,myChi.y,myChi.z）;
}
publicintaddMe（intx,inty）{
returnthis.x+x+this.y+y;
}
}
publicclassMySomeOne{
publicstaticvoidmain（Stringargs[]）{
MyChildmyChi=newMyChild（10,20,30）;
MyParentmyPar=newMyParent（10,20）;
intx=myChi.addMe（10,20,30）;
inty=myChi.addMe（myChi）;
intz=myPar.addMe（myPar）;
System.out.println（x+y+z）;
}
}
A.300
B.240
C.120
D.180
E.compileerror
F.noneoftheabove
Aisthecorrectchoice.Intheabovecode,MyChildclassoverridestheaddMe（intx,inty）methodoftheMyParentclass.AndinboththeMyChildandMyParentclass,addMe（）methodisoverloaded.Thereisnocompilationerroranywhereintheabovecode.
Onexecution,first,theobjectofMyChildclasswillbeconstructed.Pleasenotethatthereisasuper（）callfromtheconstructorofMyChildclass,whichwillcalltheconstructorofMyParentclass.ThiswillcausethevalueofzvariableofMyChildclasstobe30andx,yvariablesofMyParentclasswillbecome10and20respectively.ThenextstatementwillagaincalltheconstructorofMyParentclasswithsamexandyvalues.ThisisfollowedbyexecutionofaddMe（）methodofMyChildclasswithxas10,yas20andzas30.AlsoxandyareinheritedbyMyChildclassfromtheMyParentclass.ThusintheaddMe（）methodoftheMyChildclass,thevalueofthis.xwillbe10,this.ywillbe20andthis.zwillbe30.Thereturnvalueofthismethodwillbe"10+10+20+20+30+30",whichisequalto120.Thusxwillbecome120.
ThisisfollowedbytheinvocationoftheotheraddMe（）methodwhichtakesobjectreferenceoftheMyChildclass.Fromthismethod,themethodwhichwascalledearlierisinvoked.Thiscallisexactlythesameastheearlierone.Thusthevalueofywillalsobe120likex.
NowtheaddMe（）methodofMyParentclassisinvoked.ThismethodinvokesanotheraddMe（）methodofthesameclass.ItsequivalenttotheinvocationofaddMe（intx,inty）methodwithxas10andyas20.AlsothevalueofinstancevariablesxandyofMyParentclassis10and20respectively.Thevalueofzwillbeevaluatedto"10+10+20+20",whichisequalto60.Thusthevalueofx,yandzafteralltheinvocationswillbe120,120and60respectively.Asaresultofthisfinally,"120+120+60"whichisequalto300willbeprinted.ThusAisthecorrectchoice.
5.TheclassAssertionErrorhas"is-a"relationshipwiththeseclasses（choosetwo）
A.RuntimeException
B.Error
C.VirtualMachineError
D.IllegalAccessException
E.Throwable
BandEarecorrect.TheclassAssertionErrorisanError,whichdenotesan“incorrectcondition”asopposedtoan“unusualcondition”（Exception）.Since,theclassErrordescendsfromThrowable,AssertionErroralsohas“is-a”relationshipwithThrowable.Hereisthehierarchy–java.lang.Object
|
+-java.lang.Throwable
|
+-java.lang.ErrorException
||
+-java.lang.AssertionError+RuntimeExceptionIOException
Wanttoknowmore?
Youcanfindmoreinformationaboutthisasananswertothequestion-“WhyisAssertionErrorasubclassofErrorratherthanRuntimeException?
”at-#design-faq-error
6.Whatwillbetheresultofexecutingthefollowingcode?
1.booleana=true;
2.booleanb=false;
3.booleanc=true;
4.if（a==true）
5.if（b==true）
6.if（c==true）System.out.println（"Somethingsaretrueinthisworld"）;
7.elseSystem.out.println（"Nothingistrueinthisworld!
"）;
8.elseif（a&&（b=c））//这里是赋值，不是比较
System.out.println（"It'stooconfusingtotellwhatistrueandwhatisfalse"）;
9.elseSystem.out.println（"Heythiswon'tcompile"）;
A.Thecodewon’tcompile.
B.“somethingsaretrueinthisworld”willbeprinted
C.“heythiswon’tcompile”willbeprinted
D.Noneofthese
Discorrect.Thisisaverygoodquestiontotesttheconceptsofexecutionflowincaseofifconditions.Theruleforattachingelsestatementswithifconditionsisthesameasattachingclosebracketswithopenbrackets.Aclosebracketattacheswiththeclosestopenbracket,whichisnotalreadyclosed.Similarlyanelsestatementattacheswiththeclosestifstatement,whichdoesn'thaveanelsestatementalready,attachedtoit.Sotheelsestatementatline7attachestotheifstatementatline6.Theelsestatementatline8attachestotheifstatementatline5.Theelsestatementatline9attachestotheifstatementatline8.
Nowlet'slookattheexecution.Atline4sinceaisequaltotruetheexecutionfallstoline5.Atline5sincebisnottruetheexecutiongoestothecorrespondingelsestatementatline8.Nowitevaluatestheconditioninsidetheifstatement.Pleasenoteherethatanassignmentstatementalsohasavalueequaltothevaluebeingassigned,hence（b=c）evaluatestotrueandsubsequentlya&&（b=c）evaluatestotrueand"It'stooconfusingtotellwhatistrueandwhatisfalse"willbeprinted.HencethecorrectanswerischoiceD.
7.Whatwillhappenwhenyouattempttocompileandrunthefollowingco
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