java認證考試試題及答案
故答案為C。
12. What is the result after the following code executes?
1 short s = 0x00FD;
2 byte b = (byte)s;
3 System.out.println(b);
Select 1 correct answer:
A. Compile time error in line 1
B. Compile time error in line 2
C. 0
D. -3
E. -2
解析：考察對強制類型轉換的理解，java中，short類型為16位，占據(jù)兩個字節(jié)，byte類型為八位。第二行，對s進行了強制類型轉換，而這實際上是一個收縮基本轉換，從帶符號整數(shù)到整型T的收索轉換只是簡單地丟棄除n個最低階位以外的其它所有位，這可能導致數(shù)目或者符號的變化。
Java中所有的整數(shù)類型都具有符號位，故需要考慮二進制表示
b=0xFD=1111 1101，是一個補碼。
將最高位為1的補碼轉換為原碼的步驟為(最高位為0的補碼和原碼相同)先將補碼全部取反，然后+1
b取反后得到0000 0010，加1 ： 0000 0011，即3，考慮到這是一個附負數(shù)，最后的結果為-3
答案：D
13. Given the following method in an application:
1. public String setFileType( String fname ){
2. int p = fname.indexOf( '.' );
3. if( p > 0 ) fname = fname.substring( 0,p );
4. fname += ".TXT";
5. return fname;
6. }
and given that another part of the class has the following code:
7. String TheFile = "Program.java";
8. File F = new File( setFileType( TheFile ) );
9. System.out.println( "Created " + TheFile );
what will be printed by the statement in line 9?
Select 1 correct answer:
A. Created Program.java
B. Created Program.txt
C. Created Program.java.txt
答案：A
14. Here is the ActionEvent family tree:
java.lang.Object
|--- java.util.EventObject
|--- java.awt.AWTEvent
|---- java.awt.event.ActionEvent
Suppose we have the following code to count events and
save the most recent event:
int evtCt = 0 ;
AWTEvent lastE ;
public void saveEvent( AWTEvent evt )
{
lastE = evt ;
evtCt++ ;
}
Which of the following calls of saveEvent would run
without causing an exception?
Select all possible answers:
A. call with an AWTEvent object reference
B. call with an ActionEvent object reference
C. call with an EventObject object reference
D. call with null value
答案：ABD
15. Suppose we have two classes defined as follows:
class ApBase extends Object implements Runnable
class ApDerived extends ApBase implements Observer
Given two variables created as follows:
ApBase aBase = new ApBase() ;
ApDerived aDer = new ApDerived();
Which of the following Java code fragments will
compile and execute without error?
Select 1 correct answer:
A. Object obj = aBase ; Runnable rn = obj ;
B. Object obj = aBase ; Runnable rn = (Runnable) obj ;
C. Object obj = aBase ; Observer ob = (Observer)aBase ;
D. Object obj = aDer ; Observer ob2 = obj ;
答案：B?
16. The following lists the complete contents
of the file named Derived.java:
1. public class Base extends Object {
2. String objType ;
3. public Base(){ objType = "I am a Base type" ;
4. }
5. }
6.
7. public class Derived extends Base {
8. public Derived() { objType = "I am a Derived type";
9. }
10. public static void main(String args[] ){
11. Derived D = new Derived();
12. }
13. }
What will happen when this file is compiled?
Select 1 correct answer:
A. Two class files, Base.class and Derived.class will be created
B. The compiler will object to line 1
C. The compiler will object to line 7
解析：
答案：B
17. The following method is designed to convert an input string
to a floating point number, while detecting a bad format.
Assume that factor is correctly defined elsewhere:
public boolean strCvt( String s ){
try {
factor = Double.valueOf( s ).doubleValue();
return true ;
} catch(NumberFormatException e){
System.out.println("Bad number " + s );
factor = Double.NaN ;
}finally { System.out.println("Finally");
}
return false ; }
Which of the following descriptions of the results of various
inputs to the method are correct? Select all possible answers:
A. Input = "0.234"
Result:factor = 0.234, "Finally" is printed, true is returned.
B. Input = "0.234"
Result:factor = 0.234, "Finally" is printed, false is returned.
C. Input = null
Result:factor = NaN, "Finally" is printed, false is returned.
D. Input = null
Result:factor unchanged,"Finally" is printed,
NullPointerException is thrown.
解析：finally無論在什么情況下都會執(zhí)行。
對于double.valueof，當輸入為null(注意，不是字符串null，字符串null仍然會導致NumberFormatException)時，會導致NullPointerException
答案：A D
18. Here is the hierarchy of Exceptions related to
array index errors:
Exception
+-- RuntimeException
+-- IndexOutOfBoundsException
+-- ArrayIndexOutOfBoundsException
+-- StringIndexOutOfBoundsException
Suppose you had a method X which could throw both
array index and string index exceptions. Assuming
that X does not have any try - catch statements,
which of the following statements are correct?
A. The declaration for X must include
"throws ArrayIndexOutOfBoundsException,
StringIndexOutOfBoundsException".
B. If a method calling X catches IndexOutOfBoundsException, both
array and string index exceptions will be caught.
C. If the declaration for X includes "throwsIndexOutOfBoundsException",
any calling method must use a try - catch block.
D. The declaration for X does not have to mention exceptions.
解析：RuntimeException是運行時異常，屬于不可查異常，程序無需聲明，也無需處理。
答案：BD
19. Given the following listing of the Widget class:
1 class Widget extends Thingee{
2 static private int widgetCount = 0 ;
3 public String wName ;
4 int wNumber ;
5
6 static synchronized int addWidget(){ widgetCount++ ;
7 wName = "I am Widget # " + widgetCount ;
8 return widgetCount ;
9 }
10 public Widget(){
11 wNumber = addWidget();
12 }
13 }
What happens when we try to compile the class and use
multiple Widget objects in a program?
Select 1 correct answer:
A. The class compiles and each Widget will get a unique wNumber
and wName reflecting the order in which the Widgets were created.
B. The compiler objects to line 7
C. A runtime error occurs in the addWidget method
答案：B 原因：靜態(tài)方法操作的必須是靜態(tài)變量
20. Given the following class definition:
public class DerivedDemo extends Demo
{
int M, N, L ;
public DerivedDemo( int x, int y )
{
M = x ; N = y ;
}
public DerivedDemo( int x )
{
super( x );
}
}
Which of the following constructor signatures MUST exist
in the Demo class for DerivedDemo to compile correctly?
Select 2 correct answers:
A. public Demo( int a, int b )
B. public Demo( int c )
C. public Demo( )
答案：B C
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