java认证考试试题及答案
故答案为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：考察对强制类型转换的理解，java中，short类型为16位，占据两个字节，byte类型为八位。第二行，对s进行了强制类型转换，而这实际上是一个收缩基本转换，从带符号整数到整型T的收索转换只是简单地丢弃除n个最低阶位以外的其它所有位，这可能导致数目或者符号的变化。
Java中所有的整数类型都具有符号位，故需要考虑二进制表示
b=0xFD=1111 1101，是一个补码。
将最高位为1的补码转换为原码的步骤为(最高位为0的补码和原码相同)先将补码全部取反，然后+1
b取反后得到0000 0010，加1 ： 0000 0011，即3，考虑到这是一个附负数，最后的结果为-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无论在什么情况下都会执行。
对于double.valueof，当输入为null(注意，不是字符串null，字符串null仍然会导致NumberFormatException)时，会导致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是运行时异常，属于不可查异常，程序无需声明，也无需处理。
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：静态方法操作的必须是静态变量
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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