SCJP考试真题和解析

例题1：

　　Choose the three valid identifiers from those listed below.

　　A. IDoLikeTheLongNameClass

　　B. $byte

　　C. const

　　D. _ok

　　E. 3_case

　　解答：A, B, D

　　点评：Java中的标示符必须是字母、美元符($)或下划线(_)开头。关键字与保留字不能作为标示符。选项C中的const是Java的保留字，所以不能作标示符。选项E中的3_case以数字开头，违反了Java的规则。

　　例题2：

　　How can you force garbage collection of an object?

　　A. Garbage collection cannot be forced

　　B. Call System.gc().

　　C. Call System.gc(), passing in a reference to the object to be garbage collected.

　　D. Call Runtime.gc().

　　E. Set all references to the object to new values（null, for example）.

　　解答：A

　　点评：在Java中垃圾收集是不能被强迫立即执行的。调用System.gc()或Runtime.gc()静态方法不能保证垃圾收集器的立即执行，因为，也许存在着更高优先级的线程。所以选项B、D不正确。选项C的错误在于，System.gc()方法是不接受参数的。选项E中的方法可以使对象在下次垃圾收集器运行时被收集。
Consider the following class:

　　1. class Test(int i) {

　　2. void test(int i) {

　　3. System.out.println("I am an int.");

　　4. }

　　5. void test(String s) {

　　6. System.out.println("I am a string.");

　　7. } 
8. 
9. public static void main(String args) {

　　10. Test t=new Test();

　　11. char ch="y";

　　12. t.test(ch);

　　13. }

　　14. }

　　Which of the statements below is true?(Choose one.)

　　A. Line 5 will not compile, because void methods cannot be overridden.

　　B. Line 12 will not compile, because there is no version of test() that rakes a char argument.

　　C. The code will compile but will throw an exception at line 12.

　　D. The code will compile and produce the following output: I am an int.

　　E. The code will compile and produce the following output: I am a String.

　　解答：D

　　点评：在第12行，16位长的char型变量ch在编译时会自动转化为一个32位长的int型，并在运行时传给void test(int i)方法。

例题4：

　　Which of the following lines of code will compile without error？

　　A. 
if (i) { 
}

　　B. 
boolean b2=true; 
System.out.println("So true"); 

　　C. 
int j=2; 
System.out.println("OK");

　　D. 
int j=2; 
System.out.println("OK");

　　解答：B, C

　　点评：选项A错，因为if语句后需要一个boolean类型的表达式。逻辑操作有^、&、| 和 &&、||,但是"&|"是非法的，所以选项D不正确。

　　例题5：

　　Which two demonstrate a "has a" relationship? (Choose two)

　　A. public interface Person { }

　　public class Employee extends Person{ }

 

　　B. public interface Shape { } 

　　C. public interface Colorable { } 
{ }

　　D. public class Species{ } 

　　E. interface Component{ } 
private Component children; 

　　解答：D, E

　　点评： 在Java中代码重用有两种可能的方式，即组合（"has a"关系）和继承（"is a"关系）。"has a"关系是通过定义类的属性的方式实现的；而"is a"关系是通过类继承实现的。本例中选项A、B、C体现了"is a"关系；选项D、E体现了"has a"关系。

　　例题6：

　　Which two statements are true for the class java.util.TreeSet? (Choose two) 
A. The elements in the collection are ordered.

　　B. The collection is guaranteed to be immutable.

　　C. The elements in the collection are guaranteed to be unique.

　　D. The elements in the collection are accessed using a unique key. 
解答：A, C

　　点评：TreeSet类实现了Set接口。Set的特点是其中的元素惟一，选项C正确。由于采用了树形存储方式，将元素有序地组织起来，所以选项A也正确。

例题7：

 

　　True or False: Readers have methods that can read and return floats and doubles.

　　A. Ture

　　B. False

　　解答：B

　　点评： Reader/Writer只处理Unicode字符的输入输出。float和double可以通过stream进行I/O.

　　例题8：

　　What does the following paint() method draw?

　　1. public void paint(Graphics g) {

　　2. g.drawString("Any question", 10, 0);

　　3. }

　　A. The string "Any question?", with its top-left corner at 10,0

　　B. A little squiggle coming down from the top of the component.

　　解答：B

　　点评：drawString(String str, int x, int y)方法是使用当前的颜色和字符，将str的内容显示出来，并且最左的字符的基线从（x,y）开始。在本题中，y=0，所以基线位于最顶端。我们只能看到下行字母的一部分，即字母'y'、'q'的下半部分。

　　例题9：

　　What happens when you try to compile and run the following application? Choose all correct options.

　　1. public class Z {

　　2. public static void main(String args) {

　　3. new Z();

　　4. }

　　5.

　　6. Z() {

　　7. Z alias1 = this;

　　8. Z alias2 = this;

　　9. synchronized(alias1) {

　　10. try {

　　11. alias2.wait();

　　12. System.out.println("DONE WAITING");

　　13. }

　　14. catch (InterruptedException e) {

 

　　15. System.out.println("INTERR 

　　16. }

　　17. catch (Exception e) { 
18. System.out.println("OTHER EXCEPTION");

　　19. }

　　20. finally {

　　21. System.out.println 

　　22. }

　　23. }

　　24. System.out.println("ALL DONE");

　　25. }

　　26. }

　　A. The application compiles but doesn't print anything.

　　B. The application compiles and print "DONE WAITING"

　　C. The application compiles and print "FINALLY"

　　D. The application compiles and print "ALL DONE"

　　E. The application compiles and print "INTERRUPTED"

　　解答：A

　　点评：在Java中，每一个对象都有锁。任何时候，该锁都至多由一个线程控制。由于alias1与alias2指向同一对象Z，在执行第11行前，线程拥有对象Z的锁。在执行完第11行以后，该线程释放了对象Z的锁，进入等待池。但此后没有线程调用对象Z的notify()和notifyAll()方法，所以该进程一直处于等待状态，没有输出。

例题10：

　　Which statement or statements are true about the code listed below? Choose three.

　　1. public class MyTextArea extends TextArea {

　　2. public MyTextArea(int nrows, int ncols) {

　　3. enableEvents(AWTEvent.TEXT_ 

　　4. }

　　5.

　　6. public void processTextEvent 

　　7. System.out.println("Processing a text event.");

　　8. }

　　9. }

　　A. The source code must appear in a file called MyTextArea.java

　　B. Between lines 2 and 3, a call should be made to super(nrows, ncols) so that the new component will have the correct size.

　　C. At line 6, the return type of processTextEvent() should be declared boolean, not void.

　　D. Between lines 7 and 8, the following code should appear: return true.

　　E. Between lines 7 and 8, the following code should appear: super.processTextEvent(te).

　　解答：A, B, E

　　点评：由于类是public，所以文件名必须与之对应，选项A正确。如果不在2、3行之间加上super(nrows,ncols)的话，则会调用无参数构建器TextArea(), 使nrows、ncols信息丢失，故选项B正确。在Java2中，所有的事件处理方法都不返回值，选项C、D错误。选项E正确，因为如果不加super.processTextEvent(te)，注册的listener将不会被唤醒。

1.Which statement about the garbage collection mechanism are true? 
B. The programmer can indicate that a reference through a local variable is no longer of interest.

　　C. The programmer has a mechanism that explicity and immediately frees the memory used by Java objects. 
E. The garbage collection system never reclaims memory from objects while are still accessible to running user threads.

 

1。B、E 
答案B告诉我们程序员可以使一个本地变量失去任何意义，例如给本地变量赋值为"null"；答案E告诉我们在程序运行期间不可能完全释放内存。

1) public void method( ){ 
3) a=new String("hello world"); 
5) System.out.println(a+b+"ok"); 
7) a=b; 
9) } 
A. before line 3 
C. before line 6 
E. Before line 9

2。D 

　　3. In the class java.awt.AWTEvent,which is the parent class upon which jdk1.1 awt events are based there is a method called getID which phrase accurately describes the return value of this method? 
B. It is an indication of the nature of the cause of the event. 
D. In the case of a mouse click, it is an indication of the text under the mouse at the time of the event. 
F. It is an indication of the time at which the event occurred.

3。B 

4. Which statement about listener is true? 
B. If multiple listener be add to a single component, the event only affected one listener. 
D. The listener mechanism allows you to call an addXxxxListener method as many times as is needed, specifying as many different listeners as your design require.

4。A、D 

　　5.Give the following code: 
public static void main(String args ){ 
do{ 
}while(--l>0) 
} 
Which well be output: 
B. Doing it for l is 1 
D. Doing it for l is 0 
F. Finish

 

5。D、F 
6. Give the code fragment: 
2) case 1:System.out.println("Test 1");break; 
4) case 3:System.out.println("Test 2");break; 
6) } 
A. 1 
C. 3 

6。B.C 
7. Give incompleted method: 
2) { if(unsafe()){//do something…} 
4) } 
A. public IOException methodName() 
C. public void methodName() throw IOException 
E. public void methodName() throws Exception 
IOException异常类是Exception的子类。根据多态性的定义，IOException对象也可以被认为是Exception类型。还要注意在方法声明中抛出异常应用关键字"throws"。 
if(x>4){ 
else if (x>9){ 
else { 
Which range of value x would produce of output "Test 2"? 
B. x>4 
9 

8。D 
9. Give the following method: 
try{ 
System.out.println("Test1"); 
}finally{System.out.println("Test 3");} 
Which will display if method unsafe () run normally? 
B. Test 2 
D. Test 4

9。A、C、D 

10. Which method you define as the starting point of new thread in a class from which new the thread can be excution? 
B. public void run() 
D. public static void main(String args) 

 

10。B 

11.Given the following class definition: 
protected int i; 
this.i=i; 
} 
Select all valid answers: 
} 
} 
B(){System.out.println("i="+i);} 
D. class B{ 
} 

 

11。A 
12. Which modifier should be applied to a method for the lock of object this to be obtained prior to excution any of the method body? 
B. abstract 
D. static 

12。A 
13. The following code is entire contents of a file called Example.java,causes precisely one error during compilation: 
2) } 
4) String str; 
6) System.out.println("ok");} 
8) str=s;}} 
10) public void method(){ 
12) BaseClass b=new BaseClass("world"); 
14) }

　　Which line would be cause the error? 

13。C 

14. Which statement is correctly declare a variable a which is suitable for refering to an array of 50 string empty object? 
B. String a 
D. String a[50] 

14。A、B 
15. Give the following java source fragement: 
public class Interesting{ 
} 
A. import java.awt.*;

　　B.package mypackage 
D. public class MyClass{//do other thing…} E. class MyClass{//do something…}

 

15。A、E 
16. Give this class outline: 
private int x; 
} 
A. Change private int x to public int x 
C. Change private int x to protected int x 

16。B 
17. the piece of preliminary analsis work describes a class that will be used frequently in many unrelated parts of a project 
Which Data type would be used? 
B. int 
D. Color 

17。A、B、D 
注意，这是考试中常见的题型。 
A. protected 
C. no modifer 
18。C 

19.Which declares for native method in a java class corrected? 
B. public native void method(); 
D. public void method(){native;} 

 

19。B 
20. Which modifer should be applied to a declaration of a class member variable for the value of variable to remain constant after the creation of the object?

20。final 
21. Which is the main() method return of a application? 
B. byte 
D. void

main()方法没有返回值，所以必须用void修饰。main()方法的返回值不能任意修改。
A. String args 
C. Char args 

22。B 
23. "The Employee object is a person, An Employee has appointment store in a 　　　vector, a hire date and a number of dependent"

　　short answer: use shortest statement declare a class of Employee.

 

23。public class Employee extends Person 

24. Give the following class defination inseparate source files: 
public Example(){//do something} 
protected void method(){//do something} 
public class Hello extends Example{//member method and member variable} 
A. public void Example(){} 
C. protected void method(){} 

24。A、B、C 
25. Float s=new Float(0.9F); 
Double u=new Double(0.9); 
A. s==t 
C. s==u 
25。A、B 
1） 引用类型比较引用；基本类型比较值。 
3） 当用equals()方法进行比较时，对类File、String、Date及封装类（Wrapper Class）来说，是比较类型及内容。