例題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

   點評：Java中的標示符必須是字母、美元符($)或下劃線(_)開頭。關鍵字與保留字不能作為標示符。選項C中的const是Java的保留字，所以不能作標示符。選項E中的3_case以數(shù)字開頭，違反了Java的規(guī)則。 

   例題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

   點評：在Java中垃圾收集是不能被強迫立即執(zhí)行的。調用System.gc()或Runtime.gc()靜態(tài)方法不能保證垃圾收集器的立即執(zhí)行，因為，也許存在著更高優(yōu)先級的線程。所以選項B、D不正確。選項C的錯誤在于，System.gc()方法是不接受參數(shù)的。選項E中的方法可以使對象在下次垃圾收集器運行時被收集。 

   例題3：

   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 
   

   點評：在第12行，16位長的char型變量ch在編譯時會自動轉化為一個32位長的int型，并在運行時傳給void test(int i)方法。 

   例題4：

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

   A. int i=0;

   if (i) {

   System.out.println(“

   Hi”);

   }

   

   B.

   boolean b=true;

   boolean b2=true;

   if(b==b2) {

   System.out.println(“So true”);

   

   }

   C.

   int i=1;

   int j=2;

   if(i==1|| j==2)

   System.out.println(“OK”);

   

   D.

   int i=1;

   int j=2;

   if (i==1 &| j==2)

   System.out.println(“OK”);

   解答：B, C

   

   點評：選項A錯，因為if語句后需要一個boolean類型的表達式。邏輯操作有^、&、| 和 &&、||,但是“&|”是非法的，所以選項D不正確。 


   例題5： 
   
   Which two demonstrate a "has a" relationship? (Choose two)

   A. public interface Person { }

   public class Employee extends Person{ }

   

   B. public interface Shape { }

   public interface Rectandle extends Shape { }

   C. public interface Colorable { }

   public class Shape implements Colorable

   

   { }

   D. public class Species{ }

   public class Animal{private Species species;}

   E. interface Component{ }

   class Container implements Component{

   private Component[] children;

   

   }

   解答：D, E

   點評： 在Java中代碼重用有兩種可能的方式，即組合（“has a”關系）和繼承（“is a”關系）�！癶as a”關系是通過定義類的屬性的方式實現(xiàn)的；而“is a”關系是通過類繼承實現(xiàn)的。本例中選項A、B、C體現(xiàn)了“is a”關系；選項D、E體現(xiàn)了“has a”關系。