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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。 例题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 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. 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 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 P