11. What will be the result of executing the following code?
public static void main(String args[]){
char digit = 'a';
for (int i = 0; i < 10; i++){
switch (digit){
case 'x' :{
int j = 0;
System.out.println(j);
}
default :{
int j = 100;
System.out.println(j);
}
}
}
int i = j;
System.out.println(i);
}
A.100 will be printed 11 times.
B.100 will be printed 10 times and then there will be a runtime exception
C.The code will not compile because the variable i cannot be declared twice within the mani() method.
D.The code will not compile because the variable j cannot be declared twice within the switch statement.
E.None of these.
E is correct. The code will not compile. There is no problem with the declaration of another variable i as both the variables are in disjoint blocks (first one is inside the for loop and its scope ends with the for loop, whereas the second is outside the for loop) and, therefore, different scopes and hence valid. The problem is with the variable j. The two declarations of the variable j are perfectly valid as they are in disjoint blocks and, therefore, different scopes. The error is that both the declarations of j are not available outside the case or default statement, whereas we are trying to assign it to the variable i. Therefore the compiler objects and reports variable j not found.
12. Which of the following collection classes from java.util package are Thread safe?
A.Vector
B.ArrayList //与Vector类似，只是不同步
C.HashMap
D.Hashtable
A and D are correct. Vector and Hashtable are two collection classes that are inherently thread safe or synchronized; whereas, the classes ArrayList and HashMap are unsynchronized and must be "wrapped" via Collections.SynchronizedList or Collections.synchronizedMap if synchronization is desired.
13. What will happen when you attempt to compile and run the following code?
class MyThread extends Thread{
public void run(){
System.out.println("MyThread: run()");
}
public void start(){
System.out.println("MyThread: start()");
}
}
class MyRunnable implements Runnable{
public void run(){
System.out.println("MyRunnable: run()");
}
public void start(){
System.out.println("MyRunnable: start()");
}
}
public class MyTest {
public static void main(String args[]){
MyThread myThread = new MyThread();
MyRunnable myRunnable = new MyRunnable();
Thread thread = new Thread(myRunnable);
myThread.start();
thread.start();
}
}
A.prints: MyThread: start() followed by MyRunnable: run()
B.prints: MyThread: run() followed by MyRunnable: start()
C.prints: MyThread: start() followed by MyRunnable: start()
D.prints: MyThread: run() followed by MyRunnable: run()
E.compile time error
F.None of the above
A is the correct choice. In the above code there is not any compilation error. Thus choice E is incorrect. Inside main() method, objects of MyThread and MyRunnable class are created followed by creation of Thread with object of MyRunnable class.
Note that MyThread class extends Thread class and overrides the start() method of the Thread class. Thus on execution of "myThread.start()" statement, the start() method of the MyThread class will be executed and as a result "MyThread:start()" will be printed. Had the start() method not there in MyThread class, the start() method of the Thread class would be called which in turn would call the run() method of the MyThread class.
On execution of "thread.start();", the start() method of the Thread class would be called which in turn will call the run() method of the class which is passed to Thread constructor (i.e. MyRunnable class). Thus "MyRunnable:run()" will be printed out. Thus choice A is correct.
14. What will be the result of executing the following code?
// Filename; SuperclassX.java
package packageX;
public class SuperclassX{
protected void superclassMethodX(){}
int superclassVarX;
}
// Filename SubclassY.java
1.package packageX.packageY;
2.
3.public class SubclassY extends SuperclassX
4.{
5.SuperclassX objX = new SubclassY();
6.SubclassY objY = new SubclassY();
7.void subclassMethodY()
8.{
9.objY.superclassMethodX();
10.int i;
11.i = objY.superclassVarX;
12.}
13.}
A.Compile error at line 5.
B.Compile error at line 9.
C.Runtime exception at line 11.
D.None of these
D is correct. When no access modifier is specified for a member, it is only accessible by another class in the package where its class is defined. Even if its class is visible in another package, the member is not accessible there. In the question above the variable superclassVarX has no access modifier specified and hence it cannot be accessed in the packageY even though the class SuperclassX is visible and the protected method superclassMethodX() can be accessed. Thus the compiler will raise an error at line 11.
