Saturday, February 26, 2011

Self Test: Chapters 55 to 61

This is going to be the last self test chapter in our SCJP Exam series. The following questions will help you judge your expertise about threads that you learnt just now.


Question 1

The following block of code creates a Thread using a Runnable target:
Runnable r = new MyRunnable();
Thread SimpleThreadExample = new Thread(r);

Which of the following classes can be used to create the target, so that the preceding code compiles correctly?
A. public class MyRunnable extends Runnable{public void run(){}}
B. public class MyRunnable extends Object{public void run(){}}
C. public class MyRunnable implements Runnable{public void run(){}}
D. public class MyRunnable implements Runnable{void run(){}}
E. public class MyRunnable implements Runnable{public void start(){}}

Question 2


3. class SimpleThreadExample extends Thread {

4. public static void main(String [] args) {

5. SimpleThreadExample t = new SimpleThreadExample();

6. Thread x = new Thread(t);

7. x.start();

8. }

9. public void run() {

10. for(int i=0;i<3;++i) {

11. System.out.print(i + "..");

12. }

13. }

14. }

What is the result of this code?

A. Compilation fails

B. 1..2..3..

C. 0..1..2..3..

D. 0..1..2..

E. An exception occurs at runtime

Question 3


3. class Test {

4. public static void main(String [] args) {

5. printStuff(args);

6. }

7. public static void printStuff(String[] lines) {

8. for(int i=0;i < lines.length;i++){

9. System.out.println(lines[i]);

10. Thread.currentThread().sleep(1000);

11. }

12. }

13. }

The static method Thread.currentThread() returns a reference to the currently executing Thread object. What is the result of this code?

A. Each String in the array lines will output, with a 1-second pause between lines

B. Each String in the array lines will output, with no pause in between because this method is not executed in a Thread

C. Each String in the array lines will output, and there is no guarantee there will be a pause because currentThread() may not retrieve this thread

D. This code will not compile

E. Each String in the lines array will print, with at least a one-second pause between lines

Question 4

Assume you have a class that holds two private variables: x and y. Which of the following pairs can prevent concurrent access problems in that class? (Choose all that apply.)
A. public int read(){return x+y;}
B. public void set(int x, int y){this.x=x;this.y=y;}
C. public synchronized int read(){return x+y;}
D. public synchronized void set(int x, int y){this.x=x;this.y=y;}
E. public int read(){synchronized(x){return x+y;}}
F. public void set(int x, int y){synchronized(x){this.x=x;this.y=y;}}
G. public int read(){synchronized(x){return x+y;}}
H. public void set(int x, int y){synchronized(y){this.x=x;this.y=y;}}
I. public synchronized(this) int read(){return x+y;}
J. public synchronized(this) void set(int x, int y){this.x=x;this.y=y;}
K. public int read(){synchronized(this){return x+y;}}
L. public void set(int x, int y){synchronized(this){this.x=x;this.y=y;}}

Question 5

1. public class TestWaitMethod {
2. public static void main(String [] args) {
3. System.out.print("1 ");
4. synchronized(args){
5. System.out.print("2 ");
6. try {
7. args.wait();
8. }
9. catch(InterruptedException e){}
10. }
11. System.out.print("3 ");
12. }
13. }

What is the result of trying to compile and run this program?
A. It fails to compile because the IllegalMonitorStateException of wait() is not dealt with in line 7
B. 1 2 3
C. 1 3
D. 1 2
E. At runtime, it throws an IllegalMonitorStateException when trying to wait
F. It will fail to compile because it has to be synchronized on the this object

Question 6

Assume the following method is properly synchronized and called from a thread A on an object B:

After calling this method, when will the thread A become a candidate to get another turn at the CPU?
A. After object B is notified, or after two seconds
B. After the lock on B is released, or after two seconds
C. Two seconds after object B is notified
D. Two seconds after lock B is released

Question 7

Which are true? (Choose all that apply.)
A. The notifyAll() method must be called from a synchronized context
B. To call wait(), an object must own the lock on the thread
C. The notify() method is defined in class java.lang.Thread
D. When a thread is waiting as a result of wait(), it releases its lock
E. The notify() method causes a thread to immediately release its lock
F. The difference between notify() and notifyAll() is that notifyAll() notifies all waiting threads, regardless of the object they’re waiting on

Question 8

Given the scenario: This class is intended to allow users to write a series of messages, so that each message is identified with a timestamp and the name of the thread that wrote the message:
public class MyErrorLoggingUtilClass {
private StringBuilder contents = new StringBuilder();
public void log(String message) {
contents.append(": ");
public String getLogMessages() { return contents.toString(); }

How can we ensure that instances of this class can be safely used by multiple threads?
A. This class is already thread-safe
B. Replacing StringBuilder with StringBuffer will make this class thread-safe
C. Synchronize the log() method only
D. Synchronize the getLogMessages() method only
E. Synchronize both log() and getLogMessages()
F. This class cannot be made thread-safe

Question 9

public static synchronized void main(String[] args) throws
InterruptedException {
Thread t = new Thread();

What is the result of this code?
A. It prints X and exits
B. It prints X and never exits
C. It prints XY and exits almost immeditately
D. It prints XY with a 10-second delay between X and Y
E. It prints XY with a 10000-second delay between X and Y
F. The code does not compile
G. An exception is thrown at runtime

Question 10

class SimpleThreadExample extends Thread {
SimpleThreadExample() {
System.out.print(" SimpleThreadExample");
public void run() {
System.out.print(" aaa");
public void run(String s) {
System.out.print(" bbb");
public class TestThreads {
public static void main (String [] args) {
Thread t = new SimpleThreadExample() {
public void run() {
System.out.print(" ccc");
} }

What is the result?
A. ccc
B. SimpleThreadExample ccc
C. SimpleThreadExample aaa
D. ccc aaa
E. ccc aaa bbb
F. aaa ccc
G. Compilation fails
H. An exception is thrown at runtime

Question 11

public class ExampleThread {
synchronized void a() { doSomething(); }
static synchronized void b() { doSomething(); }
static void doSomething() {
try {
} catch (InterruptedException e) {}
public static void main(String[] args) {
final ExampleThread x = new ExampleThread();
final ExampleThread y = new ExampleThread();
Runnable runnable = new Runnable() {
public void run() {
int option = (int) (Math.random() * 4);
switch (option) {
case 0: x.a(); break;
case 1: x.b(); break;
case 2: y.a(); break;
case 3: y.b(); break;
Thread thread1 = new Thread(runnable);
Thread thread2 = new Thread(runnable);

Which of the following pairs of method invocations could NEVER be executing at the same time? (Choose all that apply.)
A. x.a() in thread1, and x.a() in thread2
B. x.a() in thread1, and x.b() in thread2
C. x.a() in thread1, and y.a() in thread2
D. x.a() in thread1, and y.b() in thread2
E. x.b() in thread1, and x.a() in thread2
F. x.b() in thread1, and x.b() in thread2
G. x.b() in thread1, and y.a() in thread2
H. x.b() in thread1, and y.b() in thread2

Question 12

public class MultipleThreadExample {
static Thread cena, orton;
public static void main(String[] args) {
cena = new Thread() {
public void run() {
try {
} catch (Exception e) {
orton = new Thread() {
public void run() {
try {
} catch (Exception e) {

Which letters will eventually appear somewhere in the output? (Choose all that apply.)
A. A
B. B
C. C
D. D
E. E
F. F
G. The answer cannot be reliably determined
H. The code does not compile

Question 13

3. public class Runner implements Runnable {
4. void go(long id) {
5. System.out.println(id);
6. }
7. public static void main(String[] args) {
8. System.out.print(Thread.currentThread().getId() + " ");
9. // insert code here
10. }
11. public void run() { go(Thread.currentThread().getId()); }
12. }

And given the following five fragments:
I. new Runner().run();
II. new Runner().start();
III. new Thread(new Runner());
IV. new Thread(new Runner()).run();
V. new Thread(new Runner()).start();

When the five fragments are inserted, one at a time at line 9, which are true? (Choose all that apply.)
A. All five will compile
B. Only one might produce the output 4 4
C. Only one might produce the output 4 2
D. Exactly two might produce the output 4 4
E. Exactly two might produce the output 4 2
F. Exactly three might produce the output 4 4
G. Exactly three might produce the output 4 2

Question 14

3. public class Awesome implements Runnable {
4. public static void main(String[] args) {
5. Thread t = new Thread(new Awesome());
6. t.start();
7. System.out.print("a1 ");
8. t.join();
9. System.out.print("a2 ");
10. }
11. public void run() {
12. System.out.print("b1 ");
13. System.out.print("b2 ");
14. }
15. }

Which are true? (Choose all that apply.)
A. Compilation fails
B. The output could be b1 b2 a1 a2
C. The output could be a1 a2 b1 b2
D. The output could be a1 b1 b2 a2
E. The output could be a1 b1 a2 b2
F. An exception is thrown at runtime

Question 15


3. class Friends {

4. static long flag = 0;

5. // insert code here

6. if(flag == 0) flag = id;

7. for(int x = 1; x < 3; x++) {

8. if(flag == id) System.out.print("yo ");

9. else System.out.print("Friend ");

10. }

11. }

12. }

13. public class FriendsHangOut implements Runnable {

14. static Friends d;

15. public static void main(String[] args) {

16. new FriendsHangOut().go();

17. }

18. void go() {

19. d = new Friends();

20. new Thread(new FriendsHangOut()).start();

21. new Thread(new FriendsHangOut()).start();

22. }

23. public void run() {


25. }

26. }

And given these two fragments:

I. synchronized void chat(long id) {

II. void chat(long id) {

When fragment I or fragment II is inserted at line 5, which are true? (Choose all that apply.)

A. An exception is thrown at runtime

B. With fragment I, compilation fails

C. With fragment II, compilation fails

D. With fragment I, the output could be yo Friend Friend yo

E. With fragment I, the output could be Friend Friend yo yo

F. With fragment II, the output could be yo Friend Friend yo


Answer 1
C is correct. The class implements the Runnable interface with a legal run() method.

A is incorrect because interfaces are implemented, not extended. B is incorrect because even though the class has a valid public void run() method, it does not implement the Runnable interface. D is incorrect because the run() method must be public. E is incorrect because the method to implement is run(), not start().

Answer 2
D is correct. The thread SimpleThreadExample will start and loop three times (from 0 to 2).

A is incorrect because the Thread class implements the Runnable interface; therefore, in line 5, Thread can take an object of type Thread as an argument in the constructor (this is NOT recommended). B and C are incorrect because the variable i in the for loop starts with a value of 0 and ends with a value of 2.

Answer 3
D is correct. The sleep() method must be enclosed in a try/catch block, or the method printStuff() must declare it throws the InterruptedException.

E is incorrect, but it would be correct if the InterruptedException was dealt with (A is too precise). B is incorrect (even if the InterruptedException was dealt with) because all Java code, including the main() method, runs in threads. C is incorrect. The sleep() method is static, it always affects the currently executing thread.

Answer 4
B and F are correct. By marking the methods as synchronized, the threads will get the lock of the this object before proceeding. Only one thread will be setting or reading at any given moment, thereby assuring that read() always returns the addition of a valid pair.

A is incorrect because it is not synchronized; therefore, there is no guarantee that the values added by the read() method belong to the same pair. C and D are incorrect; only objects can be used to synchronize on. E fails—it is not possible to select other objects (even this) to synchronize on when declaring a method as synchronized.

Answer 5
D is correct. 1 and 2 will be printed, but there will be no return from the wait call because no other thread will notify the main thread, so 3 will never be printed. It’s frozen at line 7.

A is incorrect; IllegalMonitorStateException is an unchecked exception. B and C are incorrect; 3 will never be printed, since this program will wait forever. E is incorrect because IllegalMonitorStateException will never be thrown because the wait() is done on args within a block of code synchronized on args. F is incorrect because any object can be used to synchronize on and this and static don’t mix.

Answer 6
A is correct. Either of the two events will make the thread a candidate for running again.

B is incorrect because a waiting thread will not return to runnable when the lock is released, unless a notification occurs. C is incorrect because the thread will become a candidate immediately after notification. D is also incorrect because a thread will not come out of a waiting pool just because a lock has been released.

Answer 7
A is correct because notifyAll() (and wait() and notify()) must be called from within a synchronized context. D is a correct statement.

B is incorrect because to call wait(), the thread must own the lock on the object that wait() is being invoked on, not the other way around. C is wrong because notify() is defined in java.lang.Object. E is wrong because notify() will not cause a thread to release its locks. The thread can only release its locks by exiting the synchronized code. F is wrong because notifyAll() notifies all the threads waiting on a particular locked object, not all threads waiting on any object.

Answer 8
E is correct. Synchronizing the public methods is sufficient to make this safe, so F is false. This class is not thread-safe unless some sort of synchronization protects the changing data.

B is not correct because although a StringBuffer is synchonized internally, we call append() multiple times, and nothing would prevent two simultaneous log() calls from mixing up their messages. C and D are not correct because if one method remains unsynchronized, it can run while the other is executing, which could result in reading the contents while one of the messages is incomplete, or worse.

Answer 9
G is correct. The code does not acquire a lock on t before calling t.wait(), so it throws an IllegalMonitorStateException. The method is synchronized, but it’s not synchronized on t so the exception will be thrown. If the wait were placed inside a synchronized(t) block, then the answer would have been D.

Answer 10
B is correct. The first line of main we’re constructing an instance of an anonymous inner class extending from SimpleThreadExample. So the SimpleThreadExample constructor runs and prints SimpleThreadExample. Next, main() invokes start() on the new thread instance, which causes the overridden run() method (the run() method in the anonymous inner class) to be invoked.

Answer 11

A, F, and H. A is a right answer because when synchronized instance methods are called on the same instance, they block each other. F and H can’t happen because synchronized static methods in the same class block each other, regardless of which instance was used to call the methods. (An instance is not required to call static methods; only the class.)

C could happen because synchronized instance methods called on different instances do not block each other. B, D, E, and G could all happen because instance methods and static methods lock on different objects, and do not block each other.

Answer 12
A, C, D, E, and F are correct. This may look like cena and orton are battling to cause the other to sleep() or wait()—but that’s not the case. Since sleep() is a static method, it affects the current thread, which is cena (even though the method is invoked using a reference to orton). That’s misleading but perfectly legal, and the Thread cena is able to sleep with no exception, printing A and C (after at least a 1-second delay). Meanwhile orton tries to call cena.wait()—but orton has not synchronized on cena, so calling cena.wait() immediately causes an IllegalMonitorStateException, and so orton prints D, E, and F. Although the order of the output is somewhat indeterminate (we have no way of knowing whether A is printed before D, for example) it is guaranteed that A, C, D, E, and F will all be printed in some order, eventually—so G is incorrect.

Answer 13
C and D are correct. Fragment I doesn’t start a new thread. Fragment II doesn’t compile. Fragment III creates a new thread but doesn’t start it. Fragment IV creates a new thread and invokes run() directly, but it doesn’t start the new thread. Fragment V creates and starts a new thread.

Answer 14
A is correct. The join() must be placed in a try/catch block. If it were, answers B and D would be correct. The join() causes the main thread to pause and join the end of the other thread, meaning "a2" must come last.

Answer 15
F is correct. With fragment I, the chat method is synchronized, so the two threads can’t swap back and forth. With either fragment, the first output must be yo.

Previous Chapter: Quick Review - Threads

Next Chapter: Other Topics of Importance


  1. Thanks a lot for this blog!

  2. I have a doubt about question 4.
    Shouldnt the answers be C,D,K and L?

    A and B do not have synchronized key word.E,F,G,H are not passing an int to synchronized.I and J are also not valid syntax. Please correct me if I am wrong. Thanks! Manjula


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