Java Concurrency - Quiz Questions

Ques%on
You’ve wri/en an applica%on for processing tasks. In this applica%on,
you’ve separated the cri%cal or urgent tasks from the ones that are not
cri%cal or urgent. You’ve assigned high priority to cri%cal or urgent tasks.

In this applica%on, you ﬁnd that the tasks that are not cri%cal or urgent
are the ones that keep wai%ng for an unusually long %me. Since cri%cal or
urgent tasks are high priority, they run most of the %me. Which one of
the following mul%-threading problems correctly describes this situa%on?

A. Deadlock
B. Starva1on
C. Livelock
D. Race condi1on

Answer
You’ve wri;en an applica1on for processing tasks. In this applica1on,
you’ve separated the cri1cal or urgent tasks from the ones that are not
cri1cal or urgent. You’ve assigned high priority to cri1cal or urgent tasks.

In this applica1on, you ﬁnd that the tasks that are not cri1cal or urgent are
the ones that keep wai1ng for an unusually long 1me. Since cri1cal or
urgent tasks are high priority, they run most of the 1me. Which one of the
following mul1-threading problems correctly describes this situa1on?

A. Deadlock
B. Starva%on
C. Livelock
D. Race condi1on

Explana%on
B . Starva1on
The situa1on in which low-priority threads keep wai1ng
for a long 1me to acquire the lock and execute the code
in cri1cal sec1ons is known as starva1on.

Starva%on
Starva&on describes a situa1on where a thread is unable to gain regular access to shared
resources and is unable to make progress. This happens when shared resources are made
unavailable for long periods by "greedy" threads.

Livelock
A thread oNen acts in response to the ac1on of another thread. If the other thread's
ac1on is also a response to the ac1on of another thread, then livelock may result. As with
deadlock, livelocked threads are unable to make further progress. However, the threads
are not blocked — they are simply too busy responding to each other to resume work.
This is comparable to two people a;emp1ng to pass each other in a corridor: Alphonse
moves to his leN to let Gaston pass, while Gaston moves to his right to let Alphonse pass.
Seeing that they are s1ll blocking each other, Alphone moves to his right, while Gaston
moves to his leN. They're s1ll blocking each other, so...
Source: docs.oracle.com

Ques%on
Which of the following two deﬁni%ons of Sync (when
compiled in separate ﬁles) will compile without errors?

A.
class Sync {
public synchronized void foo() {}
}
B.
abstract class Sync {
}
C.
public abstract synchronized void foo();
}
D.
interface Sync {
public synchronized void foo();
}

Answer
Which of the following two deﬁni%ons of Sync (when
compiled in separate ﬁles) will compile without errors?

A.
class Sync {
}
B.
}
C.
public abstract synchronized void foo();
}
D.
interface Sync {
public synchronized void foo();
}

Explana%on
Abstract methods (in abstract classes or interfaces)
cannot be declared synchronized , hence the op1ons C
and D are incorrect.

Ques%on
Which ONE of the following op%ons correctly makes use of
Callable that will compile without any errors?

A.
import java.u1l.concurrent.Callable;
class CallableTask implements Callable {
public int call() {
System.out.println("In Callable.call()");
return 0;
}
}
B.
class CallableTask extends Callable {
public Integer call() {
return 0;
}
}
C.
class CallableTask implements Callable<Integer>
{
return 0;
}
}
D.
class CallableTask implements Callable<Integer> {
public void call(Integer i) {
System.out.println("In Callable.call(i)");
}
}

Answer
Which one of the following op1ons correctly makes use of
Callable that will compile without any errors?

A.
class CallableTask implements Callable {
public int call() {
return 0;
}
}
B.
class CallableTask extends Callable {
return 0;
}
}
C.
import java.u%l.concurrent.Callable;
class CallableTask implements
Callable<Integer> {
return 0;
}
}
D.
class CallableTask implements Callable<Integer> {
public void call(Integer i) {
System.out.println("In Callable.call(i)");
}
}

Explana%on
C.
The Callable interface is deﬁned as follows:
public interface Callable<V> {
V call() throws Excep1on;
}

In op1on A), the call() method has the return type int , which is incompa1ble
with the return type expected for overriding the call method and so will not
compile.

In op1on B), the extends keyword is used, which will result in a compiler (since
Callable is an interface, the implements keyword should be used).

In op1on D), the return type of call() is void and the call() method also takes a
parameter of type Integer . Hence, the method declared in the interface Integer
call() remains unimplemented in the CallableTask class, so the program will not
compile.

Ques%on
Here is a class named PingPong that extends the Thread class. Which ONE of the
following PingPong class implementa%ons correctly prints “ping” from the
worker thread and then prints “pong” from the main thread?
A.
public sta1c void main(String []args) {
Thread pingPong = new PingPong();
System.out.print("pong");
}
}
B.
pingPong.run();
}
}
C.
pingPong.start();
System.out.println("pong");
}
}
D.
public sta1c void main(String []args) throws
InterruptedExcep1on{
pingPong.start();
pingPong.join();
}
}
class PingPong extends Thread {
public void run() {
System.out.println("ping ");
}
// ONE OF THE OPTIONS HERE

Answer
Here is a class named PingPong that extends the Thread class. Which ONE of the
following PingPong class implementa%ons correctly prints “ping” from the
worker thread and then prints “pong” from the main thread?
A.
}
}
B.
pingPong.run();
}
}
C.
pingPong.start();
}
}
D.
public sta%c void main(String []args) throws
InterruptedExcep%on{
pingPong.start();
pingPong.join();
}
}
public void run() {
System.out.println("ping ");
}
// ONE OF THE OPTIONS HERE

Explana%on
D .
The main thread creates the worker thread and waits for it to
complete (which prints “ping”). ANer that it prints “pong”. So,
this implementa1on correctly prints “ping pong”.

Why are the other op1ons wrong?
A. The main() method creates the worker thread, but doesn’t start it.
So, the code given in this op1on only prints “pong”.

B. The program always prints “ping pong”, but it is misleading. The
code in this op1on directly calls the run() method instead of calling
the start() method. So, this is a single threaded program: both “ping”
and “pong” are printed from the main thread.

C. The main thread and the worker thread execute independently
without any coordina1on. (Note that it does not have a call to join()
in the main method.) So, depending on which thread is scheduled
ﬁrst, you can get “ping pong” or “pong ping” printed.

Ques%on
Choose the correct op%on based on this program:

class COWArrayListTest {
public sta%c void main(String []args) {
ArrayList<Integer> aList =
new CopyOnWriteArrayList<Integer>(); // LINE A
aList.addAll(Arrays.asList(10, 20, 30, 40));
System.out.println(aList);
}
}

A. When executed the program prints the following: [10, 20, 30, 40].
B. When executed the program prints the following:
CopyOnWriteArrayList.class .
C. The program does not compile and results in a compiler error in line
marked with comment LINE A .
D. When executed the program throws a run1me excep1on
ConcurrentModiﬁca1onExcep1on .

Answer
Choose the correct op%on based on this program:

class COWArrayListTest {
ArrayList<Integer> aList =
new CopyOnWriteArrayList<Integer>(); // LINE A
aList.addAll(Arrays.asList(10, 20, 30, 40));
System.out.println(aList);
}
}

A. When executed the program prints the following: [10, 20, 30, 40].
B. When executed the program prints the following:
CopyOnWriteArrayList.class .
C. The program does not compile and results in a compiler error in line
marked with comment LINE A .
D. When executed the program throws a run1me excep1on
ConcurrentModiﬁca1onExcep1on .

Explana%on
C . The program does not compile and results in a compiler
error in the line marked with comment LINE A.

The class CopyOnWriteArrayList does not inherit from
ArrayList , so an a;empt to assign a CopyOnWriteArrayList to
an ArrayList reference will result in a compiler error. Note that
the ArrayList suﬃx in the class named CopyOnWriteArrayList
could be misleading as these two classes do not share anIS-A
rela1onship.

Ques%on
What will this code segment print?

List<Integer> ints = Arrays.asList(1, 2, 3, 4, 5);
System.out.println(ints.parallelStream()
.ﬁlter(i -> (i % 2) == 0).sequenEal()
.isParallel());

A. Prints: 2 4
B. Prints: false
C. Prints: true
D. Cannot determine the output because the program has non-
determinis1c behaviour

Answer

List<Integer> ints = Arrays.asList(1, 2, 3, 4, 5);
System.out.println(ints.parallelStream()
.ﬁlter(i -> (i % 2) == 0).sequenEal()
.isParallel());

A. Prints: 2 4
B. Prints: false
C. Prints: true
D. Cannot determine the output because the program has non-
determinis1c behaviour

Explana%on
B. Prints false

Though the created stream is a parallel stream, the
call to the sequen1al() method has made the stream
sequen1al. Hence, the call isParallel() prints false .

Ques%on
Which one of the following methods return a Future object?

A. The overloaded replace() methods declared in the ConcurrentMap
interface
B. The newThread() method declared in the ThreadFactory interface
C. The overloaded submit() methods declared in the ExecutorService
interface
D. The call() method declared in the Callable interface

Answer
Which one of the following methods return a Future object?

interface
B. The newThread() method declared in the ThreadFactory interface
C. The overloaded submit() methods declared in the ExecutorService
interface
D. The call() method declared in the Callable interface

Explana%on
C . The overloaded submit() methods declared in
ExecutorService interface. The ExecutorService interface
extends the Executor interface and provides services such as
termina1on of threads and produc1on of Future objects. Some
tasks may take considerable execu1on 1me
to complete. So, when you submit a task to the executor
service, you get a Future object.

interface remove an element from the map and return the success
status (a Boolean value) or the removed value.

B. The new Thread() is the only method declared in the
ThreadFactory interface and it returns a Thread object as the return
value

D. The call() method declared in Callable interface returns the result
of the task it executed.

Ques%on
In your applica%on, there is a producer component that keeps adding
new items to a ﬁxed-size queue; the consumer component fetches
items from that queue. If the queue is full, the producer has to wait
for items to be fetched; if the queue is empty, the consumer has to
wait for items to be added.
Which one of the following u%li%es is suitable for synchronizing the
common queue for concurrent use by a producer and consumer?

A. ForkJoinPool
B. Future
C. Semaphore
D. TimeUnit

Answer
In your applica1on, there is a producer component that keeps adding
new items to a ﬁxed-size queue; the consumer component fetches
items from that queue. If the queue is full, the producer has to wait for
items to be fetched; if the queue is empty, the consumer has to wait
for items to be added.
Which one of the following u1li1es is suitable for synchronizing the
common queue for concurrent use by a producer and consumer?

A. ForkJoinPool
B. Future
C. Semaphore
D. TimeUnit

Explana%on
C. Semaphore

The ques1on is a classic producer–consumer problem that can be
solved by using semaphores. The objects of the synchronizer class
java.u1l.concurrent.Semaphore can be used to guard the common
queue so that the producer and consumer can synchronize their
access to the queue. Of the given op1ons, semaphore is the only
synchronizer ; other op1ons are unrelated to providing
synchronized access to a queue.

A.  ForkJoinPool provides help in running a ForkJoinTask in the context of
the Fork/Join framework.
B. Future represents the result of an asynchronous computa1on whose
result will be “available in the future once the computa1on is complete.”

D. TimeUnit is an enumera1on that provides support for diﬀerent 1me
units such as milliseconds, seconds, and days.

Ques%on

class StringConcatenator {
public sta%c String result = "";
public sta%c void concatStr(String str) {
result = result + " " + str;
}
}

class StringSplitAndConcatenate {
String words[] = "the quick brown fox jumps over the lazy dog".split(" ");
Arrays.stream(words).parallel().forEach(StringConcatenator::concatStr);
System.out.println(StringConcatenator.result);
}
}

A. over jumps lazy dog the brown fox quick the
B. the quick brown fox jumps over the lazy dog
C. Prints each word in new line
D. Cannot predict output

Answer

class StringConcatenator {
public sta1c String result = "";
public sta1c void concatStr(String str) {
result = result + " " + str;
}
}

class StringSplitAndConcatenate {
String words[] = "the quick brown fox jumps over the lazy dog".split(" ");
Arrays.stream(words).parallel().forEach(StringConcatenator::concatStr);
System.out.println(StringConcatenator.result);
}
}

A. over jumps lazy dog the brown fox quick the
B. the quick brown fox jumps over the lazy dog
C. Prints each word in new line

Explana%on

When the stream is parallel, the task is split into
mul1ple sub-tasks and diﬀerent threads execute it.
The calls to forEach(StringConcatenator::concatStr)
now access the globally accessible variable result
in StringConcatenator class. Hence it results in garbled
output.

Ques%on
What is the expected output of this code segment when invoked as follows:
java -ea AtomicIntegerTest

sta%c AtomicInteger ai = new AtomicInteger(10);
ai.incrementAndGet();
ai.getAndDecrement();
ai.compareAndSet(10, 11);
assert (ai.intValue() % 2) == 0;
System.out.println(ai);
}

A. Prints: 10 11
B. Prints: 10
C. It crashes by throwing an Asser1onError
D. Prints: 9

Answer
What is the expected output of this code segment when invoked as follows:
java -ea AtomicIntegerTest

sta%c AtomicInteger ai = new AtomicInteger(10);
ai.incrementAndGet();
ai.getAndDecrement();
ai.compareAndSet(10, 11);
assert (ai.intValue() % 2) == 0;
System.out.println(ai);
}

A. Prints: 10 11
B. Prints: 10
C. It crashes by throwing an Asser%onError
D. Prints: 9

Explana%on
C.  It crashes throwing an Asser1onError .
The ini1al value of AtomicInteger is 10. Its value is
incremented by 1 aNer calling incrementAndGet ().

ANer that, its value is decremented by 1 aNer calling
getAndDecrement ( ).

The method compareAndSet (10, 11) checks if the
current value is 10, and if so sets the atomic integer
variable to value 11.

Since the assert statement checks if the atomic
integer value % 2 is zero (that is, checks if it is an even
number), the assert fails and the program results in an
Asser1onError .

Ques%on
Consider that you are developing an cards game where a person starts the
game and waits for the other players to join. Minimum 4 players are
required to start the game and as soon as all the players are available the
game has to get started. For developing such kind of game applica%on which
of the following synchroniza%on technique would be ideal:

A. Semaphore
B. Exchanger
C. Runnable
D. CyclicBarrier

Answer
Consider that you are developing an cards game where a person starts the
game and waits for the other players to join. Minimum 4 players are
required to start the game and as soon as all the players are available the
game has to get started. For developing such kind of game applica%on which
of the following synchroniza%on technique would be ideal:

A. Semaphore
B. Exchanger
C. Runnable
D. CyclicBarrier

Explana%on
D. CyclicBarrier

CyclicBarrier helps provide a synchroniza1on point
where threads may need to wait at a predeﬁned
execu1on point un1l all other threads reach that
point.

A. A Semaphore controls access to shared resources.
A semaphore maintains a counter to specify number
of resources that the semaphore controls
B. The Exchanger class is meant for exchanging data
between two threads. This class is useful when two
threads need to synchronize between each other and
con1nuously
exchange data.
C. Runnable is an interface used in crea1ng threads

Ques%on
Consider the following program and choose the correct op%on describing its
behavior

public void run() {
System.out.print("ping ");
throw new IllegalStateExcep%on();
}
public sta%c void main(String []args) throws InterruptedExcep%on {
pingPong.start();
}
}

A.  This program results in a compiler error.
B.  Prints the ping pong and excep1on in any order
C.  This program throws a run1me error.
D.  Prints pong ping

Answer
Consider the following program and choose the correct op%on describing its
behavior

public void run() {
System.out.print("ping ");
throw new IllegalStateExcep1on();
}
public sta1c void main(String []args) throws InterruptedExcep1on {
pingPong.start();
}
}

A.  This program results in a compiler error.
B.  Prints the ping pong and excep%on in any order
C.  This program throws a run1me error.
D.  Prints pong ping

Explana%on
B. Prints the ping pong and excep1on in any order

The programs executes ﬁne and the exact output
cannot be predicted as the main thread and the
worker thread execute independently without any
coordina1on.

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