11. Concurrency

Question 1

What does the java.util.concurrent.atomic package enable?

Answer 1

The java.util.concurrent.atomic package enables multithreaded applications to safely access
individual variables without locking,

Question 2

What is an atomic operation?

Answer 2

An atomic operation is one that, for all intents and purposes appears to happen all at once.

Question 3

Which classes does the atomic package provide?

Answer 3

The atomic package provides several classes for different data types, such as AtomicInteger, AtomicLong, AtomicBoolean, and AtomicReference to name a few.

Question 4

Where does CAS stand for?

Answer 4

CAS stands for Compare And Swap

Question 5

Wat does CAS do?

Answer 5

• The value is stored in count is copied to a temporary variable
• The temporary variable is incremented
• Compare the value currently in count with the original value. If it is unchanged, then swap
  the old value for the new value.
  Step 3 happens atomically, if step 3 find that some other thread has already modified the value of count, then repeat step 1-3 until we increment the field without interference.

Question 6

Which method does provides the CAS behaviour?

Answer 6

The central method in a class like AtomicInteger is the boolean compareAndSet(int expect, int
update) method which provides the CAS behavior

Question 7

What does the lock package provide?

Answer 7

• The ability to duplicate traditional synchronized blocks
• Nonblock scoped locking – obtain a lock in one method and release it in another
• Multiple wait/notify/notifyAll pools per lock – threads can select which pool they wait on
• The ability to attempt to acquire a lock and take an alternative action if locking fails
• An implementation of a multiple-reader, single-writer lock

Question 8

Why choose a lock above a synchronization?

Answer 8

The reason for you to choose to use a Lock instance instead of a traditional synchronization is that one of the very powerful features is the ability to attempt to acquire a lock. You can process a different resource (lock) and come back to the failed lock later instead of just waiting for a lock to be released and thereby making more efficient use of system resources.

Question 9

What does the tryLock() method do?

Answer 9

The tryLock() method attempts to lock the Lock instance immediately. It returns true if the locking succeeds, false if Lock is already locked. This method never blocks.

Question 10

When does a IllegalMonitorStateException get thrown?

Answer 10

If a thread attempts to release a lock that it does not own, an
IllegalMonitorStateException will be thrown.

Question 11

What is a reentrantReadWriteLock?

Answer 11

A reentrantReadWriteLock is not actually a Lock; it implements the ReadWriteLock interface. What a ReentrantReadWriteLock does is produce two specialized Lock instances, one to a read lock and the other to a write lock. What makes these locks unique is that multiple threads can hold the read lock at the same time, but only one thread can hold the write lock at a time.

Question 12

Which concurrent collections contains the concurrent package?

Answer 12

• ConcurrentHashMap
• ConcurrentLinkedDeque
• ConcurrentLinkedQueue
• ConcurrentSkipListMap
• ConcurrentSkipListSet

Question 13

What is a blocking queue?

Answer 13

A BlockingQueue is a type of shared collection that is used to exchange data between 2 or more threads while causing one or more of the threads to wait until the point in time when the data can be exchanged.

Question 14

What is the producer-consumer scenario?

Answer 14

In a producer-consumer scenario, one thread produces data, then adds it to the queue, and another thread must consume the data from the queue.

Question 15

What are the BlockingQueue implementations?

Answer 15

• ArrayBlockingQueue
• LinkedBlockingDeQue
• LinkedBlockingQueue
• PriorityBlockingQueue
• DelayQueue
• LinkedTransferQueue
• SynchronousQueue

Question 16

How does a blocking queue behave?

Answer 16

A blocking collection, depending on the method being called, may cause a thread to block until another thread calls a corresponding method on the collection. Don’t call a blocking operation in a thread unless it is safe for that thread to block.

Question 17

What does CyclicBarrier do?

Answer 17

Whereas blocking queues force threads to wait based on capacity or until a method is called on the queue, a CyclicBarrier can force threads to wait at a specific point in the execution until all threads reach that point before continuing.

Question 18

What do executers do?

Answer 18

Executors (and the ThreadPools used by them) help meet two of the same needs as Threads do:

• Creating and scheduling some java code for executing
• Optimizing the execution of that code for the hardware resources you have available.

Question 19

What is an executor?

Answer 19

An Executor (high-level multithreading) is an alternative to starting new threads.

Question 20

Which three types of ExecutorService can be created by factory methods?

Answer 20

• Cached Thread pool executors
• Fixed Thread pool executors
• Single Thread pool executors

Question 21

What does a Cached Thread pool do?

Answer 21

it will create new threads as they are needed and reuse threads that have become free.
o Threads that have been idle for 60 seconds are removed from the pool
o Without some type of external limitation, a cached thread pool may be used to
create more threads than your system can handle
 ExecutorService ex = Executors.newCachedThreadPool()

Question 22

What does a Fixed Thread pool do?

Answer 22

it is constructed using a numeric argument that specifies the number of threads used to execute tasks.
o It prevents an application from overloading a system with too many threads.

o Tasks that cannot be executed immediately are placed on an unbounded queue for
later execution
 ExecutorService ex = Executors.newFixedThreadPool(4);

Question 23

What does a Single Thread pool do?

Answer 23

it uses a single thread to execute tasks
o Tasks that cannot be executed immediately are placed on an unbounded queue for
later execution.
o Unlike a fixed thread pool executor with size of 1, a single thread executor prevents
any adjustments to the number of threads in the pool
 ExecutorService ex = Executors.newSingleThreadExecutor();

Question 24

What does a Scheduled Thread pool do?

Answer 24

This enables tasks to be executed after a delay or at repeating intervals.

Question 25

What is the benefit of using a Callable?

Answer 25

The primary benefit of using a Callable is the ability to return a result.

Question 26

What is ForkJoinPool?

Answer 26

You will typically submit a single task to a ForkJoinPool and await its completion.

Question 27

What is ForkJoinTask?

Answer 27

ForkJoinTask instance is created to represent the task that should be accomplished. This is different
from other executor services that primarily used either Runnable or Callable. A ForkJoinTask concrete
subclass has many methods, but the following are important: compute(), fork() and join().

Question 28

What is a RecursiveAction?

Answer 28

If you need to create a ForkJoinTask that does not return a result, then you should subclass
ResurciveAction. ResurciveAction extends ForkJoinTasl and has a single abtract compute method.

Question 29

What is a RecursiveTask?

Answer 29

If you need to create a ForkJoinTask that does return a result, then you should subclass
RecursiveTask.