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Concurrency in Java

  • What is concurrency?

    • Concurrency is the ability of different parts or units of a program, algorithm, or problem to be executed out-of-order or in partial order, without affecting the outcome

  • How are concurrent programs executed on a single CPU?

    • Using time-slices

  • Distinguish between concurrency and parallelism

    • Concurrency can be thought of as a way of structuring a program so that two threads can start, run and complete in overlapping time periods
    • Parallelism is the runtime behavior of some concurrent programs where two or more threads are running simultaneously

  • Differentiate processes and threads

    • Can a process access data of another process?
    • Can a thread access data of another thread?

  • What are the two main problems when dealing with thread?

    • Visibility
    • Access

  • What is the visibility problem?

  • What is the access problem?

  • What does access and visibility problems lead to?

  • What is starvation

  • What is fairness

  • What are the three causes of starvation in Java

  • What is multitasking

  • What is multi-threading

  • What is the lineage of multi-threading and multitasking

    • Time sharing system

  • Draw the life cycle of a Java thread

    • Pasted image 20230702130830

  • What are the two states of an active thread?

    • Running
    • Runnable

  • How to move from running to runnable?

    • using the yield() method

  • What does yield do?

    • yield indicates to the scheduler that the current thread is willing to relinquish its current use of the processor but it would like to get back to running ASAP
    • The scheduler is free to schedule it whenever it wants

  • What are the two types of wait in Java threads?

  • What is the range of Java thread priorities and give the name of the constants

    • MIN_PRIORITY => 1
    • MAX_PRIORITY => 10

  • What is the value given by default to thread and the name of the constant that identifies it

    • NORM_PRIORITY => 5

  • Can thread priorities guarantee the order of execution

    • No. It is platform-dependent

  • What method is used to set the priority of a thread?

    • setPriority()

  • How do you create a concurrent program in Java using the Runnable interface?

    • Create a class that implements the Runnable interface and implement the run() method
    • Create a Thread object with the constructor signature of new Thread(Runnable obj, String threadName)
    • Call the start() method

  • How do you create a concurrent program in Java extending the Thread class?

    • Create a class that extends the Thread class
    • Override the run() method
    • Create an object of the above class and call the start() method

  • What is the preferred way to do concurrency in Java - Thread vs Runnable?

    • Implementing Runnable is the preferred way to do it
    • Reasons
      • From a OOP POV we are not trying to overwrite or extend any behavior of the Thread class. Rather we just giving it something to run.
      • So Composition is the proper way to do it.
      • Another key things is Inheritance. Java doesn’t support multiple Inheritance. So if we inherit from the Thread class we can’t inherit from any other classes. But this is not the case if we are implementing the Runnable interface
      • class MyClass extends MyParent implements Runnable is possible

  • Mention some of the methods on the Thread class

    • suspend
    • stop
    • resume
    • wait
    • notify

  • How is suspend() and resume() used

    • The thread is suspended until the resume() method is invoked

  • How is wait() and notify() used

    • The thread is suspended until the notify() method is invoked on the object’s monitor

  • What is the difference in using wait(), notify() and suspend(), resume()

    • suspend(), resume() stop and start the thread
    • The purpose of wait(), notify() is completely different. wait() is used to wait for something to happen. It is usually used for synchronization

  • How do you scale when you have many threads to execute?

    • java.lang.Runnable doesn’t scale well
    • One thread for each task may add so much overhead that the point of multi-threading is moot
    • The solution is the use of thread pools
    • In Java this can be accomplished by implementing java.util.concurrent.Executor interface
    • java.util.concurrent.ExecutorService interface is used to control the execution of the thread pool

  • How do you declare a thread-safe method in java? - publicsynchronizedvoid myMethod()

  • How does a synchronized method guarantee mutual exclusion? - Locks

  • The lock acquired for an instance method is ? - object

  • The lock acquired for an static method is ? - Class

    • What are synchronized blocks?
      • They are a similar concept to a synchronized function
      • The lock of the object reference given will be acquired and mutual exclusion will be ensured

  • Differentiate between Lock and ReentrantLock - Lock is an interface - ReentrantLock is a concrete class that implements Lock plus some additional methods - ReentrantLock, as suggested by its name, is re-entrant meaning a process can acquire the lock held by itself without deadlocking

    • How to create fair locks in java?
      • Instantiate the ReentrantLock object with true as a parameter

  • We know the signature of the run is public void run() meaning nothing is returned. Suppose you want the results of the computation to be returned. How to accomplish this? - Instead of using the Runnable interface, implement the Callable interface

  • What is the Future object?

    • Used to check the status of a Callable and retrieve the results

  • What method would you use to get the results of a Callable

    • get()

  • What is the syntax used for Future with Callable?

    • Future<type> myVar = executor.submit(threadObj)

  • What is the way to implement truly parallel computation in java?

    • Using the ForkJoinTask abstract class

  • What are the two concrete classes under ForkJoinTask?

    • RecursiveAction
    • RecursiveTask

  • What is the difference between RecursiveAction and RecursiveTask

    • RecursiveTask returns the results

  • What does the invoke() method do?

    • It first calls the fork() and then waits to join()

  • What is the method that is overridden in the ForkJoinTask?

    • The compute method should be overridden

  • We know that to efficiently use many thread we use thread pools. How to use those when using the Fork-Join framework?

    • ForkJoinPool should be used