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- Java Multithreading Concurrency Interview Questions and Answers
- Interview Questions on MultiThreading
- Top 100 Java Interview Questions and Answers (Download PDF)
One of the good question on multithreading, I have recently faced in a Java interview.
We will also look into Concurrency interview questions and answers because both multithreading and concurrency go hand in hand. Thread is one of the popular topics in java interview questions. Here I am listing down most of the important java multithreading interview questions from interview perspective, but you should have good knowledge on java threads to deal with follow up questions.
Java Multithreading Concurrency Interview Questions and Answers
Multithreading and Synchronization are considered as the typical chapter in java programming. In game development companies, multithreading related interview questions are asked mostly. A list of frequently asked java multithreading and concurrency interview questions is given below.
Multithreading is a process of executing multiple threads simultaneously. Multithreading is used to obtain the multitasking. It consumes less memory and gives the fast and efficient performance. Its main advantages are:. A thread is a lightweight subprocess. It is a separate path of execution because each thread runs in a different stack frame. A process may contain multiple threads. Threads share the process resources, but still, they execute independently.
The wait method is provided by the Object class in Java. This method is used for inter-thread communication in Java. The java. Its syntax is given below. We must call the wait method otherwise it will throw java. IllegalMonitorStateException exception. Moreover, we need wait method for inter-thread communication with notify and notifyAll.
Therefore It must be present in the synchronized block for the proper and correct communication. Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks.
The scheduler then determines which task should execute next, based on priority and other factors. In Context switching the state of the process or thread is stored so that it can be restored and execution can be resumed from the same point later. Context switching enables the multiple processes to share the same CPU. The join method waits for a thread to die.
In other words, it causes the currently running threads to stop executing until the thread it joins with completes its task. Join method is overloaded in Thread class in the following ways.
The sleep method in java is used to block a thread for a particular time, which means it pause the execution of a thread for a specific time. There are two methods of doing so. When we call the sleep method, it pauses the execution of the current thread for the given time and gives priority to another thread if available.
Moreover, when the waiting time completed then again previous thread changes its state from waiting to runnable and comes in running state, and the whole process works so on till the execution doesn't complete.
No, we cannot restart the thread, as once a thread started and executed, it goes to the Dead state. Therefore, if we try to start a thread twice, it will give a runtimeException "java. Consider the following example. Yes, calling run method directly is valid, but it will not work as a thread instead it will work as a normal object. There will not be context-switching between the threads. When we call the start method, it internally calls the run method, which creates a new stack for a thread while directly calling the run will not create a new stack.
The daemon threads are the low priority threads that provide the background support and services to the user threads. There are two methods for daemon thread available in the Thread class:. No, if you do so, it will throw IllegalThreadStateException.
Therefore, we can only create a daemon thread before starting the thread. The shutdown hook is a thread that is invoked implicitly before JVM shuts down. So we can use it to perform clean up the resource or save the state when JVM shuts down normally or abruptly.
We can add shutdown hook by using the following method:. We should interrupt a thread when we want to break out the sleep or wait state of a thread. Synchronization is the capability to control the access of multiple threads to any shared resource. It is used:. Synchronization can be achieved in three ways:. The Synchronized block can be used to perform synchronization on any specific resource of the method. Only one thread at a time can execute on a particular resource, and all other threads which attempt to enter the synchronized block are blocked.
You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it. If you make any static method as synchronized, the lock will be on the class not on the object. If we use the synchronized keyword before a method so it will lock the object one thread can access an object at a time but if we use static synchronized so it will lock a class one thread can access a class at a time.
More details. The notify is used to unblock one waiting thread whereas notifyAll method is used to unblock all the threads in waiting state. Deadlock is a situation in which every thread is waiting for a resource which is held by some other waiting thread. In this situation, Neither of the thread executes nor it gets the chance to be executed.
Instead, there exists a universal waiting state among all the threads. Deadlock is a very complicated situation which can break our code at runtime. We can detect the deadlock condition by running the code on cmd and collecting the Thread Dump, and if any deadlock is present in the code, then a message will appear on cmd.
In Java, when we create the threads, they are supervised with the help of a Thread Scheduler, which is the part of JVM. Thread scheduler is only responsible for deciding which thread should be executed. Thread scheduler uses two mechanisms for scheduling the threads: Preemptive and Time Slicing. Yes, in multithreaded programming every thread maintains its own or separate stack area in memory due to which every thread is independent of each other.
If a method or class object can be used by multiple threads at a time without any race condition, then the class is thread-safe. Thread safety is used to make a program safe to use in multithreaded programming. It can be achieved by the following ways:. A Race condition is a problem which occurs in the multithreaded programming when various threads execute simultaneously accessing a shared resource at the same time.
The proper use of synchronization can avoid the Race condition. Volatile keyword is used in multithreaded programming to achieve the thread safety, as a change in one volatile variable is visible to all other threads so one variable can be used by one thread at a time. Concurrency API can be developed using the class and interfaces of java. Concurrent package. There are the following classes and interfaces in java.
The Executor Interface provided by the package java. The execute method of Executor interface is used to execute some given command. The syntax of the execute method is given below.
BlockingQueue is the subinterface of Queue that supports the operations such as waiting for the space availability before inserting a new value or waiting for the queue to become non-empty before retrieving an element from it.
The Callable interface and Runnable interface both are used by the classes which wanted to execute with multiple threads. However, there are two main differences between the both :. Lock interface is used as the synchronization mechanism. It works similar to the synchronized block. There are a few differences between the lock and synchronized block that are given below. The ExecutorService Interface is the subinterface of Executor interface and adds the features to manage the lifecycle.
Synchronous programming: In Synchronous programming model, a thread is assigned to complete a task and hence thread started working on it, and it is only available for other tasks once it will end the assigned task. Asynchronous Programming: In Asynchronous programming, one job can be completed by multiple threads and hence it provides maximum usability of the various threads. Java Callable interface: In Java5 callable interface was provided by the package java. It is similar to the Runnable interface but it can return a result, and it can throw an Exception.
It also provides a run method for execution of a thread. Java Callable can return any object as it uses Generic.
Java Future interface: Java Future interface gives the result of a concurrent process. The Callable interface returns the object of java. ExecutorServcie and ScheduledExecutorService both are the interfaces of java.
Concurrent package but scheduledExecutorService provides some additional methods to execute the Runnable and Callable tasks with the delay or every fixed time period. Java FutureTask class provides a base implementation of the Future interface. The result can only be obtained if the execution of one task is completed, and if the computation is not achieved then get method will be blocked.
If the execution is completed, then it cannot be re-started and can't be canceled. JavaTpoint offers too many high quality services. Mail us on hr javatpoint.
Interview Questions on MultiThreading
Hi Javin, Excellent piece! I have a question tho, is there a decent step by step guide out there for working through threading in Java? Reason I ask is, I find it is helpful to do assignments with the concept and while I do have knowledge about threading in java I am just not sure I have a complete picture. Again excellent work, I hope to see more articles! I would argue that for practical knowledge, familiarity with the java.
Interview Questions on MultiThreading are among my favourites. With a number of complex scenarios possible, this topic can seriously test an interview candidate. This interview guide attempt to explain the important concepts with a number of examples. Threads allow Java code to run in parallel. We are creating a Cricket Statistics Application. Let's say the steps involved in the application are.
Multithreading and Synchronization are considered as the typical chapter in java programming. In game development companies, multithreading related interview questions are asked mostly. A list of frequently asked java multithreading and concurrency interview questions is given below. Multithreading is a process of executing multiple threads simultaneously. Multithreading is used to obtain the multitasking.
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Top 100 Java Interview Questions and Answers (Download PDF)
Threads consumes CPU in best possible manner, hence enables multi processing. Multi threading reduces idle time of CPU which improves performance of application. Advantage of Thread : Suppose one thread needs 10 minutes to get certain task, 10 threads used at a time could complete that task in 1 minute, because threads can run parallely. Thread are subdivision of Process. One or more Threads runs in the context of process.
A Thread is a concurrent unit of execution. We can say that it is part of the process which can easily run concurrently with other parts of the process. Multithreading is a process of executing more than one thread simultaneously. The main advantage is:.