Here is our list of top questions from Java thread, concurrency and multi-threading. You can use this list to prepare well for your Java interview.
What is Thread in Java?
Thread is an independent path of execution. It’s way to take advantage of multiple CPU available in a machine. By employing multiple threads you can speed up CPU bound task. For example, if one thread takes 100 millisecond to do a job, you can use 10 thread to reduce that task into 10 millisecond. Java provides excellent support for multi-threading at language level, and its also one of strong selling point. For more details see here.
Difference between Thread and Process in Java?
Thread is subset of Process, in other words one process can contain multiple threads. Two process runs on different memory space, but all threads share same memory space. Don’t confuse this with stack memory, which is different for different thread and used to store local data to that thread. For more detail see this answer.
How do you implement Thread in Java?
At language level, there are two ways to implement Thread in Java. An instance of java.lang.Thread represent a thread but it need a task to execute, which is instance of interface java.lang.Runnable. Since Thread class itself implement Runnable, you can override run() method either by extending Thread class or just implementing Runnable interface. For detailed answer and discussion see this article.
When to use Runnable vs Thread in Java?
This is follow-up of previous multi-threading interview question. As we know we can implement thread either by extending Thread class or implementing Runnable interface, question arise, which one is better and when to use one? This question will be easy to answer, if you know that Java programming language doesn’t support multiple inheritance of class, but it allows you to implement multiple interface. Which means, its better to implement Runnable than extends Thread, if you also want to extend another class e.g. Canvas or CommandListener. For more points and discussion you can also refer this post.
Difference between start() and run() method of Thread class?
One of trick Java question from early days, but still good enough to differentiate between shallow understanding of Java threading model start() method is used to start newly created thread, while start() internally calls run() method, there is difference calling run() method directly. When you invoke run() as normal method, its called in the same thread, no new thread is started, which is the case when you call start() method. Read this answer for much more detailed discussion.
Difference between Runnable and Callable in Java?
Both Runnable and Callable represent task which is intended to be executed in separate thread. Runnable is there from JDK 1.0, while Callable was added on JDK 1.5. Main difference between these two is that Callable’s call() method can return value and throw Exception, which was not possible with Runnable’s run() method. Callable return Future object, which can hold result of computation. See my blog post on same topic for more in-depth answer of this question.
Difference between CyclicBarrier and CountDownLatch in Java?
Though both CyclicBarrier and CountDownLatch wait for number of threads on one or more events, main difference between them is that you can not re-use CountDownLatch once count reaches to zero, but you can reuse same CyclicBarrier even after barrier is broken. See this answer for few more points and sample code example.
What is Java Memory model?
Java Memory model is set of rules and guidelines which allows Java programs to behave deterministically across multiple memory architecture, CPU, and operating system. It’s particularly important in case of multi-threading. Java Memory Model provides some guarantee on which changes made by one thread should be visible to others, one of them is happens-before relationship. This relationship defines several rules which allows programmers to anticipate and reason behaviour of concurrent Java programs. For example, happens-before relationship guarantees :
Each action in a thread happens-before every action in that thread that comes later in the program order, this is known as program order rule.
An unlock on a monitor lock happens-before every subsequent lock on that same monitor lock, also known as Monitor lock rule.
A write to a volatile field happens-before every subsequent read of that same field, known as Volatile variable rule.
A call to Thread.start on a thread happens-before any other thread detects that thread has terminated, either by successfully return from Thread.join() or by Thread.isAlive() returning false, also known as Thread start rule.
A thread calling interrupt on another thread happens-before the interrupted thread detects the interrupt( either by having InterruptedException thrown, or invoking isInterrupted or interrupted), popularly known as Thread Interruption rule.
The end of a constructor for an object happens-before the start of the finalizer for that object, known as Finalizer rule.
If A happens-before B, and B happens-before C, then A happens-before C, which means happens-before guarantees Transitivity.
I strongly suggest to read Chapter 16 of Java Concurrency in Practice to understand Java Memory model in more detail.
What is volatile variable in Java?
volatile is a special modifier, which can only be used with instance variables. In concurrent Java programs, changes made by multiple threads on instance variables is not visible to other in absence of any synchronizers e.g. synchronized keyword or locks. Volatile variable guarantees that a write will happen before any subsequent read, as stated “volatile variable rule” in previous question. Read this answer to learn more about volatile variable and when to use them.
What is thread-safety? is Vector a thread-safe class?
(Yes, see details)
Thread-safety is a property of an object or code which guarantees that if executed or used by multiple thread in any manner e.g. read vs write it will behave as expected. For example, a thread-safe counter object will not miss any count if same instance of that counter is shared among multiple threads. Apparently, you can also divide collection classes in two category, thread-safe and non-thread-safe. Vector is indeed a thread-safe class and it achieves thread-safety by synchronizing methods which modifies state of Vector, on the other hand, its counterpart ArrayList is not thread-safe.
What is race condition in Java? Given one example?
Race condition are cause of some subtle programming bugs when Java programs are exposed to concurrent execution environment. As name suggests, race condition occurs due to race between multiple threads, if a thread which is supposed to execute first lost the race and executed second, behaviour of code changes, which surface as non-deterministic bugs. This is one of the hardest bugs to find and re-produce because of random nature of racing between threads. One example of race condition is out-of-order processing, see this answer for some more example of race conditions in Java programs.
How to stop thread in Java?
I always said that Java provides rich APIs for everything but ironically Java doesn’t provide a sure shot way of stopping thread. There was some control methods in JDK 1.0 e.g. stop(), suspend() and resume() which was deprecated in later releases due to potential deadlock threats, from then Java API designers has not made any effort to provide a consistent, thread-safe and elegant way to stop threads. Programmers mainly rely on the fact that thread stops automatically as soon as they finish execution of run() or call() method. To manually stop, programmers either take advantage of volatile boolean variable and check in every iteration if run method has loops or interrupt threads to abruptly cancel tasks. See this tutorial for sample code of stopping thread in Java.
What happens when an Exception occurs in a thread?
This is one of the good tricky Java question I have seen on interviews. In simple words, If not caught thread will die, if an uncaught exception handler is registered then it will get a call back. Thread.UncaughtExceptionHandler is an interface, defined as nested interface for handlers invoked when a Thread abruptly terminates due to an uncaught exception. When a thread is about to terminate due to an uncaught exception the Java Virtual Machine will query the thread for its UncaughtExceptionHandler using Thread.getUncaughtExceptionHandler() and will invoke the handler’s uncaughtException() method, passing the thread and the exception as arguments.
How do you share data between two thread in Java?
You can share data between threads by using shared object, or concurrent data-structure like BlockingQueue. See this tutorial to learn inter thread communication in Java. It implements Producer consumer pattern using wait and notify methods, which involves sharing objects between two threads.
Difference between notify and notifyAll in Java?
This is another tricky questions from core Java interviews, since multiple threads can wait on single monitor lock, Java API designer provides method to inform only one of them or all of them, once waiting condition changes, but they provide half implementation. There notify() method doesn’t provide any way to choose a particular thread, that’s why its only useful when you know that there is only one thread is waiting. On the other hand, notifyAll() sends notification to all threads and allows them to compete for locks, which ensures that at-least one thread will proceed further. See my blog post on similar topic for more detailed answer and code example.
Why wait, notify and notifyAll are not inside thread class?
This is a design related question, which checks what candidate thinks about existing system or does he ever thought of something which is so common but looks in-appropriate at first. In order to answer this question, you have to give some reasons why it make sense for these three method to be in Object class, and why not on Thread class. One reason which is obvious is that Java provides lock at object level not at thread level. Every object has lock, which is acquired by thread. Now if thread needs to wait for certain lock it make sense to call wait() on that object rather than on that thread. Had wait() method declared on Thread class, it was not clear that for which lock thread was waiting. In short, since wait, notify and notifyAll operate at lock level, it make sense to defined it on object class because lock belongs to object. You can also see this article for more elaborate answer of this question.
What is ThreadLocal variable in Java?
ThreadLocal variables are special kind of variable available to Java programmer. Just like instance variable is per instance, ThreadLocal variable is per thread. It’s a nice way to achieve thread-safety of expensive-to-create objects, for example you can make SimpleDateFormat thread-safe using ThreadLocal. Since that class is expensive, its not good to use it in local scope, which requires separate instance on each invocation. By providing each thread their own copy, you shoot two birds in one arrow. First, you reduce number of instance of expensive object by reusing fixed number of instances, and Second, you achieve thread-safety without paying cost of synchronization or immutability. Another good example of thread local variable is ThreadLocalRandom class, which reduces number of instances of expensive-to-create Random object in multi-threading environment. See this answer to learn more about thread local variables in Java.
What is FutureTask in Java?
FutureTask represents a cancellable asynchronous computation in concurrent Java application. This class provides a base implementation of Future, with methods to start and cancel a computation, query to see if the computation is complete, and retrieve the result of the computation. The result can only be retrieved when the computation has completed; the get methods will block if the computation has not yet completed. A FutureTask object can be used to wrap a Callable or Runnable object. Since FutureTask
also implements Runnable, it can be submitted to an Executor for execution.
Difference between interrupted and isInterrupted method in Java?
Main difference between interrupted() and isInterrupted() is that former clears the interrupt status while later does not. The interrupt mechanism in Java multi-threading is implemented using an internal flag known as the interrupt status. Interrupting a thread by calling Thread.interrupt() sets this flag. When interrupted thread checks for an interrupt by invoking the static method Thread.interrupted(), interrupt status is cleared. The non-static isInterrupted() method, which is used by one thread to query the interrupt status of another, does not change the interrupt status flag. By convention, any method that exits by throwing an InterruptedException clears interrupt status when it does so. However, it’s always possible that interrupt status will immediately be set again, by another thread invoking interrupt.
Why wait and notify method are called from synchronized block?
Main reason for calling wait and notify method from either synchronized block or method is that it made mandatory by Java API. If you don’t call them from synchronized context, your code will throw IllegalMonitorStateException. A more subtle reason is to avoid race condition between wait and notify calls. To learn more about this, check my similarly titled post here.
Why you should check condition for waiting in a loop?
Its possible for a waiting thread to receive false alerts and spurious wake up calls, if it doesn’t check the waiting condition in loop, it will simply exit even if condition is not met. As such, when a waiting thread wakes up, it cannot assume that the state it was waiting for is still valid. It may have been valid in the past, but the state may have been changed after the notify() method was called and before the waiting thread woke up. That’s why it always better to call wait() method from loop, you can even create template for calling wait and notify in Eclipse. To learn more about this question, I would recommend you to read Effective Java items on thread and synchronization.
Difference between synchronized and concurrent collection in Java?
Though both synchronized and concurrent collection provides thread-safe collection suitable for multi-threaded and concurrent access, later is more scalable than former. Before Java 1.5, Java programmers only had synchronized collection which becomes source of contention if multiple thread access them concurrently, which hampers scalability of system. Java 5 introduced concurrent collections like ConcurrentHashMap, which not only provides thread-safety but also improves scalability by using modern techniques like lock stripping and partitioning internal table. See this answer for more differences between synchronized and concurrent collection in Java.
Difference between Stack and Heap in Java?
Why do someone this question as part of multi-threading and concurrency? because Stack is a memory area which is closely associated with threads. To answer this question, both stack and heap are specific memories in Java application. Each thread has their own stack, which is used to store local variables, method parameters and call stack. Variable stored in one Thread’s stack is not visible to other. On other hand, heap is a common memory area which is shared by all threads. Objects whether local or at any level is created inside heap. To improve performance thread tends to cache values from heap into their stack, which can create problems if that variable is modified by more than one thread, this is where volatile variables comes in picture. volatile suggest threads to read value of variable always from main memory. See this article to learn more about stack and heap in Java to answer this question in greater detail.
What is thread pool? Why should you thread pool in Java?
Creating thread is expensive in terms of time and resource. If you create thread at time of request processing it will slow down your response time, also there is only a limited number of threads a process can create. To avoid both of these issue, a pool of thread is created when application starts-up and threads are reused for request processing. This pool of thread is known as “thread pool” and threads are known as worker thread. From JDK 1.5 release, Java API provides Executor framework, which allows you to create different types of thread pools e.g. single thread pool, which process one task at a time, fixed thread pool (a pool of fixed number of thread) or cached thread pool (an expandable thread pool suitable for applications with many short lived tasks). See this article to learn more about thread pools in Java to prepare detailed answer of this question.
Write code to solve Producer Consumer problem in Java?
Most of the threading problem you solved in real world are of category of Producer consumer pattern, where one thread is producing task and other thread is consuming that. You must know how to do inter thread communication to solve this problem. At lowest level, you can use wait and notify to solve this problem, and at high level you can leverage Semaphore or BlockingQueue to implement Producer consumer pattern, as shown in this tutorial.
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