Original link: http://www.importnew.com/12773.html
This article was translated from javarevisited by ImportNew - Li Guang. Welcome to the Java team. For reprints, please see the requirements at the end of the article.
Whether you're a new programmer or a veteran, you must have faced questions about threads in your interviews. An important feature of the Java language is the built-in support for concurrency, which makes Java very popular among enterprises and programmers. Most of the well-paid Java development positions require developers to be proficient in multi-threading technology and have rich experience in Java program development, debugging, and optimization, so thread-related issues are often asked in interviews.
In a typical Java interview, the interviewer will start with the basic concepts of threads, such as: why do you need to use threads, how to create threads, and what is the best way to create threads (eg: inheriting the thread class or calling the Runnable interface), and then Gradually ask concurrency issues like what challenges are encountered in the process of concurrent programming in Java, the Java memory model, which higher-order concurrency tools have been introduced in JDK1.5, the design patterns commonly used in concurrent programming, and classic multi-threading problems such as producer consumption Writers, Dining Philosophers, Readers, or Simple Bounded Buffer Problems. Just knowing the basic concepts of threads is not enough, you must know how to deal with concurrency issues such as deadlocks, race conditions, memory conflicts and thread safety. With these skills in hand, you can easily handle multi-threaded and concurrent interviews.
It is normal for many Java programmers to read the interview questions before the interview. Because collecting interview questions and exercises takes time, I collected 50 popular questions about Java multithreading and concurrency from many interviewers. I only collected relatively new interview questions and did not provide all the answers. You must be smart enough to know these questions. If you encounter a question that you don't understand, you can use Google to find the answer. If you really can't find the answer, you can ask me for help in the comments of the article. You can also find some answers in this Java thread Q&A Top 12.
50 Java Threading Interview Questions
Below are popular interview questions related to Java threading that you can use to prepare well for your interview.
1) What is a thread?
Thread is the smallest unit that the operating system can perform operation scheduling. It is included in the process and is the actual operation unit in the process. Programmers can use it for multiprocessor programming, and you can use multithreading to speed up computationally intensive tasks. For example, if it takes 100 milliseconds for one thread to complete a task, it only takes 10 milliseconds to complete the task with ten threads. Java's excellent support for multithreading at the language level is also a great selling point. For more details click here.
2) What is the difference between a thread and a process?
Threads are a subset of processes, and a process can have many threads, each executing a different task in parallel. Different processes use different memory spaces, and all threads share the same memory space. Don't confuse it with stack memory, each thread has a separate stack memory for storing local data. For more details click here.
3) How to implement threads in Java?
At the language level there are two ways. An instance of the java.lang.Thread class is a thread, but it needs to call the java.lang.Runnable interface to execute. Since the thread class itself is the Runnable interface to call, you can inherit the java.lang.Thread class or directly call the Runnable interface to reset Write the run() method to implement the thread. For more details click here.
4) Use Runnable or Thread?
This question is a follow-up to the previous question. Everyone knows that we can implement threads by inheriting the Thread class or calling the Runnable interface. The question is, which method is better? When to use it? This question is easy to answer if you know that Java does not support multiple inheritance of classes, but allows you to call multiple interfaces. So if you want to inherit other classes, of course, call the Runnable interface. For more details click here.
6) What is the difference between start() and run() methods in Thread class?
This question is often asked, but it still distinguishes the interviewee's understanding of the Java threading model. The start() method is used to start the newly created thread, and start() internally calls the run() method, which is different from calling the run() method directly. When you call the run() method, it will only be called in the original thread. If no new thread is started, the start() method will start a new thread. For more discussion click here
7) What is the difference between Runnable and Callable in Java?
Both Runnable and Callable represent tasks to be executed in different threads. Runnable has been available since JDK1.0, and Callable was added in JDK1.5. The main difference between them is that Callable's call() method can return values and throw exceptions, while Runnable's run() method does not have these functions. Callable can return a Future object loaded with the result of the calculation. My blog has more detailed instructions.
8) What is the difference between CyclicBarrier and CountDownLatch in Java?
Both CyclicBarrier and CountDownLatch can be used to make a group of threads wait for other threads. Unlike CyclicBarrier, CountdownLatch cannot be reused. Click here for more information and sample code.
9) What is the Java memory model?
The Java memory model specifies and guides Java programs to behave deterministically across different memory architectures, CPUs, and operating systems. It is especially important in multithreaded situations. The Java memory model guarantees that changes made by one thread can be seen by other threads, and there is a happen-before relationship between them. This relationship defines some rules to allow programmers to think more clearly when programming concurrently. For example, a happen-before relationship ensures that:
Code within a thread can be executed sequentially, which is called the program order rule.
For the same lock, an unlocking operation must occur before another locking operation that occurs later in time, also known as monitor locking rules.
The previous write operation to volatile precedes the next volatile read operation, also known as the volatile variable rule.
Any operation within a thread must follow the thread's start() call, also known as thread start rules.
All operations of a thread will be terminated before the thread terminates, the thread termination rule.
The finalization operation of an object must be completed after the object is constructed, which is also called the object finalization rule.
Transmissibility
I strongly recommend that you read Chapter 16 of Java Concurrent Programming in Practice to deepen your understanding of the Java memory model.
10) What is a volatile variable in Java?
volatile is a special modifier that can only be used by member variables. In the absence of synchronization classes in Java concurrent programs, multi-thread operations on member variables are transparent to other threads. Volatile variables can guarantee that the next read operation will occur after the previous write operation, which is the volatile variable rule in the previous question. Click here to see more about volatile.
11) What is thread safety? Is Vector a thread safe class? (see here for details)
If your code is in a process with multiple threads running at the same time, these threads may be running this code at the same time. If the result of each run is the same as the result of a single-threaded run, and the values of other variables are the same as expected, it is thread-safe. The same instance of a thread-safe counter class will not fail miscalculation if it is used by multiple threads. Obviously you can divide collection classes into two groups, thread-safe and non-thread-safe. Vector is thread-safe using synchronized methods, while ArrayList like it is not thread-safe.
12) What is a race condition in Java? Give an example.
Race conditions can lead to some bugs in the program in the case of concurrency. When multiple threads compete for some resources, race conditions will occur. If the first program to be executed fails to compete and is executed later, then the entire program will have some uncertain bugs. Such bugs are hard to spot and recur because of random competition between threads. An example is out-of-order processing, see answer for details.
13) How to stop a thread in Java?
Java provides a rich API but no API for stopping threads. JDK 1.0 originally had some control methods like stop(), suspend() and resume() but they were deprecated in subsequent JDK versions due to potential deadlock threats, after which the Java API designers did not provide one Compatible and thread-safe method to stop a thread. When the run() or call() method finishes executing, the thread will automatically terminate. If you want to manually terminate a thread, you can use a volatile boolean variable to exit the loop of the run() method or cancel the task to interrupt the thread. Click here to see sample code.
14) What happens when an exception occurs while a thread is running?
This is a very tricky Java interview question I encountered in an interview. Simply put, if the exception is not caught, the thread will stop executing. Thread.UncaughtExceptionHandler is an inline interface for handling sudden interruption of threads caused by uncaught exceptions. When an uncaught exception will cause the thread to be interrupted, the JVM will use Thread.getUncaughtExceptionHandler() to query the thread's UncaughtExceptionHandler and pass the thread and exception as parameters to the handler's uncaughtException() method for processing.
15) How to share data between two threads?
You can do this by sharing objects, or using concurrent data structures like blocking queues. This tutorial "Java Inter-Thread Communication" (which involves sharing objects between two threads) implements the producer-consumer model with the wait and notify methods.
16) What is the difference between notify and notifyAll in Java?
This is another tricky problem, because multiple threads can wait for a single-monitored lock, and the designers of the Java API provided some methods to notify them when the wait condition changed, but these methods are not fully implemented. The notify() method cannot wake up a specific thread, so it is only useful when a thread is waiting. And notifyAll() wakes up all threads and allows them to contend for the lock, ensuring that at least one thread can continue to run. My blog has more details and sample code.
17) Why wait, notify and notifyAll are not in thread class?
This is a design-related question that looks at the interviewer's perception of existing systems and things that are ubiquitous but don't seem reasonable. When answering these questions, you should explain why it makes sense to put these methods in the Object class, and why not to put them in the Thread class. An obvious reason is that the locks provided by JAVA are object-level rather than thread-level, and each object has a lock, which is obtained by a thread. If the thread needs to wait for some lock then it makes sense to call the wait() method on the object. If the wait() method is defined in the Thread class, it is not obvious which lock the thread is waiting for. Simply put, since wait, notify and notifyAll are lock-level operations, they are defined in the Object class because locks belong to objects. You can also check out this article to learn more.
18) What are ThreadLocal variables?
ThreadLocal is a special kind of variable in Java. Each thread has a ThreadLocal, that is, each thread has its own independent variable, and the race condition is completely eliminated. It's a good way to get thread-safe for creating expensive objects, for example you can use ThreadLocal to make SimpleDateFormat thread-safe, because that class is expensive to create and needs to create a different instance for each call so it's not worth it in local scope Using it, if you provide each thread with a unique copy of the variable, it will greatly improve efficiency. First, the creation of expensive objects is reduced through reuse. Second, you get thread safety without using expensive synchronization or immutability. Another good example of thread-local variables is the ThreadLocalRandom class, which reduces the number of expensive Random objects to create in a multithreaded environment. See the answer to learn more.
19) What is FutureTask?
In Java concurrent programs, FutureTask represents an asynchronous operation that can be canceled. It has methods for starting and canceling operations, querying whether the operation is completed, and retrieving the operation results. The result can only be retrieved when the operation is complete, and the get method will block if the operation has not yet completed. A FutureTask object can wrap objects that call Callable and Runnable. Since FutureTask also calls the Runnable interface, it can be submitted to Executor for execution.
20) What is the difference between interrupted and isInterruptedd methods in Java?
The main difference between interrupted() and isInterrupted() is that the former will clear the interrupted state while the latter will not. The interrupt mechanism of Java multi-threading is implemented with internal flags. Calling Thread.interrupt() to interrupt a thread will set the interrupt flag to true. When the interrupted thread calls the static method Thread.interrupted() to check the interrupted state, the interrupted state is cleared. The non-static method isInterrupted() is used to query the interrupted status of other threads and does not change the interrupted status flag. Simply put, any method that throws an InterruptedException will clear the interrupted state. In any case, it is possible for a thread's interrupt status to be changed by other threads calling interrupts.
21) Why wait and notify methods are called in synchronized blocks?
Mainly because the Java API mandates this, and if you don't, your code will throw an IllegalMonitorStateException. Another reason is to avoid race conditions between wait and notify.
22) Why should you check for wait conditions in loops?
Threads in the waiting state may receive false alarms and spurious wakeups, and if the waiting condition is not checked in the loop, the program will exit without meeting the end condition. Therefore, when a waiting thread wakes up, its original waiting state cannot be considered to be still valid, it may change between the time after the notify() method call and before the waiting thread wakes up. This is why using the wait() method in a loop works better, you can create a template in Eclipse to call wait and notify to give it a try. If you want to learn more about this subject, I recommend reading the threading and synchronization chapter in the book Effective Java.
23) What is the difference between synchronized collections and concurrent collections in Java?
Both synchronized collections and concurrent collections provide suitable thread-safe collections for multithreading and concurrency, but concurrent collections are more scalable. Before Java 1.5, programmers only used synchronized collections, which would cause contention when multiple threads were concurrent, hindering the scalability of the system. Java 5 introduced concurrent collections like ConcurrentHashMap, which not only provide thread safety but also improve scalability with modern techniques such as lock separation and internal partitioning. See answer for more details.
24) What is the difference between heap and stack in Java?
Why is this question classified as a multithreading and concurrency interview question? Because the stack is a memory area closely related to threads. Each thread has its own stack memory for storing local variables, method parameters and stack calls. Variables stored in one thread are invisible to other threads. The heap is a common memory area shared by all threads. Objects are created in the heap. In order to improve efficiency, the thread will get a cache from the heap to its own stack. If multiple threads use this variable, it may cause problems. At this time, the volatile variable can play a role. It requires the thread from the main Read the value of the variable from the memory. See answer for more details.
25) What is a thread pool? Why use it?
Creating a thread takes expensive resources and time. If a thread is created before a task arrives, the response time will be longer, and a process can create a limited number of threads. In order to avoid these problems, a number of threads are created to respond to processing when the program starts. They are called thread pools, and the threads in them are called worker threads. Starting from JDK1.5, the Java API provides the Executor framework so that you can create different thread pools. For example, a single thread pool, which processes one task at a time; a fixed number of thread pools or a cached thread pool (a scalable thread pool suitable for programs with many short-lived tasks). Read more in this article.
26) How to write code to solve the producer consumer problem?
In reality, many thread problems you solve belong to the producer-consumer model, that is, a thread produces tasks for other threads to consume. You must know how to communicate between threads to solve this problem. A relatively low-level way is to use wait and notify to solve this problem. A better way is to use Semaphore or BlockingQueue to implement the producer-consumer model. This tutorial has implemented it.
27) How to avoid deadlock?
Deadlock in Java multithreading Deadlock refers to a phenomenon in which two or more processes wait for each other due to competing for resources during the execution process. If there is no external force, they will not be able to advance. This is a serious problem, because deadlocks can make your program hang and fail to complete tasks. For a deadlock to occur, the following four conditions must be met:
Mutual exclusion condition: A resource can only be used by one process at a time.
Request and hold condition: When a process is blocked by requesting a resource, it will hold on to the obtained resource.
No deprivation condition: The resources that the process has obtained cannot be forcibly deprived until the resources are used up.
Circular waiting condition: A cyclic waiting resource relationship is formed between several processes.
The easiest way to avoid deadlock is to prevent the circular waiting condition, set flags and sort all resources in the system, and stipulate that all processes applying for resources must operate in a certain order (ascending or descending) to avoid deadlock. This tutorial has code samples and discussion details on avoiding deadlocks.
28) What is the difference between livelock and deadlock in Java?
This is an extension of the previous question. Livelock is similar to deadlock, except that the state of the thread or process in livelock is constantly changing. Livelock can be considered a special kind of starvation. A realistic example of a livelock is when two people meet in a narrow corridor, and both try to avoid each other to allow each other to pass, but because the direction of avoidance is the same, no one can pass the corridor in the end. Simply put, the main difference between livelock and deadlock is that the state of the former process can change but cannot continue to execute.
29) How to check if a thread owns the lock?
I never knew we could detect if a thread owns a lock until I did a phone interview. There is a method in java.lang.Thread called holdsLock() which returns true if and only if the current thread holds a lock on a specific object. You can check out this article to learn more.
30) How do you get thread stack in Java?
There are multiple ways to get the thread stack of a Java process for different operating systems. When you get the thread stack, the JVM will save the state of all threads to a log file or output to the console. On Windows you can use the Ctrl + Break key combination to get the thread stack, on Linux use the kill -3 command. You can also use the jstack tool to get it, it operates on the thread id, and you can use the jps tool to find the id.
31) Which parameter in the JVM is used to control the small stack of the thread
The problem is simple, the -Xss parameter is used to control the stack size of the thread. You can see the JVM configuration list for more information on this parameter.
32) What is the difference between synchronized and ReentrantLock in Java?
Java for a long time in the past can only achieve mutual exclusion through the synchronized keyword, which has some disadvantages. For example, you can't extend methods or block boundaries outside the lock, you can't cancel halfway through trying to acquire a lock, etc. Java 5 addresses these issues by providing more sophisticated controls through the Lock interface. The ReentrantLock class implements Lock, which has the same concurrency and memory semantics as synchronized and which is also extensible. You can check this article for more
33) There are three threads T1, T2, T3, how to ensure that they are executed in order?
In multithreading there are various ways to make threads execute in a specific order, you can use the join() method of the thread class to start another thread in one thread, and the other thread finishes executing the thread. To ensure the order of the three threads you should start the last one first (T3 calls T2, T2 calls T1), so that T1 finishes first and T3 finishes last. You can check out this article to learn more.
34) What does the yield method in the Thread class do?
The Yield method can suspend the currently executing thread object and let other threads with the same priority execute. It is a static method and only guarantees that the current thread will give up the CPU usage and cannot guarantee that other threads will be able to occupy the CPU. The thread that executes yield() may be executed again immediately after entering the suspended state. Click here to see more about the yield method.
35) What is the concurrency of ConcurrentHashMap in Java?
ConcurrentHashMap divides the actual map into several parts to achieve its scalability and thread safety. This division is obtained using the degree of concurrency, an optional parameter to the constructor of the ConcurrentHashMap class, with a default value of 16, to avoid contention in multi-threaded situations. For more concurrency and internal sizing read my article How ConcurrentHashMap works in Java.
36) What is Semaphore in Java?
Semaphore in Java is a new synchronization class which is a counting signal. Conceptually, a semaphore maintains a set of permissions. If necessary, each acquire() is blocked until a license is available, and then the license is acquired. Each release() adds a permission, potentially releasing a blocking acquirer. However, instead of using the actual license object, Semaphore just counts the number of available licenses and acts accordingly. Semaphores are often used in multithreaded code, such as database connection pooling. For more details click here.
37) If the thread pool queue is full when you submit the task. What will happen from time to time?
This is a tricky question, and many programmers will think that the task will block until the thread pool queue becomes available. In fact, if a task cannot be scheduled for execution then ThreadPoolExecutor's submit() method will throw a RejectedExecutionException.
38) What is the difference between submit() and execute() methods in Java thread pool?
Both methods can submit tasks to the thread pool. The return type of the execute() method is void, which is defined in the Executor interface, and the submit() method can return a Future object that holds the calculation result, which is defined in the ExecutorService interface. It extends the Executor interface, other thread pool classes like ThreadPoolExecutor and ScheduledThreadPoolExecutor have these methods. For more details click here.
39) What is a blocking method?
The blocking method means that the program will wait for the method to complete and do nothing else. The accept() method of ServerSocket is to wait for the client to connect. The blocking here means that before the call result is returned, the current thread will be suspended and will not return until the result is obtained. In addition, there are asynchronous and non-blocking methods that return before the task is completed. For more details click here.
40) Is Swing thread safe? Why?
You can answer with certainty that Swing is not thread-safe, but you should explain the reason for this answer even if the interviewer doesn't ask you why. When we say that swing is not thread-safe, we often refer to its components. These components cannot be modified in multiple threads. All updates to GUI components must be completed in the AWT thread. Swing provides synchronous and asynchronous callback methods. to update. Click here to see more about swing and thread safety.
41) What is the difference between invokeAndWait and invokeLater in Java?
These two methods are provided by the Swing API for Java developers to update GUI components from the current thread rather than the event dispatch thread. InvokeAndWait() synchronously updates GUI components, such as a progress bar. Once the progress is updated, the progress bar should also be changed accordingly. If the progress is tracked by multiple threads, call the invokeAndWait() method to request the event dispatch thread to update the component accordingly. The invokeLater() method calls the update component asynchronously. For more details click here.
42) Which methods in Swing API are thread safe?
This question also mentions swing and thread safety. Although components are not thread-safe, there are some methods that can be called safely by multiple threads, such as repaint(), revalidate(). JTextComponent's setText() method and JTextArea's insert() and append() methods are also thread-safe.
43) How to create Immutable object in Java?
This problem doesn't seem to have much to do with multithreading, but immutability helps simplify already complex concurrent programs. Immutable objects can be shared without synchronization, reducing the synchronization overhead of concurrent access to the object. However, Java does not have the @Immutable annotation. To create an immutable class, you must implement the following steps: initialize all members through the constructor, do not provide setter methods for variables, declare all members as private, so that direct To access these members, in the getter method, do not directly return the object itself, but clone the object and return a copy of the object. My article how to make an object Immutable in Java has a detailed tutorial, you can feel confident after reading it.
44) What is ReadWriteLock in Java?
Generally speaking, read-write locks are the result of lock separation techniques used to improve the performance of concurrent programs. ReadWriteLock in Java is an interface added in Java 5. A ReadWriteLock maintains a pair of associated locks, one for read-only operations and one for writing. In the absence of writer threads, a reader lock may be held by multiple reader threads at the same time. Write locks are exclusive, you can use ReentrantReadWriteLock in JDK to implement this rule, which supports up to 65535 write locks and 65535 read locks.
45) What is busy loop in multithreading?
A busy loop is when a programmer uses a loop to make a thread wait, unlike the traditional methods wait(), sleep() or yield() which all give up CPU control, while a busy loop does not give up the CPU, it just runs an empty loop. The purpose of this is to preserve the CPU cache, on a multi-core system, a waiting thread may wake up and run on another core, which will rebuild the cache. It can be used to avoid rebuilding the cache and reduce waiting time for rebuilds. You can check this article for more information.
46) What is the difference between volatile variable and atomic variable?
This is an interesting question. First, volatile variables and atomic variables look similar, but function differently. Volatile variables can ensure look-ahead relationships, that is, write operations will occur before subsequent read operations, but it does not guarantee atomicity. For example, if the count variable is modified with volatile, then the count++ operation is not atomic. The atomic method provided by the AtomicInteger class can make this operation atomic. For example, the getAndIncrement() method will atomically perform an incremental operation and add one to the current value. Other data types and reference variables can also perform similar operations.
47) What happens if a thread inside a synchronized block throws an exception?
This question has pitted many Java programmers. If you can think of the clue of whether the lock is released or not, there is a little hope that you will get it right. Regardless of whether your synchronized block exits normally or abnormally, the thread inside will release the lock, so I prefer the synchronized block to the lock interface, because it does not require me to spend energy to release the lock. This function can be implemented in the finally block.
48) What is the double-checked lock in the singleton pattern?
This question is often asked in Java interviews, but interviewers are only 50% satisfied with answering this question. Half of the people can't write about double-checked locking and half of the people can't say its hidden dangers and how Java 1.5 fixes it. It's actually an old way of creating thread-safe singletons, it tries to use a single lock for performance optimization when the singleton instance is first created, but it fails in JDK1.4 due to its complexity, I personally don't like it either. In any case, even if you don't like it, know about it because it's often asked. You can check the article how double checked locking on Singleton works for more information.
49) How to create a thread-safe Singleton in Java?
This is a follow-up to the question above, if you don't like double-checked locking and the interviewer asks about an alternative to creating a Singleton class, you can use the JVM's class loading and static variable initialization features to create a Singleton instance, or use an enumeration type to create a Singleton, which I love to use. You can check this article for more information.
50) Write 3 best practices for multithreading that you follow
This is my favorite question, and I'm sure you'll follow certain best practices when writing concurrent code to improve performance. Here are three best practices that I think most Java programmers should follow:
Give your thread a meaningful name. This makes it easy to find bugs or track them down. OrderProcessor, QuoteProcessor or TradeProcessor are much better names than Thread-1. Thread-2 and Thread-3, give a thread a name related to the task it is trying to accomplish, all major frameworks and even the JDK follow this best practice.
Avoid locking and narrowing the scope of synchronization Locks are expensive and context switching is more time-consuming and space-consuming, try to minimize the use of synchronization and locks, and narrow the critical section. So I prefer synchronized blocks to synchronized methods, which give me absolute control over the lock.
Use more synchronization classes and less wait and notify First, CountDownLatch, Semaphore, CyclicBarrier and Exchanger synchronization classes simplify coding operations, while it is difficult to control complex control flow with wait and notify. Second, these classes are written and maintained by the best companies, and they will continue to be optimized and improved in subsequent JDKs, and your programs can be optimized without breaking a sweat using these higher-level synchronization tools.
Use concurrent collections more than synchronized collections This is another best practice that is easy to follow and has huge benefits. Concurrent collections are more scalable than synchronized collections, so using concurrent collections is better for concurrent programming. Next time you need to use map, you should first think of using ConcurrentHashMap. My article Java Concurrent Collections has a more detailed explanation.
51) How to force start a thread?
This question is like how to force Java garbage collection. There is no way to think about it. Although you can use System.gc() for garbage collection, it is not guaranteed to be successful. There is no way to force start a thread in Java, it is controlled by the thread scheduler and Java does not publish the relevant API.
52) What is the fork join framework in Java?
The fork join framework is an efficient tool introduced in JDK7 that allows Java developers to take advantage of multiprocessors on modern servers. It is specifically designed for those that can be recursively divided into many submodules, in order to use all available processing power to improve the performance of the program. A huge advantage of the fork join framework is that it uses a work stealing algorithm, where worker threads that can complete more tasks can steal tasks from other threads to execute. You can check this article for more information.
53) What is the difference between calling wait() and sleep() methods in Java multithreading?
Both wait and sleep in Java programs cause some form of pause, and they can meet different needs. The wait() method is used for inter-thread communication. If the wait condition is true and other threads are woken up, it will release the lock, while the sleep() method only releases CPU resources or stops the current thread from executing for a period of time, but does not release the lock. You can check this article for more information.
These are the 50 popular Java multithreading and concurrency interview questions. I didn't share the answers to all the questions but provided enough hints and clues for future readers to find the answers. If you really can't find an answer to a question, contact me and I'll add it. This article can be used not only to prepare for interviews, but also to check your understanding of threading issues such as multithreading, concurrency, design patterns and race conditions, deadlocks, and thread safety. I'm going to make the question of this article a big collection of all Java multithreading questions, but it can't be done without your help, and you can share any other questions with me, including those you've been asked but haven't found yet answer the question. This article is useful for beginners as well as experienced Java developers, and you will benefit from reading it again in two or three years or even five or six years. It is especially useful for beginners because it can expand their knowledge. I will keep updating these questions. You can ask, share and answer questions in the comments at the end of the article to improve this interview question.