Difference between wait() and join() methods in Java [Answered]

Though both wait() and join() methods are used to pause the current thread and have a lot of similarities they have different purposes. One of the most obvious differences between the wait() and join() method is that the former is declared in java.lang.Object class while join() is declared in java.lang.Thread class. This means that wait() is related to the monitor lock which is held by each instance of an object and the join method is related to the thread itself. The wait() method is used in conjunction with notify() and notifyAll() method for inter-thread communication, but join() is used in Java multi-threading to wait until one thread finishes its execution.

 Another difference between the wait() and join() method is that the former must be called from a synchronized method or block but later can be called without a synchronized block in Java.

Good knowledge of different thread-related methods e.g. start and run, wait and notify, join and yield goes a long way in writing robust and correct multi-threaded, concurrent Java applications. If you don't know how to use the join method, you miss out on a writing program where one thread needs to wait for the completion of another thread before it starts.

These were some fundamental differences between the wait() and join() methods, let's see a couple of more differences and similarities in detail. Btw, if you are new to multithreading then I suggest you join a fundamental course on Java threads like Multithreading and Parallel Computing in Java from Udemy. It's also very affordable and you can get in just $9.9 on Udemy sales. 

Similarities between wait() and join()

Before looking at the difference between the wait() and join() method of thread class, let's see some of the key similarities between them:

1. Used for pausing a thread
Both wait() and join() are used to pause the current thread in Java. In the first case, the thread which calls the wait() method goes into waiting for the state while in the second case the thread which calls the join() method goes into waiting for the state until the thread on which join has been called finishes its execution.  

2. Overloaded methods
Both wait() and join() are overloaded in Java. You can find a version of both the wait() and join() which accepts a timeout parameter.

3. Interruption
Both wait() and join() can be interrupted by calling interrupt() method in Java. This means you can handle the interrupt by catching InterrruptedException in Java.

4. Non-static methods
Both wait() and join() are non-static methods and cannot be called from the static context. If you want to learn more about Thread class and its essential methods then you can further see the Complete Java Masterclass, one of the best Java courses on Udemy to learn more about how to use the join method in Java.

difference between wait and join methods in Java

Difference between wait() and join() in Java

So far you have learned what is wait() and join() method, why they are used, and some similarities between them. Now is the time to revise and find out some key differences between them.

1. Thread vs Object

The first and foremost difference between the wait() and join() method is that wait() is declared and defined in the Object class while join() is defined in the Thread class. You can further see my answer to a related question why wait and notify is declared in Object class and not in Thread class to learn more about it.

2. Synchronized Context

The second important difference between wait() and join() is that wait() must be called from synchronized context i.e. synchronized method or block otherwise it will throw IllegalMonitorStateException but there is no such requirement for calling the join() method in Java.

On the other hand, you can call the join() method with and without synchronized context in Java. The Java Fundamentals - Concurrency with Multithreading course on Pluralsight has explained this concept in more detail, you can refer to the wait, notify part to learn more.

3. Release of lock

The third difference between the wait() and join() method is that the thread which calls the wait method releases any lock held for the object on which the wait() is called, but the calling join() method doesn't release any monitor or lock.

4. Purpose

Another difference between them is that wait() is used for inter-thread communication while the join() is used for adding sequencing between multiple threads like one thread starts execution after the first thread finishes its execution.

5. Notification

You can awake a thread waiting by calling the wait() method of the object class by using notify() and notifyAll() method but you can not break the waiting imposed by join without interruption or unless the thread on which join is called has finished execution.

That's all about the difference between the wait() and join() methods in Java. Use wait() when you want inter-thread communication e.g. to solve the producer-consumer problem while using the Thread.join()  method when you want one thread to start execution only if the first thread finishes its execution.

Further Learning
Multithreading and Parallel Computing in Java
Multithreading for Senior Engineering Interviews 
Java Concurrency in Practice Course by Heinz Kabutz

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Thanks for reading this article so far. If you like this interview question and my explanation then please share it with your friends and colleagues. If you have any questions for feedback then please drop a comment. 

P. S. - If you are just started with Java and want to learn Java Multithreading but looking for a free online training course to start with then I also, suggest you check out this awesome free Java Multithreading course on Udemy. This course is absolutely free and thousands of java developers have already joined this course. 

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