How to use Multiple Threads in Java - Example

In order to use multiple threads in Java, you need to first define the task which will be executed by those threads. In order to create those task, you can either use Runnable or Callable interface. If you are just learning Java chose Runnable, it's simpler one, but if you are familiar with Java multithreading and want to leverage additional features offered by Callable e.g. it can throw an exception and it can also return value, then go ahead and use Callable. Once you have task ready, you need to create an instance of Thread class. You can create as many instances as you want, but beware don't create too many Thread instances in Java because both JVM and Operating system has a limit on how many threads you can create in Java. Crossing that limit will result in java.lang.OutOfmemoryError: could not create a native thread. For the purpose of this example, creating just three threads are enough.

How to join two threads in Java? Thread.join() example

You can join two threads in Java by using join() method from java.lang.Thread class. Why do you join threads? because you want one thread to wait for another before starts processing. It's like a relay race where the second runner waits until the first runner comes and hand over the flag to him. Remember, unlike sleep(), join() is not a static method so you need an object of java.lang.Thread class to call this method. Now, who calls and who wants and which thread dies? for example, if you have two threads in your program main thread and T1 which you have created. Now if your main thread executes this line T1.join() (main thread execute whatever you have written in the main method) then main thread will wait for T1 thread to finish its execution. The immediate effect of this call would be that main thread will stop processing immediately and will not start until T1 thread has finished. So, what will happen if T1 is not yet started and there is no one to start T1 thread? Well,  then you have a deadlock but if it's already finished then main thread will start again, provided it get the CPU back.

How to stop a thread in Java? Example

Today we're going to learn about how to stop a thread in Java. It's easy to start a thread in Java because you have a start() method but it's difficult to stop the thread because there is no working stop() method. Well, there was a stop() method in Thread class, when Java was first released but that was deprecated later. In today's Java version, You can stop a thread by using a boolean volatile variable.  If you remember, threads in Java start execution from run() method and stop, when it comes out of run() method, either normally or due to any exception. You can leverage this property to stop the thread. All you need to do is  create a boolean variable e.g. bExit or bStop. Your thread should check its value every iteration and comes out of the loop and subsequently from run() method if bExit is true.