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The join() method of Thread class is a very useful method in a situation when we want a thread to wait for another thread to finish its execution. The name join() comes from a concept of making a thread wait until a specific thread joins it after finishing its execution.

Let's understand join() method with a short code -

public static void main(String... ar)
{
Thread newThread = new Thread("Thread2"); //line1
newThread.start(); 			 //line2
newThread.join();			 //line3
System.out.println("Hello from main");
}

Code Analysis

The code is inside the main() method, hence, the main thread(default thread) runs automatically.

• At line1, we have created a thread with the name, Thread2, pointed by newThread reference.
• At line2, start() method is invoked to put Thread2 into a runnable state, in order to make it run.
• At line3, newThread.join() method call is made inside the main() method, hence it makes the main thread(default thread) wait for Thread2 to finish its execution.
• Once Thread2 finishes its execution, main thread continues its work and "Hello from main" is printed.

There are two versions of join() method -:

Method	Description
final void join()	This version of join() method makes a thread wait for another thread to finish its execution.
final void join(long millis)	This version of join() method a thread wait only for a specified time(in milliseconds) for another thread to finish its execution.

Let's understand join() method by an example

class ThJoin implements Runnable
{

public void run()  //Entry Point of the new thread.
{
Thread th=Thread.currentThread(); //Getting the reference to the currently executing thread.
try
{
	for(int i=0;i<3;i++)
	{
		Thread.sleep(1000); //This will make this thread sleep for 1000 ms.
		System.out.println(i);	
	}
	System.out.println(th.getName() + " completed");			
}
catch(InterruptedException e)
{
System.out.println("Thread Interrupted" + e);
}

}


public static void main(String... ar)
{
ThJoin newTh= new ThJoin();
Thread th= new Thread(newTh, "OtherThread");//Calling Thread's constructor & passing the object
					    //of class that  implemented  Runnable interface
					    //& the name of new thread.
th.start();

try
{
	th.join(); //main thread will wait for other thread to complete its execution and join it.

	System.out.println("Main thread ends");
}
catch(InterruptedException e)
{
	System.out.println(e);
}


}
}

Output is - :

0
1
2
Other Thread completed
Main thread completed

Program Analysis

The code is inside the main() method, hence, the main thread(default thread) runs automatically.

• In the main() method, we have created a thread with the name, OtherThread, pointed by th.
• Method call th.start() starts the execution of the OtherThread.
• Method th.join() in main(), makes the main thread wait for the OtherThread to finish its execution.
• In the run() method of OtherThread, a for-loop makes the OtherTread sleep for 1 seconds, thrice.
• Once OtherThread finishes its execution, main thread continues and "Main thread ends" is printed.

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