In Java, Thread Scheduler controls thread scheduling. But we can

use join() method on a thread to make current thread to wait for

another thread to finish.

When we use join(), the current thread stops executing. It wait for

the thread on which join() is called to finish.

This makes sure that current thread will continue only after the

thread it joined finished running. Consider following example:

Public class ThreadJoin {

Thread importantThread = new Thread(

new Runnable() {

public void run () {

//do something

}

}

);

Thread currentThread = new Thread(

new Runnable() {

public void run () {

//do something

}

}

);

importantThread.start(); // Line 1

importantThread.join(); // Line 2

currentThread.start(); // Line 3

}

In the above example, main thread is executing. On Line 1, a new

thread called importantThread is ready to run. But at Line 2, main

thread joins the importantThread. Now it lets importantTread to

finish and then it moves to Line 3. So currentThread at Line 3 will

not start till the importantThread has finished

https://www.geeksforgeeks.org/joining-threads-in-java/