Joining Threads in Java

java.lang.Thread class provides the join() method which allows one thread to wait until another thread completes its execution. If t is a Thread object whose thread is currently executing, then t.join() will make sure that t is terminated before the next instruction is executed by the program.
If there are multiple threads calling the join() methods that means overloading on join allows the programmer to specify a waiting period. However, as with sleep, join is dependent on the OS for timing, so you should not assume that join will wait exactly as long as you specify.
There are three overloaded join functions.

  1. join(): It will put the current thread on wait until the thread on which it is called is dead. If thread is interrupted then it will throw InterruptedException.
    Syntax:

    public final void join()
    
  2. join(long millis) :It will put the current thread on wait until the thread on which it is called is dead or wait for specified time (milliseconds).
    Syntax:

    public final synchronized void join(long millis)
    
  3. join(long millis, int nanos): It will put the current thread on wait until the thread on which it is called is dead or wait for specified time (milliseconds + nanos).
    Syntax:

    public final synchronized void join(long millis, int nanos)
    
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    // Java program to explain the
    // concept of joining a thread.
    import java.io.*;
      
    // Creating thread by creating the
    // objects of that class
    class ThreadJoining extends Thread
    {
        @Override
        public void run()
        {
            for (int i = 0; i < 2; i++)
            {
                try
                {
                    Thread.sleep(500);
                    System.out.println("Current Thread: "
                            + Thread.currentThread().getName());
                }
      
                catch(Exception ex)
                {
                    System.out.println("Exception has" +
                                    " been caught" + ex);
                }
                System.out.println(i);
            }
        }
    }
      
    class GFG
    {
        public static void main (String[] args)
        {
      
            // creating two threads
            ThreadJoining t1 = new ThreadJoining();
            ThreadJoining t2 = new ThreadJoining();
            ThreadJoining t3 = new ThreadJoining();
      
            // thread t1 starts
            t1.start();
      
            // starts second thread after when
            // first thread t1 has died.
            try
            {
                System.out.println("Current Thread: "
                      + Thread.currentThread().getName());
                t1.join();
            }
      
            catch(Exception ex)
            {
                System.out.println("Exception has " +
                                    "been caught" + ex);
            }
      
            // t2 starts
            t2.start();
      
            // starts t3 after when thread t2 has died.
            try
            {
                System.out.println("Current Thread: "
                     + Thread.currentThread().getName());
                t2.join();
            }
      
            catch(Exception ex)
            {
                System.out.println("Exception has been" +
                                        " caught" + ex);
            }
      
            t3.start();
        }
    }

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    output:

    Current Thread: main
    Current Thread: Thread-0
    0
    Current Thread: Thread-0
    1
    Current Thread: main
    Current Thread: Thread-1
    0
    Current Thread: Thread-1
    1
    Current Thread: Thread-2
    0
    Current Thread: Thread-2
    1
    

In the above example we can see clearly second thread t2 starts after first thread t1 has died and t3 will start its execution after second thread t2 has died.

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