By utilizing the Join method, one Thread can wait for the completion of another thread. When a thread A invokes the join method on thread B, thread A’s execution is halted until thread B finishes its execution.

Let me give you an example to make it more clear.

import java.util.ArrayList;

class ThreadB extends Thread {

private ArrayList list;

public ThreadB(ArrayList list) {

this.list = list;

}

 

@Override

public void run() {

// Add four employees to the list

list.add("Employee 1");

list.add("Employee 2");

list.add("Employee 3");

list.add("Employee 4");

}

}

 

public class Main {

public static void main(String[] args) {

ArrayList list = new ArrayList();

 

// Create an instance of ThreadB and pass the empty list

ThreadB threadB = new ThreadB(list);

 

// Start the thread

threadB.start();

 

try {

// Call join method on threadB to make the main thread wait until threadB completes

threadB.join();

} catch (InterruptedException e) {

e.printStackTrace();

}

 

// Print the list

System.out.println("List: " + list);

}

}

The output of the program:

List: [Employee 1, Employee 2, Employee 3, Employee 4]

Explanation of the steps:

1. We define a class ThreadB that extends the Thread class. It has a constructor that accepts an ArrayList to store employees.
2. Inside the run method of ThreadB, we add four employees to the provided ArrayList.
3. In the Main class, we create an empty ArrayList called list.
4. We create an instance of ThreadB called threadB and pass the empty list to it.
5. We start the threadB thread using the start method.
6. To ensure the main thread waits for threadB to complete, we call the join method on threadB.
7. If an InterruptedException occurs while joining the thread, we print the stack trace.
8. Finally, we print the contents of the list after threadB completes its execution.

The output of this program should be the list consisting of four employees. By using the join method, the main thread waits for Threadb to finish before printing the list, ensuring that the list contains all the employees added by threadB.

In conclusion, I explained the use of the join method in threads through a Java program example. It starts by introducing the join method, which allows one thread to wait for the completion of another thread. When a thread A calls the join method on thread B, thread A’s execution pauses until thread B terminates.

That the main thread needs to wait until ThreadB completes its execution. It achieves this by calling the join method on ThreadB, causing the main thread to pause until ThreadB terminates. Consequently, the output of the program will correctly display the list consisting of the four employees added by ThreadB.

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