Imporatance of join() method

The join() method in Java is used to wait for a thread to complete its execution before continuing with the execution of the current thread. It allows for synchronization and coordination between multiple threads in a program.

join() overloadable methods syntax from java.lang.Thread  class.

public final void join() throws InterruptedExceptionCode language: Java (java)

This version of join() waits for the thread it is called on to complete its execution. The calling thread will be blocked until the joined thread terminates or is interrupted. If the joined thread is interrupted, the InterruptedException will be thrown.

public final void join(long millis) throws InterruptedExceptionCode language: PHP (php)

This version of join() waits for the thread it is called on to complete its execution, but with a timeout specified in milliseconds. The calling thread will be blocked for at most the specified  millis(milli seconds) duration. If the joined thread completes before the timeout or is interrupted, the calling thread will resume execution. If the timeout expires before the joined thread completes, the InterruptedException will be thrown.

The join() method has the following importance

1.Thread Synchronization: The join() method allows one thread to wait for the completion of another thread. It ensures that the current thread does not proceed until the joined thread has finished executing. This is useful when you need to synchronize the execution of multiple threads and ensure that they complete their tasks in a specific order.

2.Coordination between Threads: In scenarios where multiple threads need to work together to accomplish a task, the join() method enables coordination. One thread can start executing and then join other threads to wait for their completion before proceeding further. This helps in achieving a synchronized and coordinated execution flow among multiple threads.

3.Waiting for Results: If a thread performs some computation or task and another thread requires the result of that computation, the join() method can be used to wait for the computation thread to finish and obtain the result. This ensures that the result is available before proceeding with further processing.

4.Threading Hierarchy: In some cases, threads are organized in a hierarchy where a parent thread creates and manages multiple child threads. The join() method allows the parent thread to wait for the completion of all its child threads before continuing its execution. This ensures that the parent thread does not finish prematurely and guarantees that all child threads have completed their tasks.

5.Exception Handling: When a thread throws an exception during its execution, it is essential to handle the exception appropriately. By using the join() method after starting a thread, you can catch any exceptions thrown by that thread within a try-catch block. This way, you can handle exceptions raised by different threads in a structured manner.

//JoinMethodDemo.java
class MyThread extends Thread{
    private String name;

    public MyThread(String name) {
        this.name = name;
    }

    @Override
    public void run() {
        System.out.println("Mr." + name + " started.");
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Mr. " + name + " finished.");
    }
}
public class JoinMethodDemo {
    public static void main(String[] args) {
        MyThread thread1 = new MyThread("Paani");
        MyThread thread2 = new MyThread("Mahesh");

        thread1.start();
        thread2.start();

        try {
            thread1.join();
            thread2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("All threads have finished executing.");
    }
}

D:\>javac JoinMethodDemo.java

D:\>java JoinMethodDemo
Mr.Paani started.
Mr.Mahesh started.
Mr. Paani finished.
Mr. Mahesh finished.
All threads have finished executing.

D:\>javac JoinMethodDemo.java

D:\>java JoinMethodDemo
Mr.Paani started.
Mr.Mahesh started.
Mr. Mahesh finished.
Mr. Paani finished.
All threads have finished executing.Code language: CSS (css)