Java Thread join()

In this guide, you will learn about the Thread join() method in Java programming and how to use it with an example.

1. Thread join() Method Overview

Definition:

The Java Thread join() method allows one thread to wait for the completion of another.

Syntax:

1) public final void join() throws InterruptedException
2) public final void join(long millis) throws InterruptedException
3) public final void join(long millis, int nanos) throws InterruptedException

Parameters:

- millis: The maximum time to wait in milliseconds.

- nanos: Additional nanoseconds to wait, ranging from 0 to 999999.

Key Points:

- The join() method allows one thread to wait for the completion of another.

- If any thread interrupts the current thread, it throws an InterruptedException.

- Overloaded join() methods with the time specified will make the current thread wait at most that amount of time for the referenced thread to complete.

- If the time is exceeded, the current thread will continue its execution.

2. Thread join() Method Example

public class ThreadJoinExample {
    public static void main(String[] args) {
        // Create a new thread
        Thread newThread = new Thread(() -> {
            for(int i = 0; i < 5; i++) {
                System.out.println("Inside new thread");
                try {
                    // Sleep for 1 second
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });

        // Start the new thread
        newThread.start();

        try {
            // Wait for newThread to complete
            newThread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("New thread has completed, resuming main thread");
    }
}

Output:

Inside new thread
Inside new thread
Inside new thread
Inside new thread
Inside new thread
New thread has completed, resuming main thread

Explanation:

In the provided example, we create a new thread (newThread) that prints "Inside new thread" 5 times and then sleeps for 1 second after each print. 

The main thread starts newThread and immediately invokes the join() method on newThread, pausing the main thread's execution until newThread has completed. 

After newThread completes its execution, the main thread resumes and prints "New thread has completed, resuming main thread".