How to Make the Main Thread Wait for Child Thread Completion in Java

The correct answer is 'C join'.

The join() method in Java allows a thread to wait until another thread completes its execution. Here's how it works:

1. Calling join(): When a thread calls join() on another thread, it essentially pauses its own execution and waits for the target thread to finish.
2. Waiting for Completion: The calling thread will block until the target thread's run() method completes.
3. Resuming Execution: Once the target thread finishes, the calling thread resumes its execution.

Example:

class MyThread extends Thread {
    public void run() {
        // Code to be executed by the child thread
        System.out.println("Child thread is running");
    }
}

public class MainThread {
    public static void main(String[] args) {
        MyThread childThread = new MyThread();
        childThread.start();
        try {
            childThread.join(); // Wait for child thread to complete
        } catch (InterruptedException e) {
            System.out.println("Main thread interrupted");
        }
        System.out.println("Main thread continues after child thread completion");
    }
}

Why Other Options Are Incorrect:

• isAlive(): This method checks if a thread is still running but doesn't make the calling thread wait for it to finish.
• sleep(): This method causes the calling thread to pause for a specified duration, but it doesn't guarantee that the target thread will have completed by the time the sleep ends.
• interrupt(): This method is used to interrupt a thread, but it doesn't directly make the calling thread wait for the target thread's completion.