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 the main thread to wait until a child thread completes its execution. Here's why it's the right choice:

• isAlive(): This method checks if a thread is still running but doesn't pause the main thread.
• sleep(): This method simply pauses the current thread for a specified duration, it doesn't depend on another thread's completion.
• join(): This method is designed for thread synchronization. When called on a child thread, the main thread will pause its execution and wait until the child thread finishes. Once the child thread completes, the main thread resumes.
• interrupt(): This method interrupts a thread's execution, it doesn't force the main thread to wait for its completion.

Example:

class MyThread extends Thread {
    public void run() {
        // Code to be executed by the child thread
    }
}

public class MainThread {
    public static void main(String[] args) {
        MyThread childThread = new MyThread();
        childThread.start();
        try {
            childThread.join(); // Main thread waits for child thread to finish
        } catch (InterruptedException e) {
            System.out.println("Interrupted: " + e.getMessage());
        }
        System.out.println("Main thread continues after child thread completes");
    }
}

In this example, the 'main' thread will wait at the 'childThread.join()' line until the 'childThread' finishes its execution. Only after that, the main thread will proceed with the remaining code.