Help with Synchronisation with a semaphore

Hey,

Recently been introduced to threads & came across a question involving the usage of semaphores. I'm just a bit confused as to how to use them and was wondering if anybody could shed a bit of guidance for me. I'll post the question to begin with:

Create a program using 2 seperate threads, each printing out a message when it reaches a particular part in its run method. Add a Java semaphore to the program and use it to ensure that whenever you run the program the point A is always reached before point B i.e. every time you run the program it behaves as follows:

$ java Points
Point A reached
Point B reached
Create and initialise your semaphore in the main method and pass it to each thread.

I'm unsure how & where to use this semaphore and also what parameter i'm supposed to give it.

My code so far is:

Code Java:

---

import java.util.concurrent.Semaphore;
 
class PointA extends Thread {
 
	public PointA() {
	}
 
	public void run() {
 
		try {
			sleep((int) (Math.random() * 100));
		} catch (InterruptedException e) {
		}
 
		System.out.println("Point A reached");
 
	}
}
 
class PointB extends Thread {
 
	public PointB() {
 
	}
 
	public void run() {
 
		try {
			sleep((int) (Math.random() * 100));
		} catch (InterruptedException e) {
		}
 
		System.out.println("Point B reached");
 
	}
}
 
public class Points {
 
	public static void main(String[] args) throws InterruptedException {
 
		PointA a = new PointA();
		PointB b = new PointB();
 
		Semaphore sem = new Semaphore(0);
 
		b.start();
		a.start();
 
		b.join();
		a.join();
	}
}

---

Thanks in advance for any help.

You can use the Semaphore in a wait/notify context...for instance, Thread B calls wait...once thread A completes it calls notify on the semaphore.

How would I do this? I don't expect you to spoon feed but I'm really lost sorry. We've been taught little to nothing about threads and semaphores and I can't seem to find any useful information to explain even the basics.
For example:

Semaphore sem = new Semaphore(1);

What number are you supposed to pass through as a parameter? And where am I supposed to call these wait(), notify() methods because every time I try, I'm getting IllegalMonitorStateExceptions

Sorry, didn't initially see you were using the Semaphore class. This class facilitates acquiring locks to a certain number of threads. In your case you could create a Semaphore of size 1, and pass this to the Threads. You can then let A acquire - this acquires the only space so when B attempts to acquire the method will block until that single permit is released. When A is finished it can call release...the acquire method in B returns and B can do its thing.

Thanks for the help. I'll post how I solved it below. By "solve" I mean it gets the required output in the sense that Point A is always reached before Point B, but would you be able to let me know if I did this correctly. Also, anytime i tried passing 1 as a parameter for the Semaphore it wouldn't work.. will only work if I pass 0. Why is this?
Cheers.

Code Java:

---

import java.util.concurrent.Semaphore;
 
class PointA extends Thread {
	private Semaphore sem;
 
	public PointA(Semaphore sem) {
		this.sem = sem;
	}
 
	public void run() {
 
		try {
			sleep((int) (Math.random() * 100));
		} catch (InterruptedException e) {
		}
		sem.release();
		System.out.println("Point A reached");
 
	}
}
 
class PointB extends Thread {
	private Semaphore sem;
 
	public PointB(Semaphore sem) {
		this.sem = sem;
	}
 
	public void run() {
		try {
			sem.acquire();
		} catch(InterruptedException e) {
			e.printStackTrace();
		}
		try {
			sleep((int) (Math.random() * 100));
		} catch (InterruptedException e) {
		}
		sem.release();
		System.out.println("Point B reached");
 
	}
}
 
public class Points {
 
	public static void main(String[] args) throws InterruptedException {
		Semaphore sem = new Semaphore(0);
		PointA a = new PointA(sem);
		PointB b = new PointB(sem);
 
 
		b.start();
		a.start();
 
		a.join();
		b.join();
	}
}

---

PointA is not acquiring the Semaphore - if you set the permit value to 1, and have the PointA class acquire the Semaphore prior to PointB, then PointB can only acquire the Semaphore after PointA releases the Semaphore.