Java concurrency: create a thread for every object in a list and then use wait()

[Filippo Restori]

Hello, i'm new.

I know this quesion is kinda strange, but I don't know how to do this in my own.

So: I have this List of Objects, and the trace of my homework says that I have to create a function that makes every object inside the list in wait(), so the thread waits on that object.
I know that 1 thread can menage 1 object (with wait()).

So do I have to create inside that function 1 Thread for every object inside the list??
If yes ok, but i'm very confused in "how to do that" ahah

[Himai Minh]

You mean this?
Assume you have 10 Runnable objects. You create 10 threads for these objects, one for each.
When 1 thread is doing a task, the rest of the 9 threads waits. Then, the first thread notifyAll threads to wake up from waiting state.

Reference:
https://howtodoinjava.com/java/multi-threading/wait-notify-and-notifyall-methods/

[Piet Souris]

Another possibility:

if you have a List of Callables, then create a Treadpool with just one Thread, and let that 'invokeAll' your list.

And welcome to the Ranch!

[Filippo Restori]

Ok ty guys, I will check some guides for thread pools so!

[Filippo Restori]

BTW for now i've done something like this, maybe you understand more than in words

public void waitAll() throws InterruptedException{
		for (Object iObject : list) {
			 new Thread(()->{
				synchronized (iObject) {
					
					try {
						
						System.out.println("I'm Thread " + Thread.currentThread().getId() + " and I wait on" + iObject);
						iObject.wait();
						System.out.println("I'm Thread " + Thread.currentThread().getId() + "  and I woke up on" + iObject);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}).start();
		}
	}

[Stephan van Hulst]

Really, before we can say anything about that code, we really need to see how the original homework problem was phrased. Can you quote it verbatim?

[Filippo Restori]

Ok, the trace is very long, but the part of the fuction waitAll() is this: "the waitAll () method allows you to wait for any of the objects in the list to be signaled by notify () or notifyAll ()"

So I assumed that i had to create a thread for every object in the list and then use wait in every innerThread

[Stephan van Hulst]

Going by just that single line, I wouldn't expect the methods to spawn any threads. It would just cause the thread that is currently executing the method to wait.

The line says "any of the objects", which implies that the current thread picks one object and waits for it, but that would be non-sensical and also not in line with the name of the method, which implies the current thread needs to wait for every object.

We're making a lot of assumptions. It would be really helpful if we could see more of the assignment. How long is "long"?

[Filippo Restori]

Ok i will send the entire trace, but is very hard to undersand I think.

It's ok if you can't help me XD

A fence is an object that has a collection of objects at its disposal and can be put on hold for any of the objects in the collection to be notified. In particular, the Fence in Java implement the following interface:

package test.exam;
public interface Fence {
      public void await() throws InterruptedException;
      public void add(Object o);
}

The add (Object) method allows you to add an object to the fence object collection.
The awaitO method allows you to wait for any of the objects in the collection to be reported using notifyQ or notifyAll (). Note that if the add (Object) method is called while a wait is in progress, then the argument passed to the add (Object) method is also immediately involved in the wait.
Write the source code of the Fencelmpl class in the test.exam package to implement the Fence interface.