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Situational use: Run tasks in ForkJoinPool vs. new Thread

Since I'm trying to understand a few of the new features provided in Java 8, I came upon the following situation:
I wish to implement asynchron method calls into my application (in JavaFX). The idea was to provide/use a seperate thread for everything related to the GUI so that background tasks wont block/delay the visible output in my application.
For the background tasks, I thought about either use a pool of threads or simply run them in the main thread of the application for now. Then, I came upon the ForkJoinPool used in the standard way by using the CompletableFuture class when doing something like this:
CompletableFuture.runAsync(task);
whereas task is a Runnable.
In most tutorials and the Javadoc, the ForkJoinPool is described as "a pool which contains threads waiting for tasks to run". Also, the ForkJoinPool is usually the size of the cores of the users machine, or doubled if hyperthreading is supported.
What advantages does the ForkJoinPool give me over the traditional Thread when I want to run a task asynchronously?

ForkJoinPool is not comparable with Threads, it's rather comparable with ThreadPools. Creating a new thread by code often bad and will lead to OutOfMemoryErrors because it is not controlled. Depending on your use case you might need to use a ForkJoinPool or a different pool but make sure you use one. And by the way, all CompletableFuture methods have overloads that allow you pass your own thread pool.
Some benefits of ForkJoinPool,
Already initialised for you and shutdown on the JVM shutdown, so you don't need to worry about that
Its size can be controlled through a VM parameter
Its perfect for compute bound task where work stealing can be beneficial, although this could be a problem depending on the case.

Difference between these two snippets using Thread and Executor for quick Java threading? [duplicate]

This question already has answers here:
When should we use Java's Thread over Executor?
(7 answers)
Closed 7 years ago.
In Java, both of the following code snippets can be used to quickly spawn a new thread for running some task-
This one using Thread-
new Thread(new Runnable() {
#Override
public void run() {
// TODO: Code goes here
}
}).start();
And this one using Executor-
Executors.newSingleThreadExecutor().execute(new Runnable(){
#Override
public void run() {
// TODO: Code goes here
}
});
Internally, what is the difference between this two codes and which one is a better approach?
Just in case, I'm developing for Android.
Now I think, I was actually looking for use-cases of newSingleThreadExecutor(). Exactly this was asked in this question and answered-
Examples of when it is convenient to use Executors.newSingleThreadExecutor()

Your second example is strange, creating an executor just to run one task is not a good usage. The point of having the executor is so that you can keep it around for the duration of your application and submit tasks to it. It will work but you're not getting the benefits of having the executor.
The executor can keep a pool of threads handy that it can reuse for incoming tasks, so that each task doesn't have to spin up a new thread, or if you pick the singleThread one it can enforce that the tasks are done in sequence and not overlap. With the executor you can better separate the individual tasks being performed from the technical implementation of how the work is done.
With the first approach where you create a thread, if something goes wrong with your task in some cases the thread can get leaked; it gets hung up on something, never finishes its task, and the thread is lost to the application and anything else using that JVM. Using an executor can put an upper bound on the number of threads you lose to this kind of error, so at least your application degrades gracefully and doesn't impair other applications using the same JVM.
Also with the thread approach each thread you create has to be kept track of separately (so that for instance you can interrupt them once it's time to shutdown the application), with the executor you can shut the executor down once and let it handle its threads itself.

The second using an ExecutorService is definitely the best approach.
ExecutorService determines how you want your tasks to run concurrently. It decouples the Runnables (or Callables) from their execution.
When using Thread, you couple the tasks with how you want them to be executed, giving you much less flexibility.
Also, ExecutorService gives you a better way of tracking your tasks and getting a return value with Future while the start method from Thread just run without giving any information. Thread therefore encourages you to code side-effects in the Runnable which may make the overall execution harder to understand and debug.
Also Thread is a costly resource and ExecutorService can handle their lifecycle, reusing Thread to run a new tasks or creating new ones depending on the strategy you defined. For instance: Executors.newSingleThreadExecutor(); creates a ThreadPoolExecutor with only one thread that can sequentially execute the tasks passed to it while Executors.newFixedThreadPool(8)creates a ThreadPoolExecutor with 8 thread allowing to run a maximum of 8 tasks in parallel.

You already have three answers, but I think this question deserves one more because none of the others talk about thread pools and the problem that they are meant to solve.
A thread pool (e.g., java.util.concurrent.ThreadPoolExecutor) is meant to reduce the number of threads that are created and destroyed by a program.
Some programs need to continually create and destroy new tasks that will run in separate threads. One example is a server that accepts connections from many clients, and spawns a new task to serve each one.
Creating a new thread for each new task is expensive; In many programs, the cost of creating the thread can be significantly higher than the cost of performing the task. Instead of letting a thread die after it has finished one task, wouldn't it be better to use the same thread over again to perform the next one?
That's what a thread pool does: It manages and re-uses a controlled number of worker threads, to perform your program's tasks.
Your two examples show two different ways of creating a single thread that will perform a single task, but there's no context. How much work will that task perform? How long will it take?
The first example is a perfectly acceptable way to create a thread that will run for a long time---a thread that must exist for the entire lifetime of the program, or a thread that performs a task so big that the cost of creating and destroying the thread is not significant.
Your second example makes no sense though because it creates a thread pool just to execute one Runnable. Creating a thread pool for one Runnable (or worse, for each new task) completely defeats the purpose of the thread-pool which is to re-use threads.
P.S.: If you are writing code that will become part of some larger system, and you are worried about the "right way" to create threads, then you probably should also learn what problem the java.util.concurrent.ThreadFactory interface was meant to solve.
Google is your friend.

According to documentation of ThreadPoolExecutor
Thread pools address two different problems: they usually provide
improved performance when executing large numbers of asynchronous
tasks, due to reduced per-task invocation overhead, and they provide a
means of bounding and managing the resources, including threads,
consumed when executing a collection of tasks. Each ThreadPoolExecutor
also maintains some basic statistics, such as the number of completed
tasks.
First approach is suitable for me if I want to spawn single background processing and for small applications.
I will prefer second approach for controlled thread execution environment. If I use ThreadPoolExecutor, I am sure that 1 thread will be running at time , even If I submit more threads to executor. Such cases are tend to happen if you consider large enterprise application, where threading logic is not exposed to other modules. In large enterprise application , you want to control the number of concurrent running threads. So second approach is more pereferable if you are designing enterprise or large scale applications.

creating and killing a thread vs using .notify () and .wait ()

Assuming a control thread has access to a bunch of threads and to the objects this thread would wait on. Which one will have a greater impact in performance if I have to start and stop, what several of these threads are doing, from this single control thread ?
Wouldn't it just be better for example to kill it via interruption and just create a new one with the same Runnable?

Creating (actually start()-ing) a new thread is relatively expensive, so from a performance perspective it would be better to use wait / notify.
Secondly, interrupt is not guaranteed to "stop" a thread. The thread may choose to ignore the interrupt ... or if it is purely CPU bound, it may not notice it at all.
There is also a third option: use an existing thread pool mechanism. For example, the ExecutorService API has a various implementations that provided bounded and unbounded thread pools. These can take care of scaling up and down, and pool shutdown. You use them by submit(...)-ing tasks as Runnable instances and you optionally get a Future that allows you to wait for the task completion.
Finally, for most concurrent programming use-cases, there are standard classes that support the use-case, and it is better to use them rather than attempting to implement from scratch; e.g. using wait / notify directly. In your case, you probably need some kind of "barrier" mechanism: java.util.concurrent.Phaser might be the one that you need.

Threads are fairly independent from one another and in most cases each thread will know better than the control thread when it's ready to terminate. Killing a thread is very abrupt thing, it's much better to wait for the threat to terminate itself cleanly.

Examples of when it is convenient to use Executors.newSingleThreadExecutor()

Could please somebody tell me a real life example where it's convenient to use this factory method rather than others?
newSingleThreadExecutor
public static ExecutorService newSingleThreadExecutor()
Creates an Executor that uses a single worker thread operating off an
unbounded queue. (Note however that if this single thread terminates
due to a failure during execution prior to shutdown, a new one will
take its place if needed to execute subsequent tasks.) Tasks are
guaranteed to execute sequentially, and no more than one task will be
active at any given time. Unlike the otherwise equivalent
newFixedThreadPool(1) the returned executor is guaranteed not to be
reconfigurable to use additional threads.
Thanks in advance.

Could please somebody tell me a real life example where it's convenient to use [the newSingleThreadExecutor() factory method] rather than others?
I assume you are asking about when you use a single-threaded thread-pool as opposed to a fixed or cached thread pool.
I use a single threaded executor when I have many tasks to run but I only want one thread to do it. This is the same as using a fixed thread pool of 1 of course. Often this is because we don't need them to run in parallel, they are background tasks, and we don't want to take too many system resources (CPU, memory, IO). I want to deal with the various tasks as Callable or Runnable objects so an ExecutorService is optimal but all I need is a single thread to run them.
For example, I have a number of timer tasks that I spring inject. I have two kinds of tasks and my "short-run" tasks run in a single thread pool. There is only one thread that executes them all even though there are a couple of hundred in my system. They do routine tasks such as checking for disk space, cleaning up logs, dumping statistics, etc.. For the tasks that are time critical, I run in a cached thread pool.
Another example is that we have a series of partner integration tasks. They don't take very long and they run rather infrequently and we don't want them to compete with other system threads so they run in a single threaded executor.
A third example is that we have a finite state machine where each of the state mutators takes the job from one state to another and is registered as a Runnable in a single thread-pool. Even though we have hundreds of mutators, only one task is valid at any one point in time so it makes no sense to allocate more than one thread for the task.

Apart from the reasons already mentioned, you would want to use a single threaded executor when you want ordering guarantees, i.e you need to make sure that whatever tasks are being submitted will always happen in the order they were submitted.

The difference between Executors.newSingleThreadExecutor() and Executors.newFixedThreadPool(1) is small but can be helpful when designing a library API. If you expose the returned ExecutorService to users of your library and the library works correctly only when the executor uses a single thread (tasks are not thread safe), it is preferable to use Executors.newSingleThreadExecutor(). Otherwise the user of your library could break it by doing this:
ExecutorService e = myLibrary.getBackgroundTaskExecutor();
((ThreadPoolExecutor)e).setCorePoolSize(10);
, which is not possible for Executors.newSingleThreadExecutor().

It is helpful when you need a lightweight service which only makes it convenient to defer task execution, and you want to ensure only one thread is used for the job.

Most efficient Java threading technique?

There seem to be a number of different ways in which one can create threads (Runnable vs Thread class) and also ThreadPools.
Are there any difference in terms of efficiency and which are the most efficient (in terms of performance) techniques for creating and pooling threads in Java?

If you need to handle many short and frequent requests it is better to use a ThreadPool so you can reuse threads already open and assign them Runnable tasks.
But when you need to launch a thread for a single task operation or instantiate a daemon thread that run for all the application time or for a long specific time then could be better create a single thread and terminate it when you don't need it anymore.

At the end of the day, they're all relying on the same underlying Thread-based mechanism to actually do the work. That means that if you are asking "what is the most efficient way to start a single thread?" the answer is, create a Thread object and call start() on it, because any other method will take some other steps before it eventually creates a Thread object and calls start() on it.
That doesn't mean that this is the best way to spawn threads, it just means that it is the most low-level way to do it from Java code. What the other ways to create threads give you is different types of infrastructure to manage the underlying Threads, so your choice of method should depend on the amount and kind of infrastructure you need.