In a simple to understand explanation, what is Runnable in Java? [closed]

Question

What is "runnable" in Java, in layman's terms? I am an AP programming student in high school, whose assignment is to do research, or seek out from others what "runnable" is (we are just getting into OOP, and haven't touched threads yet).

Mcsweeney

There is only one useful line in the API docs: "The general contract of the method run is that it may take any action whatsoever." – Trackandfield

A much more interesting research task would have been: What is "stoppable" in Java, in layman's terms. Todays Instructors are so ... – Sarcocarp

@UdoKlimaschewski - the guy is a high school student, not an undergraduate or postgraduate student. This instructor's expectations are commensurate with the level of student ... IMO. However, I agree that "first ask on SO" is NOT proper research technique for someone at the OP's level. – Edouard

Answer 1

A Runnable is basically a type of class (Runnable is an Interface) that can be put into a thread, describing what the thread is supposed to do.

The Runnable Interface requires of the class to implement the method run() like so:

public class MyRunnableTask implements Runnable {
     public void run() {
         // do stuff here
     }
}

And then use it like this:

Thread t = new Thread(new MyRunnableTask());
t.start();

If you did not have the Runnable interface, the Thread class, which is responsible to execute your stuff in the other thread, would not have the promise to find a run() method in your class, so you could get errors. That is why you need to implement the interface.

Advanced: Anonymous Type

Note that you do not need to define a class as usual, you can do all of that inline:

Thread t = new Thread(new Runnable() {
    public void run() {
        // stuff here
    }
});
t.start();

This is similar to the above, only you don't create another named class.

Maillol

This is a great answer that disproves entirely the claim that the question "cannot be reasonably answered in its current form". – Snob

I know it had been a long time but I have a question. Thread is only run one time or just keep running over and over inside program when I start a thread? – Vulgarize

The thread is run until the run method finishes, then the Thread is closed. If you want a thread to run over and over, place a loop inside the run method. – Maillol

Or simply using a Java 8 lambda function: new Thread(() -> System.out.println("hello from other thread")).start(); – Antione

Answer 2

Runnable is an interface defined as so:

interface Runnable {
    public void run();
}

To make a class which uses it, just define the class as (public) class MyRunnable implements Runnable {

It can be used without even making a new Thread. It's basically your basic interface with a single method, run, that can be called.

If you make a new Thread with runnable as it's parameter, it will call the run method in a new Thread.

It should also be noted that Threads implement Runnable, and that is called when the new Thread is made (in the new thread). The default implementation just calls whatever Runnable you handed in the constructor, which is why you can just do new Thread(someRunnable) without overriding Thread's run method.

Barneybarnhart

Are you saying there is basically no difference between inheriting Thread or implementing Runnable? Is Thread when using the Runnable used as a wrapper, as well as the executor? – Maillol

I'm saying there's no difference between [extending Thread and overriding run] and [creating a new instance of Thread with a runnable that has the same run method] – Barneybarnhart

Yeah got that. But is there really no difference, neither performance- nor otherwise? – Maillol

@AlexColeman - there is a big difference. If you extend Thread thread pooling is difficult and if you don't implement thread pooling you wear the significant cost of creating new threads all the time. By contrast, just implementing Runnable means that you can easily use things like ThreadPoolExecutorService or a 3rd-party thread pool. – Edouard