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).

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.