What are the benefits of defining interfaces in Java classes

The first step is to define a class and define the interface inside

public class Util {
Public interface Worker {
void work (int a);
}

}

Step Two: Define a class to implement the interface defined in the first step class

public class Demo implements Worker {

@Override
public void work (int a) {
System.out.println ("Hello,this word is happy" + a);

}

public static void Main (string[] args) {
Demo Ademo = new Demo ();
Ademo.work (6);
}

}

the benefits of doing this are:

1, in the Util class inside the package of the variable worker interface, that is to say, the Util class has its own algorithm: part of their own fixed,

part of it can change, and the variable part is programming an interface (where an interface is not a new design, but an idea that makes code more scalable)