Nesting Classes in an Interface Definition : Interface « Class Definition « Java Tutorial

An inner class to an interface will be static and public by default.

interface Port {
  // Methods & Constants declared in the interface...

  class Info {
    // Definition of the class...

public class MainClass {
  public static void main(String[] a) {
    Port.Info info = new Port.Info();

5.29.1.Interfaces and Abstract Classes
5.29.2.Fields and Methods in an Interface
5.29.3.To implement an interface: use the implements keyword after the class declaration
5.29.4.A Partial Interface Implementation
5.29.5.Extending Interfaces
5.29.6.Interfaces and Multiple Inheritance
5.29.7.Interfaces and Polymorphism: Using Multiple Interfaces
5.29.8.Nesting Classes in an Interface Definition
5.29.9.Encapsulating Constants in a Program
5.29.10.Multiple interfaces
5.29.11.Initializing interface fields with non-constant initializers