Interface Class Demo.

InterfaceClass.java

/*
 * Interface Class Demo.
 *
 * - An interface is a specification of method signatures.
 * - Interface is syntatically similar to class declaration,
 *   but it lacks of instance variables (no instance variables).
 * - An interface can extend one or more interfaces at one time.
 * - Interfaces cannot be instantiated,
 *   because they represent pure abstract concept.
 * - Interface can only include abstract methods.
 * - A class cannot extend an interface, it simply implements it.
 *
 * Methods of interface are public and abstract by default.
 *
 * class to class - extends
 * class to interface - implements
 * interface to interface - extends
 * class cannot extend interface
 *
 * Usage:
 *   javac InterfaceClass.java
 *   java InterfaceDemo
 */

interface Database {
  void getConnection();
}

class Impl implements Database {
  public void getConnection() {
    System.out.println("Connecting to the database.");
  }
}

class InterfaceDemo {
  public static void main(String[] args) {
    Impl i1 = new Impl();
    i1.getConnection();
  }
}