Interfaces & Abstract Classes

Interfaces
an interface is a class that only has method headers; if you use (implement) an interface, you must provide a body for every method in the header

Example of using an interface:

//here is the interface
//only method headers -- no bodies
//for the methods..

public interface Info {
      public int tryIt(int i);
      public void hiThere();
}

//here is the class which uses (implements the
//interface called Info) must provide a body for
//all methods in the interface called Info
//The class called NewInfo which implements
//the interface Info can have its own methods as well

public class NewInfo implements Info {
   private int type;

    //from interface
   public int tryIt(int i) {
      int x;
      x = i * 5;
      return x;
   }

  //from interface
   public void hiThere() {
      System.out.println("hi there!");
   }

  //regular method in NewInfo
   public void doThat(int i) {     
      int i= 5 * 9;
   }
}

Abstract Classes
used with inheritance
a normal class with the exception that their is at least one method in the abstract class that only contains a method header (a method without a body). If you use the abstract class as a parent, you must provide a body for all method headers in the parent abstract class.  In essence, a abstract class is a combination of both a normal class and a interface.  It is a placeholder in a system.

//here is the abstract class
//both method headers and methods with a body

public class Info {
      private int theOne;

      public int tryIt(int i); //an abstract method - no body

      public void hiThere() {
           System.out.println("hi there!");      }
      }
}

//here is the child class
//method headers must be written with a body
//for all abstract class from parent

public class BabyInfo extends Info {
 
      //an abstract method - provide a body for
      public int tryIt(int i) {
            int x = i + 7;
            return (x);
      }

      public void noNo() {
           System.out.println("No No!");      }
      }
}