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According to Baldwin, if you don't understand the Java interface, you don't understand Java. There is very little, if anything useful that can be done using Java without understanding and using the Java interface.

Revised: Thu Mar 31 14:33:41 CDT 2016

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Table of contents


This module is one of a series of modules designed to teach you about the essence of Object-Oriented Programming (OOP) using Java.

Viewing tip

I recommend that you open another copy of this document in a separate browser window and use the following links to easily find and view the listings while you are reading about them.


  • Listing 1 . Definition of interfaces named I1 and I2.
  • Listing 2 . Definition of the class named A.
  • Listing 3 . Definition of the class named B.
  • Listing 4 . Definition of the class named C.
  • Listing 5 . The driver class named Poly05.
  • Listing 6 . Complete program listing.


Method overloading

I covered method overloading as one form of polymorphism (compile-time polymorphism) in a previous module. I also explained automatic type conversion and the use of the cast operator for type conversion in a previous module.

Method overriding and class inheritance

I also discussed runtime polymorphism implemented through method overriding and class inheritance in previous modules.

Using the Java interface

In this module and the next, I will explain runtime polymorphism as implemented using method overriding and the Java interface.

A very important concept

In my opinion, this is one of the most important concepts in Java OOP, and the one that seems to give students the greatest amount of difficulty. Therefore, I will try to take it slow and easy. As usual, I will illustrate the concept using sample programs.

I will also tie this concept back to the concept of polymorphism using method overriding through inheritance.

A skeleton program

In this module, I will present a simple skeleton program that illustrates many of the important aspects of polymorphic behavior based on the Java interface.

Multiple inheritance and the cardinal rule

I will explain how the implementation of interfaces in Java is similar to multiple inheritance. I will explain the cardinal rule of interface implementation.

A new relationship

I will explain that objects instantiated from classes that implement the same interface have a new relationship that goes beyond the relationship imposed by the standard class hierarchy.

One object, many types

I will explain that due to the combination of the class hierarchy and the fact that a class can implement many different interfaces, a single object in Java can be treated as many different types. However, for any given type, there are restrictions on the methods that can be called on the object.

Many classes, one type

I will explain that because different classes can implement the same interface, objects instantiated from different classes can be treated as a common interface type.

Questions & Answers

who was the first nanotechnologist
Lizzy Reply
technologist's thinker father is Richard Feynman but the literature first user scientist Nario Tagunichi.
Norio Taniguchi
I need help
anyone have book of Abdel Salam Hamdy Makhlouf book in pdf Fundamentals of Nanoparticles: Classifications, Synthesis
Naeem Reply
what happen with The nano material on The deep space.?
pedro Reply
It could change the whole space science.
the characteristics of nano materials can be studied by solving which equation?
sibaram Reply
plz answer fast
synthesis of nano materials by chemical reaction taking place in aqueous solvents under high temperature and pressure is call?
hydrothermal synthesis
how can chip be made from sand
Eke Reply
is this allso about nanoscale material
are nano particles real
Missy Reply
Hello, if I study Physics teacher in bachelor, can I study Nanotechnology in master?
Lale Reply
no can't
where is the latest information on a no technology how can I find it
where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
has a lot of application modern world
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
STM - Scanning Tunneling Microscope.
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