Architecture & DesignObjects and Interfaces

Objects and Interfaces content and product recommendations are editorially independent. We may make money when you click on links to our partners. Learn More.

This series, The Object-Oriented Thought Process, is intended for someone just learning an object-oriented language and who wants to understand the basic concepts before jumping into the code, or someone who wants to understand the infrastructure behind an object-oriented language he or she is already using. These concepts are part of the foundation that any programmer will need to make the paradigm shift from procedural programming to object-oriented programming.

This is exactly what happens when a developer uses an API—with the project manager or business owner representing the law. In short, when using contracts, the developer is required to comply with the rules defined in the framework. This includes issues such as method names, number of parameters, and so on. In short, standards are created to facilitate good coding practices.

Note: The term contract is widely used in many aspects of business, including software development. Do not confuse the concept here with other possible software design concepts called contracts.

Enforcement is vital because it is always possible for a developer to break a contract. Without enforcement, a rogue developer could decide to reinvent the wheel and write his or her own code rather than use the code provided by the framework. There is little benefit to a standard if people routinely disregard or circumvent it. In Java and the .NET languages, the two ways to implement contracts are to use abstract classes and interfaces.

Designing with Object-Oriented Interfaces

Let me start right away with an example. Suppose that you work for a software development company that is writing software to play a baseball game. You will create a class called Batter and this Batter class will have a behavior (method) called swing. There are a number of programmers assigned to work on the project. One programmer is assigned the task to design and implement the LeftHandedBatter class, and one is assigned to design and implement the RightHandedBatter class. If you then send them off to their respective cubes, they may, possibly, come back with the following two class designs in Listing 1 and Listing 2.

Listing 1:

public class RightHandBatter {

   public void rightHandBatterSwing () {

      System.out.println("Righty Swing");



Listing 2:

public class Lefty {

   public void leftySwing() {

      System.out.println("Lefty Swing");



To use these classes, you create an application called TestBatter that is found in Listing 3.

Listing 3:

class TestBatter{

   public static void main(String[] args){

      Lefty lefty = new Lefty();
      RightHandBatter righty = new RightHandBatter();



Perhaps you have noticed a fundamental flaw in the management, or lack of management, involved in this project. The flaw is that the way you swing differs depending on the type of batter you are. If you are a right-handed batter, you must use the rightHandBatterSwing( ) method, and if you are a left-handed batter, you must use the leftySwing ( ) method. At first, this may not seem like much of a problem; however, the names of the methods are not necessarily consistent. In fact, the decision on the name of the methods was left totally up to the discretion of the programmers. While this may be a good way to encourage creativity, it is not a very good design decision. Now, explore this problem more closely.

As the project manager for this application, one thing that I want to ensure is that the naming conventions are standard, clearly specified, and enforced. For example, if you were a third programmer on this team with the responsibility to actually use the two batter classes, you might logically assume that when any batter swings the method would be called swing( ). While this may seem reasonable, the two programmers in this case came up with two totally different method names. Even though these differing method names would work in practice, they are not good design for several reasons, including the fact that swing( ) is a much better choice. So, how would a project manager force all the Batter classes to use a method called swing( )?

Abstract Methods

This is where the object-oriented interface comes in. Short of doing code reviews and disciplining programmers who do not follow the programming standards, you can create an interface called Batter. You can create a class diagram representing the design of this interface, simple as it is. This class diagram is found in Figure 2. Note that the class name and the method swing( ) are presented in italics, which signifies that the class and the method are abstract.

Figure 2: Batter Interface

The corresponding code is found in Listing 4.

Listing 4:

public interface Batter {

   public void swing();


By itself, this will not directly remedy the inconsistencies; however, as project manager, I now require that all programmers creating any type of Batter class must implement this Batter interface. While there is not much code in Listing 4, it illustrates a very important concept. By creating this interface, the project manager is saying:

To be a valid type of batter class, you must implement the Batter interface that requires you to implement a method called swing( ).

In effect, the Batter interface is a contract that the programmer must follow. The contract states that if you want to be a Batter in this system, you must implement the Batter interface. Abiding by this contract requires that you provide a swing( ) method. The class diagram for the entire Batter system is shown in Figure 3. Note that both the RightHandBatter class and the LeftHandBatter class must implement the swing( ) method.

Figure 3: Batter System

Thus, when the programmer who creates the right hand batter class writes code that does not conform to this model, as seen in Listing 5, you have a problem.

Listing 5:

public class RightHandBatter implements Batter {

   public void rightHandBatterSwing() {

      System.out.println("Righty Swing");



With the Batter interface in place, the programmer who attempts to compile the class in Listing 5 will receive the following compiler error:

C:column27>"C:Program FilesJavajdk1.5.0_06binjavac"
   -Xlint -classpath . RightHandBatter is not abstract and does
not override abstract method swing() in Batter
public class RightHandBatter implements Batter {
1 error

The error was generated due to the fact that the programmer who designed this class failed to meet the requirements of the contract. The content of the message is informative. The interface Batter contains a single method specification, swing( ). Note that the swing method has no implementation. By this, I mean that there are no curly braces as follows:

public void swing( ){ };

Even though there is no working code between these curly braces, the mere fact that the curly braces are present provides an implementation. It is an empty implementation; however, it is an implementation nonetheless. The code for the interface contains only the method signature and no curly braces.

public void swing( );

This syntax defines the method as abstract. When a method is abstract, this means that any class implementing or inheriting this abstract method must provide the implementation for the abstract method or that class will itself be considered abstract.

Abstract Hierarchies

For example, consider the model where you want to create subclasses of the RightHandBatter class, say a class called GoodRightHandBatter. In this case, you could implement the swing( ) method in the subclass. Note that in Figure 4, both the Batter and the RighHandBatter classes are abstract—only the final class in the hierarchy, GoodRightHandBatter is considered concrete, which means that you can instantiate it.

Figure 4: Abstract Hierarchy

The error message states that “RightHandBatter is not abstract.” This is because it is not defined as abstract. If you wanted to make RightHandBatter abstract, the code would look like this:

public abstract class RightHandBatter implements Batter {


The error message also states that “RightHandBatter does not override abstract method swing( ) is Batter“. This is the important issue in this example. The compiler is catching the fact that RightHandBatter is not satisfying the contract because it is not overriding the abstract method swing( ). Herein lies the power of the interface. By designing the appropriate interfaces and then requiring their use, software developers have a powerful tool.

To remedy the problem in the example above, a simple redesign is in order. The redesign will require the following code change as seen in Listing 6.

Listing 6:

public class RightHandBatter implements Batter {

   public void swing() {

      System.out.println("Righty Swing");



Now that the swing( ) method has been provided, the code for the RightHandBatter class will compile cleanly. The same approach must be followed for the LeftHandBatter class.

Listing 7:

public class LefttHandBatter implements Batter {

   public void swing( ) {

      System.out.println("Lefty Swing");



At this point, you can standardize the way that you have batters swing by simply using a swing( ) method regardless of what type of batter you have at the plate.

Listing 8:

class TestBatter{

   public static void main(String[] args){

      LeftHandBatter lefty   = new LeftHandBatter();
      RightHandBatter righty = new RightHandBatter();

      lefty.swing( );
      righty.swing( );


No matter who the batter is, when you want the batter to swing, you only have to say “swing”. This can cut down on any number of errors; however, perhaps the greatest power of the object-oriented interface is the increased usability. Basically, systems with well designed object-oriented interfaces are much easier to use and have less maintenance cost. To illustrate, you can try to enhance this system.

You can create a third type of batter called switchHitter, as seen in Listing 9. The class diagram representing the updated system is found in Figure 5.

Listing 9:

public class SwitchHitter implements Batter {

   public void swing() {

      System.out.println("Switch Hitter Swing");



Because the programmer implementing this class is required to implement the interface Batter, the swing( ) method is required. Thus, when the application uses the new SwitchHitter class, all the application needs to know is: If SwitchHitter is a true Batter, all I have to do is say “swing” and the SwitchHitter will swing, as seen in Listing 10.

Listing 10:

class TestBatter{

   public static void main(String[] args){

      LeftHandBatter lefty      = new LeftHandBatter();
      RightHandBatter righty    = new RightHandBatter();
      SwitchHitter switchHitter = new SwitchHitter ();



Figure 5: Batter Hierarchy

With the Batter interface in place, you can add as many different types of Batters as you like. And, no matter what the type of Batter is or does, you will know that it will have a swing( ) method.

Interfaces versus implementation

Although the object-oriented interface requires that a prescribed contract be followed, there is no way to enforce the code implementation of the contract.

For example, consider the code in Listing 11. This is a simple example. There are no object-oriented interfaces; however, there is a method defined as add( ). The danger with any method, whether it is part of an interface specification or not, is that the implementation within the method body (the actual code) is at the discretion of the programmer.

Listing 11:

class TestInterface {

   public static void main(String[] args){

      int value = 0;

      value = add(5, 6);

      System.out.println("value = " + value);


   public static int add (int num1, int num2) {

      int num = num1 - num2;

      return num;

As can be seen in the example in Listing 11, a programmer may well follow a method specification, in this case add( ); yet the implementation may be at odds with the interface. In this case, the understandable expectation is that the add( ) method will return the sum of the parameters, not the difference, as this code apparently does. There is not a compiler mechanism that can enforce this. In short, using object-oriented interfaces is a great way to enforce the user interfaces, but they will not control the implementation.


When creating an interface, all of the methods must be abstract. There is no actual implementation code in an object-oriented interface. However, there are times when you want to design a class that has both abstract and concrete methods.

In some languages, like C++, the same mechanism is used to create interfaces, which contain only abstract methods, and abstract classes, which contain both abstract and concrete methods.

In the next article, you will explore the differences between abstract classes and interfaces.


  • Just Java 2, 6th Edition. Peter van der Linden. 2004, Sun Microsystems.

About the Author

Matt Weisfeld is a faculty member at Cuyahoga Community College (Tri-C) in Cleveland, Ohio. Matt is a member of the Information Technology department, teaching programming languages such as C++, Java, C#, and .NET as well as various Web technologies. Prior to joining Tri-C, Matt spent 20 years in the information technology industry gaining experience in software development, project management, business development, corporate training, and part-time teaching. Matt holds an MS in computer science and an MBA in project management. Besides The Object-Oriented Thought Process, which is now in its second edition, Matt has published two other computer books, and more than a dozen articles in magazines and journals such as Dr. Dobb’s Journal, The C/C++ Users Journal, Software Development Magazine, Java Report, and the international journal Project Management. Matt has presented at conferences throughout the United States and Canada.

Get the Free Newsletter!

Subscribe to Developer Insider for top news, trends & analysis

Latest Posts

Related Stories