Implementing Interfaces to Define Common Behavior and Contracts in Java

In object-oriented programming (OOP), interfaces play a crucial role in defining common behavior and contracts for classes. An interface in Java can be thought of as a contract that enforces a set of rules for the classes that implement it. It allows developers to create reusable code and establish a level of consistency throughout their codebase. In this article, we will explore how to implement interfaces in Java and discuss their benefits.

Understanding Interfaces

An interface in Java is defined using the interface keyword and can contain method signatures, constants, and nested types. It serves as a blueprint for implementing classes, signaling the methods and behaviors they must provide. Unlike abstract classes, interfaces cannot have method implementations, making them purely abstract. This design decision promotes loose coupling and enables multiple inheritance, as a class can implement multiple interfaces.

Implementing an Interface

To implement an interface, a class must use the implements keyword followed by the interface name(s). The class must then provide concrete implementations for all the methods defined in the interface. By doing so, the class guarantees that it adheres to the contract specified by the interface.

Here's an example of an interface named Drawable that defines a single method draw():

public interface Drawable {
    void draw();

Let's say we have a Circle class that needs to implement the Drawable interface. We would do so like this:

public class Circle implements Drawable {
    public void draw() {
        // Provide the implementation for drawing a circle

By implementing the Drawable interface, the Circle class ensures that it provides a valid implementation for the draw() method. This allows us to treat any Circle object as a Drawable object, enabling polymorphism and encapsulating common behavior.

Achieving Polymorphism through Interfaces

Interfaces enable polymorphic behavior, where objects of different classes can be treated as instances of a common interface. Let's expand on our previous example and introduce another class called Rectangle, also implementing the Drawable interface:

public class Rectangle implements Drawable {
    public void draw() {
        // Provide the implementation for drawing a rectangle

Now, we can create an array of Drawable objects and store objects of both the Circle and Rectangle classes:

Drawable[] drawables = new Drawable[2];
drawables[0] = new Circle();
drawables[1] = new Rectangle();

By doing so, we can iterate over the drawables array and call the draw() method on each object without needing to explicitly handle different class types. This flexibility simplifies the codebase and allows for more extensible and modular designs.

Advantages of Using Interfaces

Implementing interfaces in Java offers several advantages, including:

Code Reusability

Interfaces allow developers to define common behavior once and reuse it in multiple classes. By having classes implement an interface, they inherit the specified behavior without the need for code duplication.

Loose Coupling

Using interfaces promotes loose coupling, where classes depend on abstractions instead of concrete implementations. This decoupling enhances the flexibility and maintainability of code, as classes can be modified without affecting unrelated components.

Integration with Existing Code

Interfaces are particularly useful when integrating new code with existing codebases. By designing new classes to implement existing interfaces, developers ensure compatibility and seamless integration between different parts of an application.

Multiple Inheritance

Unlike classes, which can only extend a single superclass, a class can implement multiple interfaces. This feature enables multiple inheritance in Java, allowing classes to inherit behavior from multiple sources.


Interfaces in Java are powerful tools for defining common behavior and contracts. By implementing interfaces, developers ensure that their classes adhere to specific contracts, promoting code reusability, loose coupling, and polymorphic behavior. Interfaces enhance the extensibility and maintainability of codebases, making them an essential aspect of object-oriented programming in Java.

© NoobToMaster - A 10xcoder company