Class vs. Interface

What's the Difference?

Class and interface are both important concepts in object-oriented programming. A class is a blueprint for creating objects, defining their properties and behaviors. It serves as a template that can be used to create multiple instances of objects. On the other hand, an interface is a contract that defines a set of methods that a class must implement. It provides a way to achieve multiple inheritance in Java, as a class can implement multiple interfaces. While a class can have both instance variables and methods, an interface can only have method signatures. In summary, a class represents a concrete implementation, while an interface represents a set of behaviors that a class can implement.


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InheritanceCan extend only one classCan extend multiple interfaces
ImplementationCan implement interfacesCannot implement classes or interfaces
Object CreationCan create objectsCannot create objects
ConstructorCan have constructorsCannot have constructors
FieldsCan have instance and static fieldsCan have only static final fields
MethodsCan have instance and static methodsCan have only abstract methods
Access ModifiersCan have public, protected, private, and default access modifiersCan have only public access modifier
Multiple InheritanceDoes not support multiple inheritanceSupports multiple inheritance through interfaces
VariablesCan have instance and static variablesCannot have variables
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Further Detail


When it comes to object-oriented programming, two fundamental concepts are classes and interfaces. Both play a crucial role in defining the structure and behavior of objects, but they have distinct characteristics and purposes. In this article, we will explore the attributes of classes and interfaces, highlighting their similarities and differences.

Definition and Purpose

A class in object-oriented programming is a blueprint or template for creating objects. It encapsulates data and methods that define the behavior and state of objects instantiated from it. Classes provide a way to organize and structure code, promoting reusability and modularity. They serve as the foundation for object creation and inheritance, allowing for the creation of objects with shared characteristics and behaviors.

On the other hand, an interface is a contract that defines a set of methods that a class must implement. It specifies the behavior that a class should exhibit without providing any implementation details. Interfaces enable multiple inheritance, allowing a class to implement multiple interfaces and inherit their methods. They promote loose coupling and provide a way to achieve polymorphism by allowing objects of different classes to be treated interchangeably based on their shared interface.

Attributes of Classes

Classes have several attributes that distinguish them from interfaces:

  1. State and Behavior: Classes encapsulate both state (data) and behavior (methods). They define instance variables to hold the state and methods to manipulate that state. This allows objects instantiated from classes to maintain their own unique state and exhibit specific behaviors.
  2. Constructors: Classes can have constructors, which are special methods used to initialize objects. Constructors are called when an object is created from a class and allow for the initialization of instance variables and other setup operations.
  3. Inheritance: Classes support inheritance, allowing for the creation of subclasses that inherit the attributes and behaviors of their parent class. Inheritance promotes code reuse and allows for the creation of more specialized classes.
  4. Access Modifiers: Classes can have access modifiers such as public, private, and protected, which control the visibility and accessibility of their members. This allows for encapsulation and data hiding, ensuring that the internal state of an object is not directly accessible from outside the class.
  5. Abstract Classes: Classes can be declared as abstract, meaning they cannot be instantiated directly but can only serve as a base for other classes. Abstract classes can have abstract methods, which are declared without an implementation and must be overridden by concrete subclasses.

Attributes of Interfaces

Interfaces possess unique attributes that differentiate them from classes:

  • Method Signatures: Interfaces define method signatures without providing any implementation details. They specify the name, parameters, and return type of methods that implementing classes must adhere to. This allows for the creation of classes with a common set of behaviors, regardless of their specific implementation.
  • Multiple Inheritance: Unlike classes, interfaces support multiple inheritance. A class can implement multiple interfaces, inheriting the method signatures from each interface. This enables a class to exhibit behaviors from multiple sources, promoting flexibility and code reuse.
  • Polymorphism: Interfaces provide a way to achieve polymorphism by allowing objects of different classes to be treated interchangeably based on their shared interface. This promotes loose coupling and flexibility in code design, as objects can be programmed to interact based on their common interface rather than their specific class.
  • Default Methods: Interfaces can have default methods, which provide a default implementation for a method. Default methods allow for the addition of new methods to an interface without breaking existing implementations. Implementing classes can choose to override default methods if needed.
  • Constants: Interfaces can define constants, which are static and final variables. Constants provide a way to define shared values that implementing classes can use. They promote code consistency and allow for the creation of classes with predefined values.


Classes and interfaces are fundamental concepts in object-oriented programming, each with its own attributes and purposes. Classes provide a blueprint for creating objects, encapsulating both state and behavior. They support inheritance, constructors, and access modifiers, allowing for code reuse and encapsulation. On the other hand, interfaces define a contract of methods that implementing classes must adhere to. They support multiple inheritance, default methods, and constants, promoting flexibility and polymorphism.

Understanding the attributes of classes and interfaces is crucial for designing effective and maintainable object-oriented systems. By leveraging the strengths of both classes and interfaces, developers can create well-structured, reusable, and flexible code.

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