Abstract class vs interface

This post describes the significance of Abstract class and interface and the differences between them.

5/22/20233 min read

In object-oriented programming, both abstract classes and interfaces play vital roles in defining the structure and behavior of classes. While they share some similarities, they also have distinct characteristics and purposes. In this article, we will explore the differences between abstract classes and interfaces, understanding when and how to use each construct appropriately.

1. Definition

An abstract class is a class that cannot be instantiated and is primarily used as a blueprint for other classes. It may contain both abstract and non-abstract methods and can provide partial implementations of its methods. Abstract classes serve as a foundation for derived classes to inherit common attributes and behaviors while allowing each derived class to implement its specific functionalities.

On the other hand, an interface is a contract or a set of rules that a class must adhere to. It defines a collection of abstract methods that a class implementing the interface must implement. Interfaces are used to establish a common behavior among unrelated classes, promoting code reusability and allowing multiple inheritance-like capabilities through interface implementation.

2. Method Implementation

Abstract classes can have both abstract and non-abstract methods. Abstract methods are declared without an implementation and must be implemented by any concrete class that extends the abstract class. Non-abstract methods in an abstract class can have a defined implementation and can be directly used by derived classes. This allows abstract classes to provide common functionality that can be inherited by multiple derived classes, reducing code duplication.

Interfaces, on the other hand, only define abstract methods. All methods declared in an interface are implicitly abstract and must be implemented by any class that implements the interface. Interfaces do not provide any method implementations. Instead, they establish a contract that specifies the methods a class must implement to fulfill the interface requirements.

3. Inheritance

In Java, a class can extend only one abstract class, as Java supports single inheritance. This means that a class can inherit from an abstract class to obtain its attributes and behaviors. By extending an abstract class, a derived class forms an "is-a" relationship with the abstract class. This allows the derived class to inherit the abstract class's fields, non-abstract methods, and even provide implementations for its abstract methods.

In contrast, a class can implement multiple interfaces in Java, as it supports multiple inheritance through interfaces. By implementing an interface, a class forms a "has-a" relationship with the interface. The class must provide implementations for all the methods declared in the interface. Implementing multiple interfaces allows a class to inherit the contract and behavior defined by each interface, enabling code reuse across unrelated classes.

4. Accessibility and Flexibility

Abstract classes can have different access modifiers for their members, allowing flexibility in defining visibility within the class hierarchy. Abstract classes can also have constructors, which facilitate the initialization of their derived classes. However, a class inheriting from an abstract class must use the "extends" keyword.

Interfaces, on the other hand, are inherently public. All methods declared in an interface are public by default, and interfaces cannot have constructors. When a class implements an interface, it must explicitly define the implemented methods as public. Additionally, a class implementing an interface uses the "implements" keyword.

5. Usage Scenarios

Abstract classes are typically used when there is a need to define common attributes and behaviors for a group of related classes. They are useful in scenarios where code reuse and inheritance play a significant role. Abstract classes provide a level of abstraction and allow for the creation of a base class that can be extended and specialized by derived classes.

Interfaces are best suited when there is a need to define a contract or establish common behavior across

unrelated classes. They are used to enforce a specific set of methods that a class must implement, promoting loose coupling and allowing multiple unrelated classes to conform to the same interface.

When choosing between an abstract class and an interface, consider the relationship between the classes, the level of code reuse needed, and the desired flexibility in inheritance and implementation.


Abstract classes and interfaces are essential concepts in object-oriented programming. Abstract classes provide a way to create a common base for derived classes and allow for code reuse and inheritance. Interfaces, on the other hand, define contracts that unrelated classes must adhere to, promoting loose coupling and multiple inheritance-like capabilities. By understanding the differences between abstract classes and interfaces, developers can make informed design decisions and effectively structure their software systems.

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