Understanding Java: Interface vs Abstract Class

Introduction

In the world of object-oriented programming, Java offers two powerful features - interfaces and abstract classes. Both are used to define contracts for classes, but they differ significantly in terms of usage and behavior. In this post, we will delve into the concepts of interfaces and abstract classes in Java, exploring their unique characteristics, differences, and use cases.

What is an Interface?

An interface in Java is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. It is a way to ensure that a class adheres to a certain contract or protocol. A class implements an interface using the implements keyword.

Here's an example of an interface:

Example in java

interface Animal { 
    void eat(); 
    void sleep(); 
}

An interface provides a high level of abstraction, as it allows us to define what a class should do, but not how it should do it.

What is an Abstract Class?

An abstract class, on the other hand, is a class that cannot be instantiated and is always used as a base class. It can contain both abstract methods (methods without a body) and concrete methods (methods with an implementation). Abstract classes are defined using the abstract keyword.

Here's an example of an abstract class:

Example in java

abstract class Animal { 
    abstract void eat(); 
    abstract void sleep(); 
    void breathe() { 
        // implementation of breathe method 
    } 
}

Abstract classes provide a mechanism to encapsulate common functionalities for subclasses.

Interface vs Abstract Class: Key Differences

Basis of Comparison	Interface	Abstract Class
Implementation	A class can implement multiple interfaces.	A class can extend only one abstract class.
Default Methods	Interfaces cannot provide any code, just the method signatures, except for default and static methods (Java 8 onwards).	Abstract classes can provide complete code, default code and/or just the details that need to be overridden.
Accessibility	Members of an interface are public by default.	An abstract class can have members with all access modifiers (private, protected, etc.).
Constructor	Interfaces cannot have constructors.	Abstract classes can have constructors.
Performance	Interfaces can be slower and require more memory due to extra indirection.	Abstract classes can be faster due to the direct nature of method calls.
Type of Methods	Only abstract methods can be declared, except for default and static methods (Java 8 onwards).	Both abstract and non-abstract methods can be declared.
Type of Variables	Only final and static variables can be declared.	Abstract classes can have both final and non-final as well as static and non-static variables.
Multiple Inheritance	Interfaces support multiple inheritance. A class can implement multiple interfaces.	Abstract classes do not support multiple inheritance. A class can extend only one abstract class.
Functional Programming	With the introduction of functional programming in Java 8, interfaces can now have default and static methods.	Abstract classes cannot define functional programming methods unless they also implement an interface.
Use Case	Useful when different implementations share method signatures. It provides a form of multiple inheritances.	Useful when different implementations share common state (data members) and behavior (methods).
Flexibility	More flexible than abstract classes as one class can implement multiple interfaces.	Less flexible as a class can extend only one abstract class due to Java's single inheritance rule.
Fields	Interfaces cannot contain fields.	Abstract classes can contain fields, thus allowing you to define behavior and state.
Implementation	Forces a class to implement all its declared methods.	Allows a class to choose which methods to implement.
Inheritance	Interfaces can inherit from multiple interfaces using the `extends` keyword.	Abstract classes can extend other classes or abstract classes using the `extends` keyword.
Implementation Overhead	Implementing an interface requires writing method implementations for all declared methods.	Extending an abstract class allows for selectively overriding abstract methods and implementing concrete methods.
Extensibility	Adding a new method to an interface can potentially break implementations in existing classes.	Adding a new method to an abstract class does not affect existing subclasses.
Size and Complexity	Interfaces tend to be smaller and less complex as they only define method signatures.	Abstract classes can have a broader scope, encapsulating data and behavior.
Design Focus	Interfaces focus on defining contracts and specifying behavior for a wide range of unrelated classes.	Abstract classes focus on providing a common base for related classes, emphasizing code reusability.
Usage in Libraries	Interfaces are commonly used in libraries and frameworks to define APIs and allow for customization.	Abstract classes are used when providing partial implementations and common functionality to subclasses.
Type Compatibility	An object can be polymorphically referenced by an interface, allowing for more flexibility in type usage.	Abstract classes provide a more specific type hierarchy and restrict the polymorphic references to the abstract class type and its subclasses.

When to Use Interfaces and Abstract Classes

Use an interface when:

You expect unrelated classes to implement your interface. For example, interfaces Comparable and Cloneable are implemented by many unrelated classes.
You want to specify the behavior of a particular data type, but not concerned about who implements its behavior.
You want to take advantage of multiple inheritances of types.

Use an abstract class when:

You want to share code among several closely related classes.
You expect classes that extend your abstract class to have many common methods or fields or require access modifiers other than public.
You want to declare non-static or non-final fields.

Conclusion

Both interfaces and abstract classes are essential parts of Java, and they have their specific use cases. Understanding when to use an interface and when to use an abstract class is crucial in the design of an object-oriented system. As a rule of thumb, an interface defines a contract for functionality, while an abstract class provides a partial implementation for functionality shared among closely related classes.