Differences Between Interfaces and Abstract Classes in Java

Using relevant properties highlight the differences between interfaces and abstract classes In Java.

In Java, both interfaces and abstract classes are used to achieve abstraction, but they have distinct characteristics and use cases. Here are the key differences between interfaces and abstract classes:

1. Definition and Purpose

• Interface: An interface is a reference type in Java that can contain only constants, method signatures, default methods, static methods, and nested types. Interfaces are used to define a contract that implementing classes must follow.
• Abstract Class: An abstract class is a class that cannot be instantiated on its own and can contain both abstract methods (without a body) and concrete methods (with a body). Abstract classes are used to provide a common base for subclasses.

2. Method Implementation

• Interface: All methods in an interface are implicitly public and abstract (except for default and static methods). Prior to Java 8, interfaces could not have any method implementations. From Java 8 onwards, interfaces can have default methods with implementations.
• Abstract Class: An abstract class can have both abstract methods (which must be implemented by subclasses) and concrete methods (which can be inherited as-is).

3. Multiple Inheritance

• Interface: A class can implement multiple interfaces, allowing for a form of multiple inheritance. This is useful for defining capabilities that can be shared across different classes.
• Abstract Class: A class can extend only one abstract class (single inheritance). This means that a class cannot inherit from multiple abstract classes.

4. Fields and State

• Interface: Interfaces can only have static final variables (constants). They cannot have instance variables or maintain state.
• Abstract Class: Abstract classes can have instance variables and can maintain state. They can also have constructors.

5. Accessibility Modifiers

• Interface: All methods in an interface are public by default. Fields are also public, static, and final.
• Abstract Class: Abstract classes can have various access modifiers (public, protected, private) for their methods and fields, allowing for more control over visibility.

6. Use Cases

• Interface: Use interfaces when you want to define a contract for classes that may not share a common ancestor or when you want to achieve multiple inheritance of type.
• Abstract Class: Use abstract classes when you want to provide a common base with shared code and state, and when you want to enforce a certain structure among subclasses.

7. Example

Here’s a simple example to illustrate the differences:

// Interface example
interface Animal {
    void makeSound(); // implicitly public and abstract
}

// Abstract class example
abstract class Mammal {
    abstract void giveBirth(); // abstract method
    void breathe() { // concrete method
        System.out.println("Breathing...");
    }
}

// Implementing the interface
class Dog implements Animal {
    public void makeSound() {
        System.out.println("Bark");
    }
}

// Extending the abstract class
class Whale extends Mammal {
    void giveBirth() {
        System.out.println("Giving birth to a calf");
    }
}

Summary

In summary, interfaces are best suited for defining capabilities and contracts, while abstract classes are ideal for sharing code and state among related classes. The choice between using an interface or an abstract class depends on the specific design requirements of your application.