Abstract vs. Interface

Attribute	Abstract	Interface
Definition	Abstract class can have both abstract and non-abstract methods.	Interface can only have abstract methods.
Multiple Inheritance	Cannot implement multiple abstract classes.	Can implement multiple interfaces.
Implementation	Subclasses extend abstract classes.	Classes implement interfaces.
Variables	Can have instance variables.	Cannot have instance variables.
Constructors	Can have constructors.	Cannot have constructors.

Definition

In Java, both abstract classes and interfaces are used to achieve abstraction, but they have some key differences. An abstract class is a class that cannot be instantiated on its own and may contain abstract methods that must be implemented by its subclasses. On the other hand, an interface is a reference type in Java that is similar to a class but only contains abstract methods and constants. Classes can implement multiple interfaces, but they can only extend one abstract class.

Usage

Abstract classes are typically used when a class needs to define some common behavior that can be shared among its subclasses. By marking a class as abstract, you are indicating that it is incomplete and should be extended by other classes. Interfaces, on the other hand, are used to define a contract for classes that implement them. They are useful for defining a set of methods that a class must implement, regardless of its inheritance hierarchy.

Extending vs. Implementing

One of the key differences between abstract classes and interfaces is how they are extended or implemented by other classes. Abstract classes are extended using the "extends" keyword, while interfaces are implemented using the "implements" keyword. This means that a class can only extend one abstract class, but it can implement multiple interfaces. This allows for more flexibility in designing class hierarchies.

Default Implementation

Another important distinction between abstract classes and interfaces is that abstract classes can provide default implementations for methods, while interfaces cannot. This means that when a class extends an abstract class, it can choose to override some or all of the methods, but it is not required to do so. On the other hand, when a class implements an interface, it must provide concrete implementations for all of the methods defined in the interface.

Access Modifiers

Abstract classes can have constructors, member variables, and methods with various access modifiers, such as public, private, protected, or package-private. This allows for more control over the visibility of class members and methods. Interfaces, on the other hand, can only have public static final constants and public abstract methods. All methods in an interface are implicitly public and abstract, and all variables are implicitly public, static, and final.

Multiple Inheritance

Java does not support multiple inheritance for classes, meaning a class can only extend one superclass. However, a class can implement multiple interfaces, allowing it to inherit behavior from multiple sources. This is a key advantage of interfaces over abstract classes when it comes to achieving multiple inheritance in Java. By implementing multiple interfaces, a class can define its behavior based on the contracts specified by each interface.

When to Use Abstract Classes

Abstract classes are best suited for situations where you have a base class that contains some common behavior that can be shared among its subclasses. If you have a class hierarchy where some methods are common across multiple subclasses, you can define those methods in an abstract class and have the subclasses extend it. Abstract classes are also useful when you want to provide default implementations for some methods that can be overridden by subclasses.

When to Use Interfaces

Interfaces are ideal for defining contracts that classes must adhere to. If you have a set of methods that multiple classes need to implement, you can define those methods in an interface and have the classes implement it. Interfaces are also useful when you want to achieve polymorphism without the need for a common base class. By implementing multiple interfaces, a class can exhibit different behaviors based on the contracts specified by each interface.

Conclusion

In conclusion, abstract classes and interfaces are both important tools for achieving abstraction in Java, but they serve different purposes and have distinct characteristics. Abstract classes are used to define common behavior and provide default implementations, while interfaces are used to define contracts and achieve multiple inheritance. Understanding the differences between abstract classes and interfaces is crucial for designing effective class hierarchies and promoting code reusability in Java.