Deciding between an abstract class and interface is vital for reusable and flexible object-oriented code. But what exactly is the difference, and when should each be used?
In this comprehensive guide, we’ll clarify abstract classes vs interfaces in depth so you can make optimal decisions in your own projects.
Here’s what we’ll cover:
- Key Differences Summarized
- Abstract Class Explanation
- Interface Explanation
- Implementation Examples
- Performance Considerations
- Alternative Interface Approaches
- When to Use Each One
- FAQ
So let’s get started!
Key Differences Summarized
First, the major differences between abstract classes and interfaces:
| Criteria | Abstract Class | Interface |
|---|---|---|
| Implementation | Contains both abstract and non-abstract methods | Cannot contain method implementations |
| Instantiation | Cannot be directly instantiated | Cannot be directly instantiated |
| Inheritance | A class can inherit only 1 abstract class | A class can inherit multiple interfaces |
| Access Modifiers | Can specify access modifiers for methods/properties | Members are public by default |
| Extension | Can be extended to add new methods | Adding methods breaks implementing classes |
To summarize:
- Abstract classes provide partial implementations to subclasses
- Interfaces only provide method/property signatures as a contract
Understanding these key traits will guide your usage of each.
Now let’s unpack what exactly abstract classes and interfaces entail…
Abstract Class Explanation
Abstract classes act as templates for concrete subclass extension. They allow code reuse without dictating exact implementation details to subtypes.
For example:
public abstract class Animal {
public void eat() {
// generic eat implementation
}
// Abstract subclass must implement
public abstract void makeSound();
}Here Animal defines reused eat() logic while leaving makeSound() up to concrete descendants:
public class Dog extends Animal {
@Override
public void makeSound() {
System.out.println("Bark!");
}
}
public class Cat extends Animal {
@Override
public void makeSound() {
System.out.println("Meow!");
}
} Now Dog and Cat simply plug in their specific sounds.
Key properties of abstract classes:
- Prevent direct instantiation with the
abstractkeyword - Can contain both abstract and non-abstract methods
- Abstract methods have no body, just a signature
- Non-abstract methods provide reuseable implementations
- Promote polymorphism and code reuse through inheritance
- Concrete subclasses must implement all abstract methods
Overall, abstract classes strike a balance between reusable logic and subclass flexibility.
Interface Explanation
Contrast this with interfaces, which define functionality contracts without any method bodies:
public interface Barkable {
// Signature only - no implementation
void bark();
}Any class implementing Barkable must contain a bark() method:
public class Dog implements Barkable {
@Override
public void bark() {
System.out.println("Woof!");
}
} Key aspects of interfaces:
- Defined with the
interfacekeyword - Cannot be instantiated directly
- Only contain method and property signatures
- No access modifiers – members are public
- Implemented by concrete classes with method bodies
- Classes can implement multiple interfaces
Fundamentally, interfaces enforce a protocol across disparate classes without dictating specifics. This loose coupling promotes flexibility.
Now that we’ve clarified abstract classes and interfaces independently, let’s look at code examples across languages…
Implementation Examples
While details vary by language, the concepts transfer broadly.
Consider how we implement the same abstract class and interface in Java, C++, C# and Python:
Abstract Class Examples
Here is how an abstract Vehicle parent with reuseable functionality would be defined:
Java
public abstract class Vehicle {
private String type;
public abstract void start();
public void brake() {
System.out.println("Applying brakes!");
}
}
public class Car extends Vehicle {
@Override
public void start() {
System.out.println("Turning ignition key");
}
} C++
class Vehicle {
public:
string type;
virtual void start() = 0;
void brake() {
cout << "Applying brakes!\n";
}
};
class Car : public Vehicle {
public:
void start() override {
cout << "Turning ignition key\n";
}
}; C#
public abstract class Vehicle {
public string Type { get; set; }
public abstract void Start();
public void Brake() {
Console.WriteLine("Applying brakes!");
}
}
public class Car : Vehicle {
public override void Start() {
Console.WriteLine("Turning ignition key");
}
}Python
from abc import ABC, abstractmethod
class Vehicle(ABC):
def __init__(self, type):
self.type = type
@abstractmethod
def start(self):
pass
def brake(self):
print("Applying brakes!")
class Car(Vehicle):
def __init__(self, type):
super().__init__(type)
def start(self):
print("Turning ignition key")Despite language variation, the abstract class concepts remain consistent:
- Inheritable template with partial implementation
- Abstract methods without bodies
- Concrete subclasses overriding to provide specifics
Interface Examples
Similarly, interfaces share many core traits across languages:
Java
public interface Driveable {
void turnOn();
void accelerate();
}
public class Car implements Driveable {
public void turnOn() {
System.out.println("Turning ignition key");
}
public void accelerate() {
System.out.println("Pressing accelerator");
}
}C++
class Driveable {
public:
virtual void turnOn() = 0;
virtual void accelerate() = 0;
};
class Car : public Driveable {
public:
void turnOn() override {
cout << "Turning ignition key\n";
}
void accelerate() override {
cout << "Pressing accelerator\n";
}
};C#
public interface Driveable {
void TurnOn();
void Accelerate();
}
public class Car : Driveable {
public void TurnOn() {
Console.WriteLine("Turning ignition key");
}
public void Accelerate() {
Console.WriteLine("Pressing accelerator");
}
}Python
from abc import ABC, abstractmethod
class Driveable(ABC):
@abstractmethod
def turn_on(self):
pass
@abstractmethod
def accelerate(self):
pass
class Car(Driveable):
def turn_on(self):
print("Turning ignition key")
def accelerate(self):
print("Pressing accelerator") The syntax differs but core concepts remain:
- Signature-only methods
- Implemented by concrete classes
- No access specifiers – public by default
This forces adhering classes to focus on what is done rather than how.
Now that we’ve seen interfaces and abstract classes in practice across languages, let’s compare performance tradeoffs.
Performance Considerations
In terms of run-time performance:
- Interfaces tend to be faster in languages like C# and Java since calls resolve at compile-time. There is less overhead compared to abstract classes
- Abstract classes can be slower due to virtual method invocation and extra checks required at runtime. But they also provide more metadata available for JIT optimizations.
So while microbenchmarks might show interfaces faster, in practice this rarely matters except for very performance-sensitive code.
And interfaces have downsides to be aware of:
- More memory needed with extra indirections per call
- Added complexity when calls span languages due to marshaling
Weighing all factors, abstract classes likely incur slightly more overhead but not enough to warrant choosing interfaces universally.
Alternative Interface Techniques
Some languages provide alternatives to simulate multiple inheritance interfaces:
C++ Pure Virtual Classes
We can emulate interfaces in C++ with pure virtual base classes:
class Interface {
public:
virtual void method1() = 0;
virtual void method2() = 0;
};
class Implementation : public Interface {
public:
void method1() override {
//implementation
}
void method2() override {
//implementation
}
};While not an interface per se, this achieves similar versioning flexibility.
Python ABC module
Python’s built-in ABC module offers @abstractmethod decorators to define interface-like abstract methods:
from abc import ABC, abstractmethod
class MyInterface(ABC):
@abstractmethod
def method1(self):
pass
@abstractmethod
def method2(self):
pass
class Implementation(MyInterface):
def method1(self):
#implementation
def method2(self):
#implementationSo multiple techniques exist beyond base interfaces, with varying tradeoffs.
When to Use Each One
Given the pros and cons, when should you apply each?
Use Abstract Classes for:
- Closely related class inheritance hierarchies
- Reusing common logic between subclasses
- Flexible method extension in the future
Use Interfaces for:
- Unrelated classes needing common APIs
- Decoupling class dependencies
- Standardizing disparate class capabilities
In essence:
- Interfaces – What classes can do
- Abstract Classes – How classes partly achieve it
You can also leverage both simultaneously – interfaces define expectations that abstract classes implement before sub-classing.
But focus first on what problem you’re fundamentally trying to solve with abstraction.
FAQ
Here are some common abstract class and interface questions:
Can an abstract class implement an interface?
Yes, abstract classes can implement interfaces, providing method implementations to inheriting concrete sub-classes.
When should I use an abstract class vs interface?
Use abstract classes for closely related inherited class functionality and reusable code templates. Use interfaces when you need to define capability contracts across very different components.
What is the difference between an abstract class and interface?
Key differences are that abstract classes allow both abstract and non-abstract methods to enable code reuse. Interfaces solely provide method/property signatures without any implementation details whatsoever.
Can I change an abstract class to an interface or vice versa?
You can refactor an abstract class into an interface and vice versa. But beware this will break dependent subclasses and require code changes. Only do so when absolutely necessary.
Can I create an instance of an abstract class or interface?
No. Because they omit implementation details, you cannot directly instantiate an abstract class or interface alone. They must first be inherited into a concrete subclass which you then instantiate.
What happens if I don’t override an abstract class’ methods?
The compiler will throw an error saying you must override all inherited abstract methods by providing an implementation body before instantiating a subclass.
Summary
We’ve covered a lot of ground comparing abstract classes and interfaces! Let’s recap:
- Abstract classes promote inheritance for concrete implementation extension
- Interfaces enforce capability contracts across disparate classes
Both limit direct instantiation and force subclassing to utilize functionality.
Major differences center on implementation flexibility vs loose coupling. Abstract classes allow both abstract and concrete methods for code reuse among subtypes. Interfaces solely define public method and property signatures without detailing implementation.
Language support also varies – Java and C# offer native syntax for interfaces while C++ and Python must emulate them. But core concepts transfer broadly.
Choosing between the two depends on your specific reuse, coupling and inheritance needs. You can even leverage both in tandem!
I hope this guide gives you a solid grasp of abstract class vs interface differences and when to apply each principle. Let me know if you have any other questions!
