Why Can't Interface Extend a Class? The Core Concept Explained

The Basics of Interfaces and Classes in OOP

Alright, let’s dive straight into this. If you're learning object-oriented programming (OOP) or already working with languages like Java or TypeScript, you’ve probably asked yourself at some point: “Why can’t an interface extend a class?” It's a totally valid question. I know I was confused about it when I first started learning OOP.

So, here’s the deal: in OOP, a class defines the blueprint for creating objects, including methods and properties. It’s concrete, meaning you can instantiate it. On the other hand, an interface is more abstract. It only defines what methods or properties an object should have but doesn’t provide any implementation. It's more of a contract, a promise, if you will.

But here’s where the confusion often sets in. If interfaces and classes serve different roles, why can't one extend the other? Let's break this down.

The Difference Between Extending and Implementing

Before I get into the specifics, let’s make sure we understand what "extend" and "implement" mean in OOP.

• Extend: This means a class can inherit from another class, taking on its properties and methods. It allows you to create a new class based on an existing one.

• Implement: This is what an interface does. A class can implement an interface, meaning it must provide the actual methods and properties declared in the interface.

So, why is this important? Well, it all boils down to the fact that interfaces are about defining a contract, while classes are the actual implementation.

Why Can't an Interface Extend a Class?

Here’s the fundamental reason why an interface can’t extend a class: it would break the separation of concerns between interfaces and classes. Classes provide the implementation, while interfaces define the structure. Mixing the two would create confusion about where the line should be drawn between what’s defined (interface) and what’s implemented (class).

Think about it: when you extend a class, you’re inheriting its logic and state, which are concrete. But an interface, by definition, is supposed to be abstract, purely focused on the "what" an object can do, not the "how."

The Role of Abstraction

This distinction is critical in OOP because it encourages better abstraction. An interface is a way to decouple your code, so different classes can agree on a set of behaviors but don’t necessarily need to share implementation. If interfaces could extend classes, we’d be muddying the waters of abstraction, mixing implementation and structure in ways that could lead to tightly coupled, less flexible code.

I remember discussing this with my colleague Mike recently, and he put it this way: "If interfaces could extend classes, we’d end up with situations where classes are forcing their implementation details on the interfaces, and that would ruin the flexibility we get from having clean, separate contracts."

The Power of Composition Over Inheritance

One of the core principles of OOP is that composition is often preferred over inheritance. The idea is that you should compose objects using smaller, more reusable pieces of functionality, rather than relying heavily on deep inheritance chains. Extending classes makes things more rigid. On the other hand, interfaces allow you to compose behavior across multiple classes without forcing them into a rigid hierarchy.

When interfaces can’t extend classes, it reinforces the idea that interfaces are for defining behaviors and classes are for defining how to execute those behaviors.

Real-World Analogy: Why Interfaces Shouldn’t Extend Classes

Let’s take a real-world analogy to understand this better. Imagine you’re building a car, and you have a blueprint for a sports car (a class) and a blueprint for a vehicle (an interface). Now, if the vehicle interface could extend the sports car, the vehicle would suddenly inherit the details of the sports car’s design—like the engine type or specific performance features—when it should just focus on the basic structure of what makes any vehicle (car, truck, etc.) work.

In simple terms, an interface should define what the car should do (drive, stop, accelerate) but not how those things should be implemented (sports car vs. truck).

What Happens If You Try to Make an Interface Extend a Class?

In most languages, trying to make an interface extend a class will result in a compiler error or runtime exception. This is because the interface can’t inherit the implementation details of the class. The whole point of interfaces is to keep things abstract and flexible, not bound to specific implementations.

I learned this the hard way while working on a project in Java. I mistakenly tried to have an interface extend a class while trying to build a class library for multiple types of user authentication. The compiler didn’t like it at all, and it took me a while to figure out why. That was a frustrating moment, but it taught me a lot about the importance of respecting the principles of OOP!

Conclusion: Understanding the OOP Principles

To sum it all up: interfaces can’t extend classes because it would violate the core concepts of abstraction and flexibility in object-oriented programming. Interfaces define what needs to be done, while classes provide the how. By keeping these two concepts separate, you can create more maintainable, flexible, and decoupled code.

Remember: composition, not inheritance. Stick with interfaces for structure and classes for implementation, and you’ll be on your way to writing cleaner, more effective code. Trust me, it’s worth the effort!

Is 165 cm normal for a 15 year old?

The predicted height for a female, based on your parents heights, is 155 to 165cm. Most 15 year old girls are nearly done growing. I was too. It's a very normal height for a girl.

Is 160 cm too tall for a 12 year old?

How Tall Should a 12 Year Old Be? We can only speak to national average heights here in North America, whereby, a 12 year old girl would be between 137 cm to 162 cm tall (4-1/2 to 5-1/3 feet). A 12 year old boy should be between 137 cm to 160 cm tall (4-1/2 to 5-1/4 feet).

How tall is a average 15 year old?

Average Height to Weight for Teenage Boys - 13 to 20 Years

How to get taller at 18?

Staying physically active is even more essential from childhood to grow and improve overall health. But taking it up even in adulthood can help you add a few inches to your height. Strength-building exercises, yoga, jumping rope, and biking all can help to increase your flexibility and grow a few inches taller.

Is 5.7 a good height for a 15 year old boy?

Generally speaking, the average height for 15 year olds girls is 62.9 inches (or 159.7 cm). On the other hand, teen boys at the age of 15 have a much higher average height, which is 67.0 inches (or 170.1 cm).

Can you grow between 16 and 18?

Most girls stop growing taller by age 14 or 15. However, after their early teenage growth spurt, boys continue gaining height at a gradual pace until around 18. Note that some kids will stop growing earlier and others may keep growing a year or two more.

Can you grow 1 cm after 17?

Even with a healthy diet, most people's height won't increase after age 18 to 20. The graph below shows the rate of growth from birth to age 20. As you can see, the growth lines fall to zero between ages 18 and 20 ( 7 , 8 ). The reason why your height stops increasing is your bones, specifically your growth plates.