Are You Using Interfaces the Right Way?

Let’s be honest. Many developers use interfaces without fully understanding their purpose. Ask yourself:

• Are you using interfaces just to list method names?
• As a replacement for public static final constants?
• Because a design pattern told you to?
• Or simply because you’ve seen others do it?

If yes, it’s time to rethink what interfaces are truly for — and how they can make your code more flexible, testable, and scalable.

What Is an Interface Really For?

In object-oriented programming (OOP), an interface defines a contract — a set of behaviors that a class must implement. It’s not about organizing methods. It’s about enabling polymorphism: different objects of the same type perform the same action but produce different results.

Let’s focus on how interfaces are used in Java, especially in service layers and common design patterns.

Why Do We Create Interfaces Before Implementing Classes?

You’ve likely seen this pattern:

• Define an interface first
• Then create a class that implements it

But why bother? After all, we could just write the class directly and everything would still work — no errors, no complaints.

If you think interfaces are just for listing method names, you’re missing their true purpose.

The Real Reason: Flexibility Through Dependency Injection

Interfaces are essential for enabling Dependency Injection (DI) and following the Dependency Inversion Principle — the “I” in SOLID.

Let’s take a real-world example:

POS Machine Example

Imagine a POS machine that can process Visa, MasterCard, and UnionPay. Each card is a dependency. The POS machine should not care which card it’s using — it should rely on a shared interface like Card.

If a new card type is introduced, you simply implement the Card interface. The POS machine doesn’t need to change at all.

Without DI and interfaces, each card would require its own custom POS machine — rigid, expensive, and hard to maintain.

Interfaces as Specifications in Libraries and Frameworks

Interfaces also serve as specification contracts in frameworks and libraries.

Take Spring Security, for example. It allows you to customize user retrieval logic by implementing the UserDetailsService interface. You don’t need to modify the framework — just plug in your own logic.

This is how frameworks stay flexible and extensible.

Interfaces aren’t just for listing methods. They’re contracts that make your code flexible, testable, and scalable.

Best Practices for Using Interfaces

• Not every method belongs in an interface — internal or private logic should stay out.
• Keep interfaces focused — don’t lump everything into one giant interface.
• Avoid using interfaces just to “see method names” — that’s not their purpose.
• Don’t use interfaces as a replacement for public static final constants — use enums or final classes instead.
• Design interfaces to support testing and future changes.