Ever seen List<Set<Integer>> rowSets = new ArrayList<>() and wondered "Why not new List<Set<Integer>>()?" This fundamental question about interfaces vs concrete classes trips up many Java learners. Let's clear it up through a real conversation.
"Interfaces are for frameworks, right?" Wrong! While frameworks love interfaces, they're solving a much broader problem. Here are 7 real-world scenarios where YOU will use interfaces—no framework required.
In Part 1, we learned why Spring Data JPA repositories must be interfaces. Now let's understand the fundamental difference between classes and interfaces in Java, and when to use each.
In Parts 1 and 2, we learned what interfaces are and when to use them. Now let's dive deep into the OOP principles that make interfaces essential for large-scale applications. This isn't about avoiding code duplication—it's about fundamental software design.
"Why can't I use class instead of interface for UserRepository?" Because Java's type system has strict rules: class extends class, interface extends interface, class implements interface. When you try class UserRepository extends JpaRepository, Java sees JpaRepository is an interface and throws an error. Let's understand the three-level inheritance chain and who implements what.
Trying to create a Spring Data repository by extending JpaRepository with a class instead of an interface? It won't work. Here's why Spring Data JPA requires interfaces and what happens behind the scenes.
"Can't we just create a utility class with static methods instead of using interfaces? Isn't the interface approach just fancy?" Great question! Yes, you could use utility classes - but you'd be solving only 50% of the problem. Let's see what happens in practice and why interfaces aren't fancy, they're necessary.