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Intermediate 18 min readModule: Module 6: Interfaces, Records & Sealed Classes

Interfaces, Default Methods & Sealed Classes

Implement multiple interfaces, declare immutable Records, and restrict subclassing with 'sealed'.

What You Will Learn in This Lesson

  • Multiple interface implementation (implements A, B)
  • Default method implementations in interfaces
  • Sealed classes (sealed class Shape permits Circle, Square) for exhaustive pattern matching

Introduction & Core Concept

Interfaces define behavioral contracts. Sealed classes allow authors to explicitly declare which classes are permitted to extend them, enabling compiler-enforced exhaustiveness.
WHY DOES THIS MATTER IN THE REAL WORLD?

Sealed classes prevent third-party code from extending internal domain models in unexpected ways.

Sealed Class Hierarchy

java
java
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public sealed interface Result permits Success, Failure {}
 
public record Success(String data) implements Result {}
public record Failure(String error) implements Result {}
 
// Pattern matching in Java 21
// switch (result) { case Success s -> ... case Failure f -> ... }

Line-by-Line Technical Breakdown

1Interfaces can have static methods and private helper methods as well as default methods.

Try It Yourself (Interactive Editor)

Modify the code in real-time and click Run to test live browser output and console logs.

Intelligent Code Runner & Live Sandbox[JAVA]
JAVA SOURCE EDITOR
Interactive Live Code

Industry Best Practices & Professional Standards

  • Use sealed interfaces to model algebraic data types (ADTs) in Java.

Lesson Summary & Core Takeaways

  • Interfaces and sealed classes provide total control over class contracts and hierarchies.