Understanding Sealed Classes in Java

Sealed class: A class or interface that explicitly restricts which types may extend or implement it.

Sealed classes matter because they let the type system express a rule that older Java often left in comments: only these variants are valid.

1public sealed interface PaymentResult
2        permits PaymentResult.Success, PaymentResult.Failure {
3
4    record Success(String confirmationId) implements PaymentResult {}
5    record Failure(String reason) implements PaymentResult {}
6}

That is stronger than documentation alone. It tells both the compiler and the reader that the set of possibilities is closed.

Why This Changes Design

Closed hierarchies are common in real systems:

• finite workflow states
• fixed command types
• known result variants
• domain events with a bounded family

Before sealed classes, Java often modeled these with:

• open interfaces that were “not really open”
• enums with too little structure
• inheritance hierarchies controlled only by convention

Sealed types give a better middle ground.

Where Sealed Types Fit Well

They are strongest when the domain genuinely has a known set of variants. Examples include:

• parsing results
• success and failure outcomes
• AST node families
• protocol message families
• policy decisions

In those cases, open extension would be a bug, not a feature.

Where They Fit Poorly

Avoid sealed hierarchies when a model is expected to be extended by:

• third-party plugins
• customer integrations
• framework consumers
• independent modules with their own domain types

Sealing a hierarchy that should stay open only pushes extensibility problems somewhere else.

Relationship to Classic Patterns

Sealed classes influence:

• State
• Visitor
• Interpreter
• Factory Method for closed product families

They do not replace those patterns completely, but they often make the domain boundary clearer and the implementation safer.

Practical Rule

Use sealed classes when the domain says “these are the only legal variants.” If the hierarchy is meant to be extensible by others, do not seal it just because the feature exists.