[mperor]

What you'll learn

• How a sealed interface plus a closed set of record implementations models data without inheritance tricks
• How to deconstruct nested records directly in a switch with record patterns
• How a when guard lets you special-case a pattern before falling back to the general one
• Why a sealed hierarchy makes switch exhaustiveness a compile-time guarantee instead of a runtime hope

Why it matters

This combination (sealed types + records + pattern-matching switch) is the core of "data-oriented programming" in modern Java: it replaces instanceof/cast chains and the visitor pattern with exhaustive, compiler-checked switches, so adding a new case in the data model is caught immediately everywhere it's handled.

Read more

• JEP 441: Pattern Matching for switch
• JEP 440: Record Patterns
• JEP 409: Sealed Classes

Quiz

Given

sealed interface Expr permits Const, Add, Mul {}
record Const(double value) implements Expr {}
record Add(Expr left, Expr right) implements Expr {}
record Mul(Expr left, Expr right) implements Expr {}

and eval switching with case Mul(Const c, Expr right) when c.value() == 0 -> 0; before the generic case Mul(Expr left, Expr right) -> eval(left) * eval(right); — does eval(new Mul(new Const(0), new Add(new Const(1), new Const(1)))) evaluate the Add branch?

Answer

No. The guarded `Mul(Const c, Expr right) when c.value() == 0` pattern matches first and returns `0` immediately, so `eval` is never called on the `Add` subexpression — multiplying by a constant zero short-circuits without evaluating the other operand.

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Note: this sandbox only has JDK 21 installed, and its network policy blocks both the Gradle toolchain auto-provisioner (foojay) and a direct JDK 25 download, so ./gradlew :FirstSteps:test could not be run locally for this PR. The code was reviewed carefully against the language spec; CI (which has JDK 25) will verify it compiles and passes. Marking as draft until that's confirmed.

Closes #89

[github-actions]

Test Results

276 tests  +2   273 ✅ +2   6s ⏱️ ±0s
 80 suites +1     3 💤 ±0 
 80 files   +1     0 ❌ ±0 

Results for commit f486fcb. ± Comparison against base commit e635139.

This pull request skips 1 and un-skips 1 tests.

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testSerializationSafe()

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testThreadSafe()