Add sealed module: modelling domains with sealed types and exhaustive switch

Payment model, permits and the three modifiers, default-branch trap, generic and recursive hierarchies, visitor comparison, run-time enforcement, modules. Transcripts from JDK 21, 25 and 27.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TF9JWFvJSNm6HVzswzZU5a

sealed/broken/AnonymousSealed.java

@@ -0,0 +1,7 @@
+public class AnonymousSealed {
+    sealed interface Payment permits Card {}
+    record Card(long amount) implements Payment {}
+    static Payment sneaky() {
+        return new Payment() {};                          // an anonymous class is a subtype nobody listed
+    }
+}

sealed/broken/MissingModifier.java

@@ -0,0 +1,4 @@
+public class MissingModifier {
+    sealed interface Payment permits Card {}
+    class Card implements Payment {}                      // must say final, sealed or non-sealed
+}

sealed/broken/NoSubclass.java

@@ -0,0 +1,3 @@
+public class NoSubclass {
+    sealed interface Payment {}                           // no permits clause and no subtype in this file
+}

sealed/broken/NonSealedWrongPlace.java

@@ -0,0 +1,4 @@
+public class NonSealedWrongPlace {
+    interface Plain {}
+    non-sealed class Oops implements Plain {}             // non-sealed only makes sense under a sealed supertype
+}

sealed/broken/NotPermitted.java

@@ -0,0 +1,6 @@
+public class NotPermitted {
+    sealed interface Payment permits Card, Upi {}
+    record Card(long amount) implements Payment {}
+    record Upi(long amount) implements Payment {}
+    record Cheque(long amount) implements Payment {}      // not in the permits clause
+}

sealed/broken/VisitorAdded.java

@@ -0,0 +1,17 @@
+public class VisitorAdded {
+    interface Payment { <R> R accept(Visitor<R> v); }
+    interface Visitor<R> { R visitCard(Card c); R visitUpi(Upi u); R visitWallet(Wallet w); }   // Wallet added here
+    record Card() implements Payment { public <R> R accept(Visitor<R> v) { return v.visitCard(this); } }
+    record Upi() implements Payment { public <R> R accept(Visitor<R> v) { return v.visitUpi(this); } }
+    record Wallet() implements Payment { public <R> R accept(Visitor<R> v) { return v.visitWallet(this); } }
+
+    // Two existing operations, neither knows about Wallet: each one now fails to compile.
+    static class Fee implements Visitor<Long> {
+        public Long visitCard(Card c) { return 2L; }
+        public Long visitUpi(Upi u) { return 0L; }
+    }
+    static class Label implements Visitor<String> {
+        public String visitCard(Card c) { return "card"; }
+        public String visitUpi(Upi u) { return "upi"; }
+    }
+}

sealed/broken/WalletNotHandled.java

@@ -0,0 +1,16 @@
+public class WalletNotHandled {
+    sealed interface Payment permits Card, Upi, NetBanking, Wallet {}     // Wallet added
+    record Card(long amount) implements Payment {}
+    record Upi(long amount) implements Payment {}
+    record NetBanking(long amount) implements Payment {}
+    record Wallet(long amount) implements Payment {}
+
+    // Same switch as before, with no default and no case for Wallet.
+    static long fee(Payment p) {
+        return switch (p) {
+            case Card c -> c.amount() * 2 / 100;
+            case Upi u -> 0;
+            case NetBanking n -> 10;
+        };
+    }
+}

sealed/broken/xpkg/a/Base.java

@@ -0,0 +1,2 @@
+package a;
+public sealed interface Base permits b.Impl {}

sealed/broken/xpkg/b/Impl.java

@@ -0,0 +1,2 @@
+package b;
+public final class Impl implements a.Base {}