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/run.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# Sealed classes and interfaces: permits rules, exhaustive switch without default, the traps around them, and what the JVM enforces at run time.
+# Regenerates every file in output/.   JDK25=/path JDK27=/path [JDK21=/path] ./run.sh
+set -uo pipefail
+unset JAVA_TOOL_OPTIONS
+HERE="$(cd "$(dirname "$0")" && pwd)"
+JDK25="${JDK25:-/opt/jdks/jdk-25.0.4.1+1}"; JDK27="${JDK27:-/opt/jdks/jdk27}"; JDK21="${JDK21:-}"
+OUT="$HERE/output"; mkdir -p "$OUT"; B="$(mktemp -d)"; trap 'rm -rf "$B"' EXIT
+cd "$HERE"
+t() { "$1/bin/java" -version 2>&1 | sed -n 2p | sed 's/.*(build \(.*\))/\1/'; }
+mk() { mktemp -d -p "$B"; }
+# compile + run
+cr() { local jdk="$1" src="$2" cls="$3" o; o="$(mk)"
+  echo "\$ javac $src && java $cls     ($(t "$jdk"))"
+  "$jdk/bin/javac" -d "$o" "$src" 2>&1 && "$jdk/bin/java" -cp "$o" "$cls" 2>&1; }
+# compile only, show diagnostics
+co() { local jdk="$1" src="$2" o; o="$(mk)"
+  echo "\$ javac $src     ($(t "$jdk"))"
+  "$jdk/bin/javac" -d "$o" "$src" 2>&1 | grep -v '^Note:'; }
+all3() { local fn="$1"; shift; [ -n "$JDK21" ] && { "$fn" "$JDK21" "$@"; echo; }; "$fn" "$JDK25" "$@"; echo; "$fn" "$JDK27" "$@"; }
+
+# 01 - the payment model
+{ all3 cr src/PaymentDemo.java PaymentDemo; } > "$OUT/01-payment-demo.txt"
+
+# 02 - permits, the three modifiers, reflection and the class-file attribute
+{ all3 cr src/Reflect.java Reflect; echo; all3 cr src/ImplicitPermits.java ImplicitPermits; echo
+  o="$(mk)"; "$JDK25/bin/javac" -d "$o" src/PaymentDemo.java
+  echo "\$ javap -v -cp . 'PaymentDemo\$Payment' | grep -A3 PermittedSubclasses     ($(t "$JDK25"))"
+  "$JDK25/bin/javap" -v -cp "$o" 'PaymentDemo$Payment' | grep -A3 '^PermittedSubclasses'; } > "$OUT/02-permits-and-modifiers.txt"
+
+# 03 - the compile errors, one file each
+{ for f in WalletNotHandled NotPermitted MissingModifier NoSubclass NonSealedWrongPlace AnonymousSealed; do all3 co broken/$f.java; echo; done
+  xp() { local jdk="$1" o; o="$(mk)"; echo "\$ javac broken/xpkg/a/Base.java broken/xpkg/b/Impl.java     ($(t "$jdk"))"
+    "$jdk/bin/javac" -d "$o" broken/xpkg/a/Base.java broken/xpkg/b/Impl.java 2>&1; }
+  all3 xp; } > "$OUT/03-compile-errors.txt"
+
+# 04 - a default branch turns the check off
+{ all3 cr src/WithDefault.java WithDefault; } > "$OUT/04-default-trap.txt"
+
+# 05 - generic and recursive hierarchies
+{ all3 cr src/ResultDemo.java ResultDemo; echo; all3 cr src/ExprDemo.java ExprDemo; } > "$OUT/05-generic-and-recursive.txt"
+
+# 06 - the visitor, before sealed types
+{ all3 cr src/VisitorVersion.java VisitorVersion; echo; all3 co broken/VisitorAdded.java; } > "$OUT/06-visitor.txt"
+
+# 07 - what the JVM enforces at run time (a class file whose permits clause was different when it was compiled)
+rt() { local jdk="$1" o; o="$(mk)"; mkdir "$o/v1" "$o/evil"
+  "$jdk/bin/javac" -d "$o/v1" sep/v1/*.java sep/Main.java 2>&1; "$jdk/bin/javac" -d "$o/evil" sep/evil/*.java 2>&1
+  cp "$o/evil/Hack.class" "$o/v1/"
+  echo "\$ java -cp v1+Hack.class Main     (Hack was compiled against a Payment that permitted it)     ($(t "$jdk"))"
+  "$jdk/bin/java" -cp "$o/v1" Main 2>&1 | head -3; }
+{ all3 rt; } > "$OUT/07-runtime-enforcement.txt"
+
+# 08 - sealed across packages needs a named module
+mm() { local jdk="$1" o; o="$(mk)"
+  echo "\$ javac -d out \$(find multi/demo.pay -name '*.java') && java -p out -m demo.pay/pay.app.Main     ($(t "$jdk"))"
+  "$jdk/bin/javac" -d "$o" $(find multi/demo.pay -name '*.java') 2>&1 && "$jdk/bin/java" -p "$o" -m demo.pay/pay.app.Main 2>&1; }
+{ all3 mm; } > "$OUT/08-modules.txt"
+echo done