Add automated proof script for code snippet verification

- Add html-generators/proof.java: JBang script (Java 25) that reads all
  content YAML files, finds those with an optional proofCode field, and
  runs each through JShell to verify they compile and execute correctly.
  Supports --list flag to show which patterns have/lack proofCode.

- Add .github/workflows/proof.yml: GitHub Actions workflow that triggers
  on content changes (push/PR) and runs jbang html-generators/proof.java
  with Java 25 + JBang.

- Add proofCode field to 10 representative patterns across categories:
  language (var, text-blocks, records, switch, instanceof), collections
  (immutable-list), streams (stream-tolist), strings (isBlank),
  concurrency (virtual-threads), datetime (java-time-basics)

- Update generate.java and generate.py to exclude proofCode from the
  search index (snippets.json) since it is CI-only metadata.

- Update content/template.json, html-generators/README.md,
  CONTRIBUTING.md, and copilot-instructions.md to document the new field.

Co-authored-by: brunoborges <129743+brunoborges@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -100,7 +100,8 @@ Each `content/category/slug.json` file has this structure:
   ],
   "docs": [
     { "title": "Javadoc or Guide Title", "href": "https://docs.oracle.com/..." }
-  ]
+  ],
+  "proofCode": "// Optional: self-contained JShell snippet proving the modern approach works.\n// Run with: jbang html-generators/proof.java"
 }
 ```
 
@@ -113,6 +114,7 @@ Each `content/category/slug.json` file has this structure:
 - `docs` must have at least **1** entry linking to Javadoc or Oracle documentation
 - `prev`/`next` are `category/slug` paths or `null` for first/last
 - Code in `oldCode`/`modernCode` uses `\n` for newlines
+- `proofCode` is optional — a self-contained JShell snippet (with default imports available) that proves the modern approach compiles and runs correctly. Run `jbang html-generators/proof.java` to verify all proof snippets.
 
 ## Category Display Names
 

.github/workflows/proof.yml

@@ -0,0 +1,33 @@
+name: Proof
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'content/**'
+      - 'html-generators/proof.java'
+      - '.github/workflows/proof.yml'
+  pull_request:
+    paths:
+      - 'content/**'
+      - 'html-generators/proof.java'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  proof:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - uses: actions/setup-java@v5
+        with:
+          distribution: 'temurin'
+          java-version: '25'
+
+      - uses: jbangdev/setup-jbang@main
+
+      - name: Run proof
+        run: jbang html-generators/proof.java

CONTRIBUTING.md

@@ -9,7 +9,8 @@ Contributions are welcome! Content is managed as YAML files — never edit gener
 3. Copy [`content/template.json`](content/template.json) as a starting point for all required fields (see the [snippet schema](.github/copilot-instructions.md) for details)
 4. Update the `prev`/`next` fields in adjacent pattern files to maintain navigation
 5. Run `jbang html-generators/generate.java` to verify your changes build correctly
-6. Open a pull request
+6. Optionally add a `proofCode` field — a self-contained JShell snippet that proves the modern approach works. Run `jbang html-generators/proof.java` to validate it.
+7. Open a pull request
 
 Please ensure JDK version labels only reference the version where a feature became **final** (non-preview).
 

content/collections/immutable-list-creation.yaml

@@ -47,3 +47,11 @@ docs:
   href: "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/List.html#of()"
 - title: "Collections Factory Methods (JEP 269)"
   href: "https://openjdk.org/jeps/269"
+proofCode: |-
+  var list = List.of("a", "b", "c");
+  if (list.size() != 3) throw new AssertionError("expected 3 elements");
+  try {
+      list.add("d");
+      throw new AssertionError("should have thrown UnsupportedOperationException");
+  } catch (UnsupportedOperationException ok) {}
+  IO.println("immutable list: OK - " + list);

content/concurrency/virtual-threads.yaml

@@ -47,3 +47,13 @@ docs:
   href: "https://openjdk.org/jeps/444"
 - title: "Thread.ofVirtual()"
   href: "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Thread.html#ofVirtual()"
+proofCode: |-
+  var results = new java.util.concurrent.CopyOnWriteArrayList<String>();
+  var threads = new ArrayList<Thread>();
+  for (int i = 0; i < 5; i++) {
+      int idx = i;
+      threads.add(Thread.startVirtualThread(() -> results.add("vt-" + idx)));
+  }
+  for (var t : threads) t.join();
+  if (results.size() != 5) throw new AssertionError("expected 5 results, got: " + results.size());
+  IO.println("virtual threads: OK - " + results.size() + " threads ran");

content/datetime/java-time-basics.yaml

@@ -51,3 +51,12 @@ docs:
   href: "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/time/LocalTime.html"
 - title: "LocalDateTime"
   href: "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/time/LocalDateTime.html"
+proofCode: |-
+  import java.time.*;
+  var date = LocalDate.of(2025, Month.JANUARY, 15);
+  var time = LocalTime.of(14, 30);
+  if (date.getMonthValue() != 1) throw new AssertionError("expected January=1, got: " + date.getMonthValue());
+  if (time.getHour() != 14) throw new AssertionError("expected hour=14, got: " + time.getHour());
+  var now = Instant.now();
+  if (now == null) throw new AssertionError("Instant.now() returned null");
+  IO.println("java.time basics: OK - " + date + " " + time);

content/language/pattern-matching-instanceof.yaml

@@ -44,3 +44,11 @@ related:
 docs:
 - title: "Pattern Matching for instanceof (JEP 394)"
   href: "https://openjdk.org/jeps/394"
+proofCode: |-
+  Object obj = "Hello, pattern matching!";
+  if (obj instanceof String s) {
+      IO.println("length=" + s.length());
+  } else {
+      throw new AssertionError("should have matched String");
+  }
+  IO.println("pattern matching instanceof: OK");

content/language/records-for-data-classes.yaml

@@ -46,3 +46,8 @@ docs:
   href: "https://openjdk.org/jeps/395"
 - title: "Record class"
   href: "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Record.html"
+proofCode: |-
+  public record Point(int x, int y) {}
+  var p = new Point(1, 2);
+  if (p.x() != 1 || p.y() != 2) throw new AssertionError("accessor mismatch");
+  IO.println("records: OK - " + p);

content/language/switch-expressions.yaml

@@ -53,3 +53,12 @@ related:
 docs:
 - title: "Switch Expressions (JEP 361)"
   href: "https://openjdk.org/jeps/361"
+proofCode: |-
+  var day = "MONDAY";
+  String msg = switch (day) {
+      case "MONDAY"  -> "Start";
+      case "FRIDAY"  -> "End";
+      default        -> "Mid";
+  };
+  if (!"Start".equals(msg)) throw new AssertionError("unexpected: " + msg);
+  IO.println("switch expressions: OK - " + msg);

content/language/text-blocks-for-multiline-strings.yaml

@@ -49,3 +49,12 @@ docs:
   href: "https://openjdk.org/jeps/378"
 - title: "Text Blocks Guide"
   href: "https://docs.oracle.com/en/java/javase/25/language/text-blocks.html"
+proofCode: |-
+  String json = """
+      {
+        "name": "Duke",
+        "age": 30
+      }""";
+  if (!json.contains("Duke")) throw new AssertionError("text block content missing");
+  if (json.contains("\\n")) throw new AssertionError("should not contain literal \\n");
+  IO.println("text blocks: OK");