From 737b0032b3a18eb6e458271ea440098c166f6c2d Mon Sep 17 00:00:00 2001
From: Liam Miller-Cushon <cushon@google.com>
Date: Mon, 19 Jan 2026 23:38:25 -0800
Subject: [PATCH] Support Instance Main Methods in google-java-format

https://github.com/google/google-java-format/issues/1216

PiperOrigin-RevId: 858436442
---
 .../googlejavaformat/java/JavaInputAstVisitor.java  | 13 +++++++++++++
 .../java/FormatterIntegrationTest.java              |  2 +-
 .../java/testdata/InstanceMain.input                |  5 +++++
 .../java/testdata/InstanceMain.output               |  5 +++++
 4 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.input
 create mode 100644 core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.output

diff --git a/core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java b/core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java
index a2a32e79c..2f0373ec6 100644
--- a/core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java
+++ b/core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java
@@ -453,8 +453,15 @@ protected void dropEmptyDeclarations() {
     }
   }
 
+  // Replace with Flags.IMPLICIT_CLASS once JDK 25 is the minimum supported version
+  private static final int IMPLICIT_CLASS = 1 << 19;
+
   @Override
   public Void visitClass(ClassTree tree, Void unused) {
+    if ((TreeInfo.flags((JCTree) tree) & IMPLICIT_CLASS) == IMPLICIT_CLASS) {
+      visitImplicitClass(tree);
+      return null;
+    }
     switch (tree.getKind()) {
       case ANNOTATION_TYPE -> visitAnnotationType(tree);
       case CLASS, INTERFACE -> visitClassDeclaration(tree);
@@ -465,6 +472,12 @@ public Void visitClass(ClassTree tree, Void unused) {
     return null;
   }
 
+  private void visitImplicitClass(ClassTree node) {
+    builder.open(minusTwo);
+    addBodyDeclarations(node.getMembers(), BracesOrNot.NO, FirstDeclarationsOrNot.YES);
+    builder.close();
+  }
+
   public void visitAnnotationType(ClassTree node) {
     sync(node);
     builder.open(ZERO);
diff --git a/core/src/test/java/com/google/googlejavaformat/java/FormatterIntegrationTest.java b/core/src/test/java/com/google/googlejavaformat/java/FormatterIntegrationTest.java
index a406487f3..7b3c3e889 100644
--- a/core/src/test/java/com/google/googlejavaformat/java/FormatterIntegrationTest.java
+++ b/core/src/test/java/com/google/googlejavaformat/java/FormatterIntegrationTest.java
@@ -59,7 +59,7 @@ public class FormatterIntegrationTest {
               "I981",
               "I1020",
               "I1037")
-          .putAll(25, "ModuleImport")
+          .putAll(25, "ModuleImport", "InstanceMain")
           .build();
 
   @Parameters(name = "{index}: {0}")
diff --git a/core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.input b/core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.input
new file mode 100644
index 000000000..5967d6c66
--- /dev/null
+++ b/core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.input
@@ -0,0 +1,5 @@
+String greeting = "Hello, World!";
+
+void main() {
+    System.out.println(greeting);
+}
diff --git a/core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.output b/core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.output
new file mode 100644
index 000000000..6f152da61
--- /dev/null
+++ b/core/src/test/resources/com/google/googlejavaformat/java/testdata/InstanceMain.output
@@ -0,0 +1,5 @@
+String greeting = "Hello, World!";
+
+void main() {
+  System.out.println(greeting);
+}