Fix a bug calculating the file name in the absense of directories.

PiperOrigin-RevId: 814799893

Author: mkruskal-google

java/core/src/main/java/com/google/protobuf/GeneratorNames.java

@@ -139,7 +139,8 @@ static String getDefaultFileClassName(
       FileDescriptorProtoOrBuilder file, boolean useOldOuterClassnameDefault) {
     // Replicates the logic of ClassNameResolver::GetFileDefaultImmutableClassName.
     String name = file.getName();
-    name = name.substring(name.lastIndexOf('/'));
+    // The `+ 1` includes the case where no '/' is present.
+    name = name.substring(name.lastIndexOf('/') + 1);
     name = underscoresToCamelCase(stripProto(name));
     return useOldOuterClassnameDefault ? name : name + "Proto";
   }

java/core/src/test/java/com/google/protobuf/GeneratorNamesTest.java

@@ -234,6 +234,15 @@ public void getFileClassName_weirdExtension() {
         .isEqualTo("BarNotproto");
   }
 
+  @Test
+  public void getFileClassName_noDirectory() {
+    // This isn't exercisable in blazel because all our tests are in a directory.
+    assertThat(
+            GeneratorNames.getFileClassName(
+                FileDescriptorProto.newBuilder().setName("bar.proto").build()))
+        .isEqualTo("Bar");
+  }
+
   @Test
   public void getFileClassName_conflictingName2024() {
     // This isn't exercisable in blazel because conflicts trigger a protoc error in edition 2024.