When record is nested by a non-record type (e.g. interface) use simple builder name

avaje-record-builder-core/src/main/java/io/avaje/recordbuilder/internal/ClassBodyBuilder.java

@@ -19,7 +19,8 @@ static String createClassStart(
       TypeElement type,
       String typeParams,
       boolean isImported,
-      String packageName) {
+      String packageName,
+      String builderName) {
 
     if (type.getEnclosingElement() instanceof TypeElement) {
       isImported = true;
@@ -54,9 +55,6 @@ static String createClassStart(
     final String build =
         build(components).transform(s -> numberOfComponents > 6 ? "\n        " + s : s);
 
-    final var builderName =
-        ProcessorUtils.shortType(utype.mainType()).replace(".", "$") + "Builder";
-
     final var shortName = type.getSimpleName().toString();
     return ClassTemplate.classTemplate(
         packageName,

avaje-record-builder-core/src/main/java/io/avaje/recordbuilder/internal/RecordProcessor.java

@@ -141,9 +141,7 @@ private void readElement(TypeElement type, BuilderPrism prism) {
         unnamed
             ? ""
             : packageElement.getQualifiedName().toString() + (isImported ? ".builder" : "");
-    var builderName =
-        ProcessorUtils.shortType(UType.parse(type.asType()).mainType()).replace(".", "$")
-            + "Builder";
+    var builderName = builderName(type);
     try (var writer =
         new Append(
             createSourceFile((unnamed ? "" : packageName + ".") + builderName).openWriter())) {
@@ -153,14 +151,28 @@ private void readElement(TypeElement type, BuilderPrism prism) {
               .map(Object::toString)
               .collect(joining(", "))
               .transform(s -> s.isEmpty() ? s : "<" + s + ">");
-      writer.append(ClassBodyBuilder.createClassStart(type, typeParams, isImported, packageName));
+      writer.append(ClassBodyBuilder.createClassStart(type, typeParams, isImported, packageName, builderName));
 
       methods(writer, typeParams, builderName, components, prism);
     } catch (final IOException e) {
       throw new UncheckedIOException(e);
     }
   }
 
+  private static String builderName(TypeElement type) {
+    var name = ProcessorUtils.shortType(UType.parse(type.asType()).mainType());
+    int pos = name.lastIndexOf('.');
+    if (pos > 0 && isNotNestedRecord(type.getEnclosingElement())) {
+      return name.substring(pos + 1) + "Builder";
+    }
+    return name.replace(".", "$") + "Builder";
+  }
+
+  private static boolean isNotNestedRecord(Element enclosingElement) {
+    ElementKind kind = enclosingElement.getKind();
+    return kind != ElementKind.RECORD;
+  }
+
   private void methods(
       Append writer,
       String typeParams,

blackbox-test-records/src/main/java/io/avaje/recordbuilder/nested/SomeInterface.java

@@ -0,0 +1,19 @@
+package io.avaje.recordbuilder.nested;
+
+import io.avaje.recordbuilder.RecordBuilder;
+
+public interface SomeInterface {
+
+  @RecordBuilder
+  record MyParams(
+    String param1,
+    String param2
+  ) {
+
+    public static MyParamsBuilder builder() {
+      // builder is called MyParamsBuilder rather than SomeInterface$MyParamsBuilder
+      // ... because the enclosed type is not a record
+      return MyParamsBuilder.builder();
+    }
+  }
+}

blackbox-test-records/src/test/java/io/avaje/recordbuilder/nested/SomeInterfaceTest.java

@@ -0,0 +1,21 @@
+package io.avaje.recordbuilder.nested;
+
+import io.avaje.recordbuilder.nested.SomeInterface.MyParams;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class SomeInterfaceTest {
+
+  @Test
+  void test() {
+
+    var params = MyParams.builder()
+      .param1("p1")
+      .param2("p2")
+      .build();
+
+    assertThat(params.param1()).isEqualTo("p1");
+    assertThat(params.param2()).isEqualTo("p2");
+  }
+}