New J2K: Add conversion to promote classes with only companions into objects

j2k/newSrc/org/jetbrains/kotlin/j2k/ConversionsRunner.kt

@@ -38,6 +38,7 @@ object ConversionsRunner {
         InnerClassConversion(),
         ModifiersConversion(context),
         StaticsToCompanionExtractConversion(),
+        ClassToObjectPromotionConversion(),
         PolyadicExpressionConversion(),
         BinaryExpressionConversion()
     )

j2k/newSrc/org/jetbrains/kotlin/j2k/NewCodeBuilder.kt

@@ -329,12 +329,6 @@ class NewCodeBuilder {
                     printer.printWithNoIndent(type.name)
                 else -> printer.printWithNoIndent("Unit /* TODO: ${type::class} */")
             }
-            when (type.nullability) {
-                Nullability.Nullable -> printer.printWithNoIndent("?")
-                Nullability.Default -> printer.printWithNoIndent("?")// /* TODO: Default */")
-                else -> {
-                }
-            }
             if (type is JKParametrizedType && type.parameters.isNotEmpty()) {
                 printer.par(ANGLE) {
                     renderList(type.parameters) {
@@ -342,6 +336,12 @@ class NewCodeBuilder {
                     }
                 }
             }
+            when (type.nullability) {
+                Nullability.Nullable -> printer.printWithNoIndent("?")
+                Nullability.Default -> printer.printWithNoIndent("?")// /* TODO: Default */")
+                else -> {
+                }
+            }
         }
 
         override fun visitTypeElement(typeElement: JKTypeElement) {

j2k/newSrc/org/jetbrains/kotlin/j2k/conversions/ClassToObjectPromotionConversion.kt

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.j2k.conversions
+
+import org.jetbrains.kotlin.j2k.tree.JKClass
+import org.jetbrains.kotlin.j2k.tree.JKKtPrimaryConstructor
+import org.jetbrains.kotlin.j2k.tree.JKTreeElement
+import org.jetbrains.kotlin.j2k.tree.impl.JKClassImpl
+import org.jetbrains.kotlin.utils.addToStdlib.firstIsInstanceOrNull
+
+class ClassToObjectPromotionConversion : RecursiveApplicableConversionBase() {
+    override fun applyToElement(element: JKTreeElement): JKTreeElement {
+        if (element is JKClass && element.classKind == JKClass.ClassKind.CLASS) {
+            val constructor = element.declarationList.firstIsInstanceOrNull<JKKtPrimaryConstructor>()
+            val companion = element.declarationList.firstOrNull { it is JKClass && it.classKind == JKClass.ClassKind.COMPANION }
+            if (companion != null && (constructor == null || constructor.parameters.isEmpty())) {
+                val declarations = element.declarationList - companion - listOfNotNull(constructor)
+                if (declarations.isEmpty()) {
+                    companion as JKClass
+                    companion.invalidate()
+                    element.invalidate()
+                    return recurse(
+                        JKClassImpl(element.modifierList, element.name, element.inheritance, JKClass.ClassKind.OBJECT)
+                            .apply {
+                                declarationList = companion.declarationList
+                            }
+                    )
+                }
+            }
+        }
+        return recurse(element)
+    }
+}

j2k/newSrc/org/jetbrains/kotlin/j2k/tree/jk.kt

@@ -38,7 +38,7 @@ interface JKFile : JKTreeElement, JKBranchElement {
     var declarationList: List<JKDeclaration>
 }
 
-interface JKClass : JKDeclaration, JKModifierListOwner {
+interface JKClass : JKDeclaration, JKModifierListOwner, JKBranchElement {
     val name: JKNameIdentifier
 
     val inheritance: JKInheritanceInfo