[Kotlin] Improve field nullability based on (required) (#7658)

* [Kotlin] Only generate nullable return types if the field is not required

* [Kotlin] Fix generated code formatting according to kotlin style guide

Co-authored-by: Derek Bailey <derekbailey@google.com>
Co-authored-by: Paulo Pinheiro <paulovictor.pinheiro@gmail.com>

src/idl_gen_kotlin.cpp

@@ -191,10 +191,11 @@ class KotlinGenerator : public BaseGenerator {
     auto r_type = GenTypeGet(field.value.type);
     if (field.IsScalarOptional() ||
         // string, structs and unions
-        (base_type == BASE_TYPE_STRING || base_type == BASE_TYPE_STRUCT ||
+        (!field.IsRequired() &&
-         base_type == BASE_TYPE_UNION) ||
+         (base_type == BASE_TYPE_STRING || base_type == BASE_TYPE_STRUCT ||
+          base_type == BASE_TYPE_UNION)) ||
         // vector of anything not scalar
-        (base_type == BASE_TYPE_VECTOR &&
+        (base_type == BASE_TYPE_VECTOR && !field.IsRequired() &&
          !IsScalar(field.value.type.VectorType().base_type))) {
       r_type += "?";
     }
@@ -998,7 +999,15 @@ class KotlinGenerator : public BaseGenerator {
                     OffsetWrapper(
                         writer, offset_val,
                         [&]() { writer += "obj.__assign({{seek}}, bb)"; },
-                        [&]() { writer += "null"; });
+                        [&]() {
+                          if (field.IsRequired()) {
+                            writer +=
+                                "throw AssertionError(\"No value for "
+                                "(required) field {{field_name}}\")";
+                          } else {
+                            writer += "null";
+                          }
+                        });
                   });
             }
             break;
@@ -1008,12 +1017,30 @@ class KotlinGenerator : public BaseGenerator {
             // val Name : String?
             //     get() = {
             //         val o = __offset(10)
-            //         return if (o != 0) __string(o + bb_pos) else null
+            //          return if (o != 0) {
+            //              __string(o + bb_pos)
+            //          } else {
+            //              null
+            //          }
             //     }
             // ? adds nullability annotation
             GenerateGetter(writer, field_name, return_type, [&]() {
               writer += "val o = __offset({{offset}})";
-              writer += "return if (o != 0) __string(o + bb_pos) else null";
+              writer += "return if (o != 0) {";
+              writer.IncrementIdentLevel();
+              writer += "__string(o + bb_pos)";
+              writer.DecrementIdentLevel();
+              writer += "} else {";
+              writer.IncrementIdentLevel();
+              if (field.IsRequired()) {
+                writer +=
+                    "throw AssertionError(\"No value for (required) field "
+                    "{{field_name}}\")";
+              } else {
+                writer += "null";
+              }
+              writer.DecrementIdentLevel();
+              writer += "}";
             });
             break;
           case BASE_TYPE_VECTOR: {
@@ -1038,7 +1065,11 @@ class KotlinGenerator : public BaseGenerator {
             GenerateFun(writer, field_name, params, return_type, [&]() {
               auto inline_size = NumToString(InlineSize(vectortype));
               auto index = "__vector(o) + j * " + inline_size;
-              auto not_found = NotFoundReturn(field.value.type.element);
+              auto not_found =
+                  field.IsRequired()
+                      ? "throw IndexOutOfBoundsException(\"Index out of range: "
+                        "$j, vector {{field_name}} is empty\")"
+                      : NotFoundReturn(field.value.type.element);
               auto found = "";
               writer.SetValue("index", index);
               switch (vectortype.base_type) {

tests/MyGame/Example/Monster.kt

@@ -68,10 +68,14 @@ class Monster : Table() {
             false
         }
     }
-    val name : String?
+    val name : String
         get() {
             val o = __offset(10)
-            return if (o != 0) __string(o + bb_pos) else null
+            return if (o != 0) {
+                __string(o + bb_pos)
+            } else {
+                throw AssertionError("No value for (required) field name")
+            }
         }
     val nameAsByteBuffer : ByteBuffer get() = __vector_as_bytebuffer(10, 1)
     fun nameInByteBuffer(_bb: ByteBuffer) : ByteBuffer = __vector_in_bytebuffer(_bb, 10, 1)

tests/MyGame/Example/Stat.kt

@@ -31,7 +31,11 @@ class Stat : Table() {
     val id : String?
         get() {
             val o = __offset(4)
-            return if (o != 0) __string(o + bb_pos) else null
+            return if (o != 0) {
+                __string(o + bb_pos)
+            } else {
+                null
+            }
         }
     val idAsByteBuffer : ByteBuffer get() = __vector_as_bytebuffer(4, 1)
     fun idInByteBuffer(_bb: ByteBuffer) : ByteBuffer = __vector_in_bytebuffer(_bb, 4, 1)