GSON-based DSL causing a NPE - kotlin

Question

I've been trying to create a Kotlin DSL for creating GSON JsonObjects with a JSON-like syntax. My builder looks like this
import com.google.gson.JsonArray
import com.google.gson.JsonElement
import com.google.gson.JsonObject
import com.google.gson.JsonPrimitive
class JsonBuilder(builder: JsonBuilder.() -> Unit) {
init {
builder()
}
val result = JsonObject()
infix fun String.to(property: Number) = result.addProperty(this, property)
infix fun String.to(property: Char) = result.addProperty(this, property)
infix fun String.to(property: Boolean) = result.addProperty(this, property)
infix fun String.to(property: String) = result.addProperty(this, property)
infix fun String.to(property: JsonElement) = result.add(this, property)
infix fun String.to(properties: Collection<JsonElement>) {
val arr = JsonArray()
properties.forEach(arr::add)
result.add(this, arr)
}
operator fun String.invoke(builder: JsonObject.() -> Unit) {
val obj = JsonObject()
obj.builder()
result.add(this, obj)
}
}
fun json(builder: JsonBuilder.() -> Unit) = JsonBuilder(builder).result
And my test looks like this
fun main() {
val json = json {
"name" to "value"
"obj" {
"int" to 1
}
"true" to true
}
println(json)
}
However, upon execution it causes a NullPointerException pointing to the first String extension function used, which I don't find very descriptive as I don't see anything being nullable up to that point. Moreover, I don't see how it really differs from the regular execution which of course doesn't cause a NPE.
val json = JsonObject()
json.addProperty("name", "value")
val obj = JsonObject()
obj.addProperty("int", 1)
json.add("obj", obj)
json.addProperty("true", true)
My question is what's exactly causing the exception (and how to prevent it).

Answer 1

The issue is that you've specified the initialiser block earlier than the result object, causing it to be null when you come to use it - this can be visualised by the following (decompiled output of your code).
public JsonBuilder(#NotNull Function1 builder) {
Intrinsics.checkParameterIsNotNull(builder, "builder");
super();
builder.invoke(this);
this.result = new JsonObject();
}
Therefore, the solution is to move the declaration and initialisation of result earlier than the initialiser block.
class JsonBuilder(builder: JsonBuilder.() -> Unit) {
val result = JsonObject()
init {
builder()
}
// ...
}
And the result is now...
{"name":"value","int":1,"obj":{},"true":true}
EDIT: You'll also want to allow chaining with your DSL, and fix a bug you currently have.
operator fun String.invoke(builder: JsonBuilder.() -> Unit) {
val obj = JsonBuilder(builder).result
result.add(this, obj)
}
Which produces the correct result of
{"name":"value","obj":{"int":1},"true":true}