Extension functions to convert object to json and json to object using Moshi. It also works with List, ArrayList, Collection etc.

ExtMoshi.kt

object MoshiUtil {
    fun getMoshi(): Moshi {
        return Moshi.Builder()
            .add(MutableCollectionJsonAdapter.FACTORY)
            .build()
    }
}

inline fun <reified T> String?.getObjFromJson(): T? {
    if (this == null) return null

    Timber.e("getObjFromJson: $this")

    return try {
        val jsonAdapter = MoshiUtil.getMoshi().adapter(T::class.java).lenient()

        jsonAdapter.fromJson(this)
        
        // Or, if you us it in the Navigation Component as url:
        // jsonAdapter.fromJson(this.urlDecode())
    } catch (e: Exception) {
        Timber.e(e)

        null
    }
}

inline fun <reified T> T?.getJsonFromObj(): String? {
    if (this == null) return null

    Timber.e("getJsonFromObj: $this")

    return try {
        val jsonAdapter = MoshiUtil.getMoshi().adapter(T::class.java).lenient()

        jsonAdapter.toJson(this)
        
        // Or, if you us it in the Navigation Component as url:
        // jsonAdapter.toJson(this).urlEncode()
    } catch (e: Exception) {
        Timber.e(e)

        null
    }
}

fun String.urlEncode(): String = URLEncoder.encode(this, "utf-8")

fun String.urlDecode(): String = URLDecoder.decode(this, "utf-8")

MoshiExtensionsExample.kt

// To Object
val json = "..."
val user: User = json.getObjFromJson<User>()

// To Json String
val userJson: String = user.getJsonFromObj<User>()

MutableCollectionJsonAdapter.kt

import com.squareup.moshi.*
import com.squareup.moshi.JsonAdapter.Factory
import timber.log.Timber
import java.io.IOException
import java.lang.reflect.Type
import java.util.*

/**
 * Converts collection types to JSON arrays containing their converted contents.
 *
 * Code is inspired from:
 * https://github.com/square/moshi/blob/master/moshi/src/main/java/com/squareup/moshi/CollectionJsonAdapter.java
 */
internal abstract class MutableCollectionJsonAdapter<C : MutableCollection<T>?, T> private constructor(
    private val elementAdapter: JsonAdapter<T>
) : JsonAdapter<C>() {
    abstract fun newCollection(): C

    @Throws(IOException::class)
    override fun fromJson(reader: JsonReader): C {
        val result = newCollection()
        reader.beginArray()
        while (reader.hasNext()) {
            result?.add(elementAdapter.fromJson(reader)!!)
        }
        reader.endArray()
        return result
    }

    @Throws(IOException::class)
    override fun toJson(writer: JsonWriter, value: C?) {
        writer.beginArray()
        for (element in value!!) {
            elementAdapter.toJson(writer, element)
        }
        writer.endArray()
    }

    override fun toString(): String {
        return "$elementAdapter.collection()"
    }

    companion object {
        val FACTORY = Factory { type, annotations, moshi ->
            val rawType = Types.getRawType(type)
            Timber.e("rawType: $rawType")
            if (annotations.isNotEmpty()) return@Factory null
            if (rawType == ArrayList::class.java || rawType == MutableList::class.java || rawType == MutableCollection::class.java) {
                return@Factory newArrayListAdapter<Any>(
                    type,
                    moshi
                ).nullSafe()
            }
            null
        }

        private fun <T> newArrayListAdapter(
            type: Type,
            moshi: Moshi
        ): JsonAdapter<MutableCollection<T>> {
            val elementType = Types.collectionElementType(type, MutableCollection::class.java)
            val elementAdapter: JsonAdapter<T> = moshi.adapter(elementType)
            return object : MutableCollectionJsonAdapter<MutableCollection<T>, T>(elementAdapter) {
                override fun newCollection(): MutableCollection<T> {
                    return ArrayList()
                }
            }
        }
    }
}