Android Moshi Adapter Convert Null To Empty String With JsonQualifier (Kotlin)

May 14, 2018

The following adapter convert reading of json string such as {"name":null} to name = "".

Create a NullToEmptyString adapter class. I am using a @JsonQualifier as I don’t want to apply this adapter to all String.

@Retention(AnnotationRetention.RUNTIME)
@JsonQualifier
annotation class NullToEmptyString

class NullToEmptyStringAdapter {
    @ToJson
    fun toJson(@NullToEmptyString value: String?): String? {
        return value
    }

    @FromJson
    @NullToEmptyString
    fun fromJson(@Nullable data: String?): String? {
        return data ?: ""
    }
}

Apply NullToEmptyStringAdapter.

data class PhotoPin (
        @Json(name = "name") @NullToEmptyString var name: String = ""
        )

Setup Moshi

val moshi = Moshi.Builder()
        .add(NullToEmptyStringAdapter())
        .add(KotlinJsonAdapterFactory())
        .build()

NOTE: KotlinJsonAdapterFactory is required (for Kotlin) else the behaviour differ.

Test Code

@RunWith(AndroidJUnit4::class)
class MoshiTest {
    lateinit var moshi: Moshi
    lateinit var adapter: JsonAdapter<PhotoPin>

    @Before
    fun init() {
        moshi = Moshi.Builder()
            .add(NullToEmptyStringAdapter())
            .add(KotlinJsonAdapterFactory())
            .build()
        adapter = moshi.adapter<PhotoPin>(PhotoPin::class.java)
    }


    @Test
    fun testNameNull() {
        var item = adapter.fromJson("""{"name":null}""")!!

        assertThat(item.name, Is(""))
    }
}