JSON parsing with Gson/Moshi in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Gson parsing code?

Given this Kotlin data class and Gson parsing code, what will be the value of user.name after parsing?

Android Kotlin

data class User(val id: Int, val name: String)

val json = "{\"id\":1, \"name\":\"Alice\"}"
val user = Gson().fromJson(json, User::class.java)
println(user.name)

A "Alice"

B Throws JsonSyntaxException

C null

D "id"

💡 Hint

Check how Gson maps JSON keys to data class properties.

Gson maps JSON keys to matching Kotlin data class properties by name. The JSON key "name" matches the property name, so user.name is "Alice".

📝 Syntax

intermediate

Which Moshi adapter code correctly parses a JSON array of strings?

You want to parse this JSON array: ["apple", "banana", "cherry"] into a Kotlin List. Which Moshi adapter code is correct?

A

val moshi = Moshi.Builder().build()
val adapter = moshi.adapter<List<String>>(String::class.java)
val fruits = adapter.fromJson(json)

B

val moshi = Moshi.Builder().build()
val adapter = moshi.adapter<List<String>>(List::class.java)
val fruits = adapter.fromJson(json)

C

val moshi = Moshi.Builder().build()
val type = Types.newParameterizedType(List::class.java, String::class.java)
val adapter = moshi.adapter<List<String>>(type)
val fruits = adapter.fromJson(json)

D

val moshi = Moshi.Builder().build()
val adapter = moshi.adapter<List<String>>()
val fruits = adapter.fromJson(json)

💡 Hint

Moshi needs a parameterized type for generic collections.

Moshi requires a parameterized type to parse generic collections like List. Using Types.newParameterizedType creates the correct type for the adapter.

❓ lifecycle

advanced

What happens if you parse invalid JSON with Gson?

Consider this code snippet parsing invalid JSON with Gson. What is the expected behavior?

Android Kotlin

val json = "{\"id\": 1, \"name\": Alice}" // Missing quotes around Alice
val user = Gson().fromJson(json, User::class.java)

A Returns a User object with name set to null

B Throws com.google.gson.JsonSyntaxException

C Returns a User object with name set to "Alice"

D Returns null without exception

💡 Hint

Check how Gson handles malformed JSON strings.

Gson throws JsonSyntaxException when the JSON string is malformed, such as missing quotes around string values.

🧠 Conceptual

advanced

Why use @Json annotation in Moshi data classes?

You have a JSON key named user_name but your Kotlin property is userName. Why use @Json(name = "user_name") annotation?

Android Kotlin

data class User(@Json(name = "user_name") val userName: String)

A To ignore the <code>user_name</code> key during parsing

B To rename the Kotlin property to <code>user_name</code> in the compiled code

C To convert the property to uppercase during parsing

D To map the JSON key <code>user_name</code> to the Kotlin property <code>userName</code>

💡 Hint

Think about how JSON keys and Kotlin properties can have different names.

The @Json annotation tells Moshi to map the JSON key user_name to the Kotlin property userName, enabling correct parsing despite naming differences.

🔧 Debug

expert

Why does this Moshi parsing code throw a NullPointerException?

Given this Kotlin data class and Moshi parsing code, why does user.address.city cause a NullPointerException?

Android Kotlin

data class Address(val city: String)
data class User(val name: String, val address: Address?)

val json = "{\"name\":\"Bob\"}" // address key missing
val moshi = Moshi.Builder().build()
val adapter = moshi.adapter(User::class.java)
val user = adapter.fromJson(json)
println(user!!.address!!.city)

A Because the JSON is missing the address key, so address is null and accessing city causes NullPointerException

B Because Moshi cannot parse nested data classes

C Because the city property is not nullable

D Because the adapter is not built correctly

💡 Hint

Check what happens when a JSON key is missing for a nullable property.

The JSON does not include the address key, so Moshi sets the address property to null. Accessing address!!.city causes a NullPointerException because address is null.