JSON parsing with Gson/Moshi in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Gson in Android development?

Gson is a library that helps convert JSON strings into Kotlin objects and vice versa. It makes handling JSON easy and automatic.

beginner

What is Moshi and how is it different from Gson?

Moshi is another JSON library for Android that is modern and supports Kotlin features better, like null safety and Kotlin data classes. It is often preferred for Kotlin projects.

beginner

How do you parse a JSON string into a Kotlin data class using Gson?

You create a Gson object and call fromJson(jsonString, YourDataClass::class.java). This converts the JSON string into your Kotlin object.

intermediate

What annotation does Moshi use to map JSON keys to Kotlin properties?

Moshi uses the @Json(name = "json_key") annotation to map JSON keys to Kotlin properties when the names differ.

beginner

Why is it important to use data classes with Moshi or Gson?

Data classes automatically provide useful functions like equals(), hashCode(), and toString(), and they work well with JSON libraries for easy parsing and serialization.

Which library is known for better Kotlin support and null safety in JSON parsing?

A Moshi

B Retrofit

C Jackson

D Gson

Moshi is designed with Kotlin in mind and supports Kotlin features like null safety better than Gson.

How do you convert a JSON string to a Kotlin object using Gson?

A Moshi().fromJson(jsonString, DataClass::class.java)

B Gson().toJson(jsonString, DataClass::class.java)

C Gson().fromJson(jsonString, DataClass::class.java)

D JsonParser.parse(jsonString)

The fromJson() method of Gson converts JSON string to Kotlin object.

Which annotation is used in Moshi to rename JSON keys?

A @SerializedName

B @Json(name = "key")

C @JsonKey

D @JsonProperty

Moshi uses @Json(name = "key") to map JSON keys to Kotlin properties.

What Kotlin feature helps Moshi parse JSON easily?

A Extension functions

B Sealed classes

C Companion objects

D Data classes

Data classes provide a simple structure that Moshi can easily map JSON data into.

Which of these is NOT a function provided automatically by Kotlin data classes?

A parseJson()

B equals()

C toString()

D hashCode()

parseJson() is not automatically provided by data classes; it's a function from JSON libraries.

Explain how to parse a JSON string into a Kotlin object using Gson.

Think about the method that converts JSON text to an object.

Describe the advantages of using Moshi over Gson in Kotlin projects.

Focus on Kotlin-specific features and annotations.

Practice

1. What is the main purpose of using Gson or Moshi in Android Kotlin apps?

easy

A. To convert JSON text into Kotlin objects

B. To create user interfaces

C. To manage app navigation

D. To handle database queries

Solution

1. Step 1: Understand JSON parsing role

   Gson and Moshi are libraries that help convert JSON strings into Kotlin objects for easier data handling.

2. Step 2: Identify the correct purpose

   They do not create UI, manage navigation, or handle databases directly.

3. Final Answer:

   To convert JSON text into Kotlin objects -> Option A

4. Quick Check:

   JSON parsing = Convert JSON to objects [OK]

Hint: Gson/Moshi = JSON to Kotlin objects [OK]

Common Mistakes:

• Thinking Gson/Moshi create UI components
• Confusing JSON parsing with database handling
• Assuming they manage app navigation

2. Which of the following is the correct way to define a Kotlin data class for this JSON?
{"name":"Alice","age":30}

easy

A. class Person(var name: String, var age: String)

B. data class Person(val name: String, val age: Int)

C. data class Person(name: String, age: Int)

D. data class Person(val name: String, val age: String)

Solution

1. Step 1: Match JSON keys and types

   The JSON has keys "name" (String) and "age" (Int).

2. Step 2: Check Kotlin data class syntax

   Correct syntax uses 'data class' with val properties and types matching JSON.

3. Final Answer:

   data class Person(val name: String, val age: Int) -> Option B

4. Quick Check:

   Data class with val and correct types = data class Person(val name: String, val age: Int) [OK]

Hint: Use val and exact types matching JSON keys [OK]

Common Mistakes:

• Using var instead of val unnecessarily
• Wrong property types (e.g., age as String)
• Missing 'val' keyword in data class

3. Given this code snippet using Gson:

val json = "{\"name\":\"Bob\", \"age\":25}"
val person = Gson().fromJson(json, Person::class.java)
println(person.name)

What will be printed?

medium

A. null

B. 25

C. Person

D. Bob

Solution

1. Step 1: Parse JSON to Person object

   Gson converts the JSON string into a Person object with name="Bob" and age=25.

2. Step 2: Print the name property

   println(person.name) outputs the name value, which is "Bob".

3. Final Answer:

   Bob -> Option D

4. Quick Check:

   person.name = "Bob" [OK]

Hint: fromJson maps JSON keys to object properties [OK]

Common Mistakes:

• Printing age instead of name
• Expecting class name instead of property value
• Assuming null if parsing fails

4. What is wrong with this Moshi parsing code?

val json = "{\"name\":\"Eve\", \"age\":28}"
val moshi = Moshi.Builder().build()
val adapter = moshi.adapter(Person::class.java)
val person = adapter.fromJson(json)
println(person?.name)

Assuming Person is a correct data class, why might this code fail at runtime?

medium

A. Person class is not a data class

B. JSON string is malformed

C. Missing Moshi Kotlin adapter for data class support

D. Using println instead of Log

Solution

1. Step 1: Check Moshi setup for Kotlin data classes

   Moshi requires adding the KotlinJsonAdapterFactory to support Kotlin features like default values and nullability.

2. Step 2: Identify missing adapter

   The code builds Moshi without .add(KotlinJsonAdapterFactory()), so parsing may fail or produce null.

3. Final Answer:

   Missing Moshi Kotlin adapter for data class support -> Option C

4. Quick Check:

   Moshi needs KotlinJsonAdapterFactory for data classes [OK]

Hint: Add KotlinJsonAdapterFactory to Moshi builder [OK]

Common Mistakes:

• Not adding KotlinJsonAdapterFactory
• Assuming JSON string is invalid
• Confusing println with logging

5. You have this JSON array:

[{"id":1,"title":"Task 1"},{"id":2,"title":"Task 2"}]

How do you parse it into a Kotlin List<Task> using Gson, assuming data class Task(val id: Int, val title: String)?

hard

A. Use TypeToken() {}.type with Gson.fromJson

B. Call Gson.fromJson(json, Task::class.java)

C. Parse each JSON object manually with JSONObject

D. Use Gson.toJson(json, List::class.java)

Solution

1. Step 1: Understand parsing JSON arrays

   Gson needs a Type object to parse generic types like List<Task>.

2. Step 2: Use TypeToken to get List<Task> type

   TypeToken() {}.type provides the correct type for Gson.fromJson to parse the array.

3. Final Answer:

   Use TypeToken() {}.type with Gson.fromJson -> Option A

4. Quick Check:

   Use TypeToken for generic list parsing [OK]

Hint: Use TypeToken for lists with Gson [OK]

Common Mistakes:

• Parsing array as single object
• Manually parsing instead of using Gson
• Using toJson instead of fromJson