JSON parsing with Gson/Moshi in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Gson instance for JSON parsing.

Android Kotlin

val gson = GsonBuilder().[1]().create()

Drag options to blanks, or click blank then click option'

AserializeNulls

BtoJson

CfromJson

DsetPrettyPrinting

Attempts:

3 left

💡 Hint

Common Mistakes

Using toJson or fromJson here causes errors because they are Gson methods, not GsonBuilder.

The setPrettyPrinting() method enables formatted JSON output when using Gson.

2fill in blank

medium

Complete the code to parse a JSON string into a Kotlin data class using Gson.

Android Kotlin

val user: User = gson.[1](jsonString, User::class.java)

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AparseJson

Bserialize

CfromJson

DtoJson

Attempts:

3 left

💡 Hint

Common Mistakes

Using toJson tries to convert an object to JSON, not parse JSON.

The fromJson() method converts a JSON string into an object of the specified class.

3fill in blank

hard

Fix the error in this Moshi adapter creation code by filling the blank.

Android Kotlin

val moshi = Moshi.Builder().build()
val adapter = moshi.adapter([1]::class.java)

Drag options to blanks, or click blank then click option'

AUser

BString

Cuser

DJsonAdapter

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name instead of a class causes compile errors.

The adapter method needs the class type, so User::class.java is correct. Using lowercase user is a variable, not a class.

4fill in blank

hard

Fill both blanks to parse a JSON array string into a list of User objects using Gson.

Android Kotlin

val listType = object : TypeToken<[1]<[2]>>() {}.type
val users: List<User> = gson.fromJson(jsonArrayString, listType)

Drag options to blanks, or click blank then click option'

AList

BUser

CArray

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using Array instead of List causes type mismatch errors.

To parse a JSON array, we use List of User as the type token.

5fill in blank

hard

Fill all three blanks to create a Moshi adapter for a list of User objects and parse JSON.

Android Kotlin

val moshi = Moshi.Builder().build()
val type = Types.newParameterizedType([1]::class.java, [2]::class.java)
val adapter = moshi.adapter<List<[3]>>(type)
val users = adapter.fromJson(jsonArrayString)

Drag options to blanks, or click blank then click option'

AList

BUser

CArray

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using Array instead of List or mismatching types causes runtime errors.

We use List as the parameterized type, and User as the class inside the list for Moshi parsing.

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