Json

Learn Json.encodeToString and decodeFromString.

The Json object

Json is the object that produces and reads JSON with kotlinx.serialization. Json.encodeToString(value) produces JSON, Json.decodeFromString<T>(text) reads JSON.

Example: val json = Json.encodeToString(user). This turns user into {"name":"Alex"}. With Json { prettyPrint = true } you configure it. The default Json object produces compact JSON. decodeFromString<User>(json) declares the target class via the type parameter.

decodeFromString

decodeFromString turns a JSON string into a Kotlin object. The type parameter declares the target class.

Example: val user = Json.decodeFromString<User>("{"name":"Alex"}"). This turns the JSON string into a User object. JSON field names must match Kotlin property names. Mismatched fields throw. With ignoreUnknownKeys = true unknown fields are skipped. This is useful for external JSON.

Exercise Use Json encodeToString

Define @Serializable data class User(val name: String). Inside main, set val user = User("Alex"), produce JSON with val json = Json.encodeToString(user), and print println("JSON produced").

Show example solution

@Serializable
data class User(val name: String)

fun main() {
    val user = User("Alex")
    val json = Json.encodeToString(user)
    println("JSON produced")
}

Test yourself

Complete the encodeToString call

val json = Json.____(user)

Show answer

val json = Json.encodeToString(user)

Match Json methods

encodeToString

Object to JSON
decodeFromString

JSON to object
Json

Configuration
prettyPrint

Readable output
Which direction does Json.decodeFromString work?
- Object to JSON
- JSON to object
- Object to XML
- XML to object
Show answer

JSON to object — decodeFromString turns a JSON string into an object.

Next lesson: @SerialName

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