JSON to Kotlin
Convert between data formats instantly with this json to kotlin. Transform your data for different applications and programming languages.
Features
- Instant Conversion: Transform data formats in milliseconds
- Preserve Structure: Maintains data relationships and hierarchy
- Error Handling: Clear feedback on conversion issues
- Copy Output: One-click copying of converted data
Example
Input:
``
{"id": 1, "name": "John", "tags": ["dev", "admin"]}
`
Output:
`
data class Root(
val id: Int,
val name: String,
val tags: List
)
`
Tips
Uses Kotlin data classes- val for immutable properties
- Works with kotlinx.serialization or Moshi
How to Use
1. Paste your JSON in the input area
2. Click convert to transform the data
3. Review the converted output
4. Copy or download the result
Best Practices
- Validate your source data before conversion
- Check the output for any formatting issues
- Test with a small sample before converting large files
- Keep backups of original data
JSON to a Kotlin Data Class
kotlinx.serialization is compile-time and needs no reflection, which is what makes it
work on Kotlin/Native and Kotlin/JS as well as the JVM:
`kotlin
@Serializable
data class User(
val id: Int,
val name: String,
val tags: List = emptyList()
)
val user = Json.decodeFromString(json)
`
Kotlin's null safety is the real benefit here: a non-nullable field that is missing or null in
the JSON throws at parse time rather than producing a null that explodes three layers later.
Default values make a field optional. Json { ignoreUnknownKeys = true } is almost always
needed against a real API, since kotlinx.serialization rejects unknown keys by default.
JSON Types in Kotlin
| JSON | Kotlin |
|---|---|
| object | data class |
| array | List |
| string | String |
| number | Int, Long or Double |
| true / false | Boolean |
| null | null — only for nullable types |
A missing or null value for a non-nullable property throws at parse time, which is the point of using Kotlin here.Generate the Type, Do Not Hand-Write It
Deriving a type from a sample payload is a starting point and not a specification. A sample
cannot tell you which fields are optional, which strings are really enums, what the bounds
are, or which numbers are money. Treat generated types as a first draft, then:
Mark optional fields explicitly. A field absent from your sample is not necessarily
required.
- Narrow strings to unions where the API documents a fixed set.
- Widen numbers where precision matters — decimals for money, strings for large IDs.
- Decide on unknown fields. Ignoring them is forgiving; rejecting them catches API
changes early. Pick deliberately.Numbers Are the Recurring Problem
A JSON number is an IEEE 754 double. Three consequences that bite in production:
Twitter hit the first one publicly: 64-bit tweet IDs arrived in JavaScript rounded, so the
API began sending an Value What happens Integers above 2⁵³ Silently lose precision — send IDs as strings Money as a float 0.1 + 0.2 = 0.30000000000000004 Leading zeros 007 is invalid JSON; "007" is a stringNaN and InfinityNot valid JSON at all alongside every id.
Keys, Order and Duplicates
Objects are formally unordered, though every JavaScript engine preserves insertion order for
string keys — with one exception: integer-like keys sort numerically and come first.
`javascript
JSON.stringify({ b: 1, 2: 2, a: 3 }); // {"2":2,"b":1,"a":3}
`
Duplicate keys are not an error in the spec, and JSON.parse` keeps the last one. Two
parsers can legitimately disagree about which value wins, which has been the basis of real
request-smuggling attacks.