Qs Kotlin

qs-kotlin Usage Assistant

Help users parse and build query strings with the Kotlin/JVM qs-kotlin package, the Android qs-kotlin-android wrapper, and the optional OkHttp, Ktor, and Spring Web integration modules. Focus on user application code and interoperability outcomes, not repository maintenance.

Start With Inputs

Before producing a final snippet, collect only the missing details that change the code:

• Runtime: Kotlin/JVM, Android, Java, java.net.URI, OkHttp, Ktor, Spring Web, tests, backend code, or generated example.
• Direction: decode an incoming query string, encode Kotlin or Java data, or normalize query-string handling around an existing URL/request object.
• The actual query string or data structure when available.
• Target API convention for lists: indexed brackets, empty brackets, repeated keys, or comma-separated values.
• Whether the query may include a leading ?, dot notation, literal dots in keys, duplicate keys, custom delimiters, comma-separated lists, null flags, ISO-8859-1/legacy charset behavior, Android AAR requirements, or untrusted user input.

Do not over-ask when the desired behavior is obvious. State assumptions in the answer and give the user a concrete snippet they can paste.

Installation

Use the core JVM artifact for normal Kotlin/JVM and most Android projects:

dependencies {
    implementation("io.github.techouse:qs-kotlin:<version>")
}

Use the Android AAR wrapper when the project specifically wants an AAR coordinate and Android Gradle Plugin metadata:

dependencies {
    implementation("io.github.techouse:qs-kotlin-android:<version>")
}

Add only the optional integration artifact for the framework URL helper the application uses. Each integration module depends on the core artifact.

OkHttp:

dependencies {
    implementation("io.github.techouse:qs-kotlin-okhttp:<version>")
}

Ktor:

dependencies {
    implementation("io.github.techouse:qs-kotlin-ktor:<version>")
}

Spring Web:

dependencies {
    implementation("io.github.techouse:qs-kotlin-spring-web:<version>")
}

The library targets Java 17. Android apps with minSdk < 26 that use java.time transitively may need core library desugaring in the app module.

Public API

Prefer package-level functions in Kotlin:

import io.github.techouse.qskotlin.decode
import io.github.techouse.qskotlin.encode
import io.github.techouse.qskotlin.models.DecodeOptions
import io.github.techouse.qskotlin.models.EncodeOptions

For Java, use the generated QS facade and Java-friendly builders:

import io.github.techouse.qskotlin.QS;
import io.github.techouse.qskotlin.models.DecodeOptions;
import io.github.techouse.qskotlin.models.EncodeOptions;

Kotlin extension helpers are also available:

import io.github.techouse.qskotlin.toQueryMap
import io.github.techouse.qskotlin.toQueryString

val params = "a[b]=c".toQueryMap()
val query = mapOf("a" to mapOf("b" to "c")).toQueryString()

Decode a java.net.URI through its raw query component:

import io.github.techouse.qskotlin.decodeQsQuery
import java.net.URI

val params = URI("https://example.com/?a%5Bb%5D=c").decodeQsQuery()

Use QS.decodeQsQuery(uri) from Java. This helper deliberately reads URI.rawQuery; do not pass URI.query or URI.getQuery() to qs because those accessors percent-decode before qs splits query pairs.

Optional framework helpers live in integration packages:

import io.github.techouse.qskotlin.okhttp.addQsQueryParameters
import io.github.techouse.qskotlin.ktor.appendQsQueryParameters
import io.github.techouse.qskotlin.ktor.parseQsQuery
import io.github.techouse.qskotlin.spring.web.queryQs

Base Patterns

Decode a query string into nested Kotlin values:

import io.github.techouse.qskotlin.decode

val params =
    decode("a[b][c]=d&tags[]=kotlin&tags[]=android")

check(
    params ==
        mapOf(
            "a" to mapOf("b" to mapOf("c" to "d")),
            "tags" to listOf("kotlin", "android"),
        )
)

Encode nested Kotlin values into a query string:

import io.github.techouse.qskotlin.encode

val query =
    encode(
        mapOf(
            "a" to mapOf("b" to mapOf("c" to "d")),
            "tags" to listOf("kotlin", "android"),
        )
    )

check(query == "a%5Bb%5D%5Bc%5D=d&tags%5B0%5D=kotlin&tags%5B1%5D=android")

Java callers should prefer builders for options:

import io.github.techouse.qskotlin.QS;
import io.github.techouse.qskotlin.enums.ListFormat;
import io.github.techouse.qskotlin.models.EncodeOptions;
import java.util.List;
import java.util.Map;

String query =
    QS.encode(
        Map.of("tags", List.of("kotlin", "android")),
        EncodeOptions.builder().listFormat(ListFormat.REPEAT).build());

assert query.equals("tags=kotlin&tags=android");

Decode Recipes

Use these options with decode(query, DecodeOptions(...)) in Kotlin or QS.decode(query, DecodeOptions.builder()...build()) in Java:

• Leading question mark: ignoreQueryPrefix = true.
• java.net.URI: use uri.decodeQsQuery(options), which reads rawQuery and returns an empty map for absent, empty, or opaque query components.
• Dot notation such as a.b=c: allowDots = true.
• Double-encoded literal dots in keys such as name%252Eobj.first=John: decodeDotInKeys = true.
• Duplicate keys: duplicates = Duplicates.COMBINE keeps all values as a list; use Duplicates.FIRST or Duplicates.LAST to collapse plain duplicate keys. Bracket-list keys such as items[] always combine.
• Bracket lists: enabled by default; set parseLists = false to treat list syntax as map keys.
• Empty list tokens such as foo[]: allowEmptyLists = true.
• List limits: default listLimit is 20; indices greater than or equal to the limit become map keys. Duplicate keys, mixed notation, and flat comma values contribute cumulatively. Exact-limit results remain lists; soft overflow preserves every value in a numeric-keyed map, while throwOnLimitExceeded = true throws. A negative limit always overflows or throws.
• Comma-separated values such as a=b,c: comma = true.
• Object/scalar conflicts such as a[b]=c&a=d: strictMerge = true wraps the conflict in a list; set strictMerge = false for legacy key-as-true behavior.
• Tokens without = as null: strictNullHandling = true.
• Custom delimiters: delimiter = Delimiter.SEMICOLON, delimiter = StringDelimiter(";"), or delimiter = RegexDelimiter("[;,]").
• Legacy charset input: charset = StandardCharsets.ISO_8859_1; use charsetSentinel = true when a form may include utf8=... to signal the real charset.
• HTML numeric entities: interpretNumericEntities = true, usually with ISO-8859-1 or charset sentinel handling.
• Untrusted input: keep depth, parameterLimit, and listLimit bounded; use strictDepth = true and throwOnLimitExceeded = true when callers need hard failures instead of soft limiting.

Example for a request query:

import io.github.techouse.qskotlin.decode
import io.github.techouse.qskotlin.enums.Duplicates
import io.github.techouse.qskotlin.models.DecodeOptions

val params =
    decode(
        "?filter.status=open&tag=kotlin&tag=android",
        DecodeOptions(
            ignoreQueryPrefix = true,
            allowDots = true,
            duplicates = Duplicates.COMBINE,
        ),
    )

check(params == mapOf("filter" to mapOf("status" to "open"), "tag" to listOf("kotlin", "android")))

Encode Recipes

Use these options with encode(data, EncodeOptions(...)) in Kotlin or QS.encode(data, EncodeOptions.builder()...build()) in Java:

• List style defaults to ListFormat.INDICES: tags%5B0%5D=kotlin&tags%5B1%5D=android.
• Empty brackets: listFormat = ListFormat.BRACKETS.
• Repeated keys: listFormat = ListFormat.REPEAT.
• Comma-separated values: listFormat = ListFormat.COMMA.
• Single-item comma lists that must round-trip as lists: commaRoundTrip = true.
• Drop null items before comma-joining lists: commaCompactNulls = true.
• Dot notation for nested maps: allowDots = true.
• Literal dots in keys: encodeDotInKeys = true; set allowDots = true when nested paths should use dot notation.
• Add a leading ?: addQueryPrefix = true.
• Custom pair delimiter: delimiter = Delimiter.SEMICOLON or delimiter = StringDelimiter(";").
• Preserve readable bracket/dot keys while encoding values: encodeValuesOnly = true.
• Disable percent encoding entirely for debugging or documented examples: encode = false.
• Emit null without =: strictNullHandling = true.
• Omit null keys: skipNulls = true.
• Emit empty lists as foo[]: allowEmptyLists = true.
• Omit selected values: use skipNulls = true for null values, return null from a FunctionFilter, or remove those entries before calling encode.
• Legacy form spaces as +: format = Format.RFC1738; the default is Format.RFC3986, which emits spaces as %20.
• Legacy charset output: charset = StandardCharsets.ISO_8859_1; use charsetSentinel = true to prepend the utf8=... sentinel.
• Custom behavior: use encoder, dateSerializer, sort, or filter when the target API needs special scalar encoding, date formatting, stable key order, or selected fields.

Example for an API that expects repeated keys:

import io.github.techouse.qskotlin.encode
import io.github.techouse.qskotlin.enums.ListFormat
import io.github.techouse.qskotlin.models.EncodeOptions

val query =
    encode(
        mapOf(
            "q" to "query strings",
            "tag" to listOf("kotlin", "android"),
        ),
        EncodeOptions(
            listFormat = ListFormat.REPEAT,
            addQueryPrefix = true,
        ),
    )

check(query == "?q=query%20strings&tag=kotlin&tag=android")

Framework Integrations

Use the framework helpers when callers need to append qs-kotlin output to URL builder types without losing duplicate keys, name-only values, bracket notation, or existing query parameters. Pass EncodeOptions the same way as encode. The helpers append already encoded names and values through encoded query APIs to avoid double-encoding %5B into %255B.

OkHttp:

import io.github.techouse.qskotlin.okhttp.addQsQueryParameters
import okhttp3.HttpUrl.Companion.toHttpUrl

val url =
    "https://api.example.com/products"
        .toHttpUrl()
        .addQsQueryParameters(mapOf("filter" to mapOf("name" to "John")))

check(url.toString() == "https://api.example.com/products?filter%5Bname%5D=John")

Use HttpUrl.Builder.addQsQueryParameters(value, options) when the caller already has a builder. Use HttpUrl.addQsQueryParameters(value, options) to return a new immutable URL and leave the original unchanged.

Ktor client or shared URL building:

import io.github.techouse.qskotlin.ktor.appendQsQueryParameters
import io.ktor.http.URLBuilder

val url =
    URLBuilder("https://api.example.com/products")
        .appendQsQueryParameters(mapOf("tags" to listOf("a", "b")))
        .build()

check(url.encodedQuery == "tags%5B0%5D=a&tags%5B1%5D=b")

Ktor server request parsing:

import io.github.techouse.qskotlin.ktor.parseQsQuery

val params = call.request.parseQsQuery()

check(params == mapOf("filter" to mapOf("name" to "John")))

parseQsQuery intentionally reads ApplicationRequest.queryString() and then calls decode, so prefer it over decoded Ktor parameter collections when bracket notation, duplicate keys, or original percent escapes matter.

Spring Web:

import io.github.techouse.qskotlin.spring.web.queryQs
import org.springframework.web.util.UriComponentsBuilder

val uri =
    UriComponentsBuilder.fromUriString("https://api.example.com/products")
        .queryQs(mapOf("filter" to mapOf("name" to "John Doe")))
        .build(true)
        .toUri()

check(uri.toString() == "https://api.example.com/products?filter%5Bname%5D=John%20Doe")

Always finish Spring queryQs usage with build(true).toUri() or build(true).toUriString(). Do not use build().toUri(), encode().build(), or build().encode() after queryQs, because Spring will double-encode qs-kotlin's already encoded output. queryQs requires EncodeOptions.encode = true; raw unencoded output is not safe with Spring's encoded-component path.

There is no dedicated Retrofit integration. When full qs-kotlin fidelity is needed, build an OkHttp HttpUrl with qs-kotlin-okhttp and pass it to Retrofit with @Url.

Java Interop

Use builders from Java to avoid long Kotlin data-class constructors:

import io.github.techouse.qskotlin.QS;
import io.github.techouse.qskotlin.enums.Duplicates;
import io.github.techouse.qskotlin.models.DecodeOptions;
import java.util.List;
import java.util.Map;

Map<String, Object> params =
    QS.decode(
        "?tag=kotlin&tag=android",
        DecodeOptions.builder()
            .ignoreQueryPrefix(true)
            .duplicates(Duplicates.COMBINE)
            .build());

assert params.equals(Map.of("tag", List.of("kotlin", "android")));

Use Java-friendly functional interfaces for custom callbacks:

• JDecoder for decode customization.
• JValueEncoder for encode customization.
• JDateSerializer for LocalDateTime serialization.
• Java Comparator instances through EncodeOptions.builder().sort(...).

Combinations To Check

Warn or adjust before giving code for these cases:

• DecodeOptions(decodeDotInKeys = true, allowDots = false) is invalid.
• parameterLimit must be positive, or Int.MAX_VALUE for effectively unlimited parsing.
• throwOnLimitExceeded = true turns parameter and list limit violations into IndexOutOfBoundsException; without it, parameter parsing stops at the limit and list overflows fall back to numeric-keyed maps. Flat comma values are checked before value decoding; a comma group assigned through []= counts as one outer list element.
• strictDepth = true throws on well-formed depth overflow; with the default false, the remainder beyond depth is kept as a trailing key segment.
• Built-in charset handling supports only StandardCharsets.UTF_8 and StandardCharsets.ISO_8859_1; other encodings require a custom encoder or decoder.
• EncodeOptions.encoder is ignored when encode = false.
• Combining encodeValuesOnly = true and encodeDotInKeys = true encodes only dots in keys; values remain otherwise unchanged.
• DecodeOptions.comma parses simple comma-separated values, but does not decode nested map syntax such as a={b:1},{c:d}.
• encode(null), scalar roots, empty maps, and empty containers generally produce an empty string.
• The JDK and many web frameworks flatten duplicates or nested query syntax. Prefer decode or QS.decode on the raw query string when qs-style nested or repeated values matter.
• For java.net.URI, prefer decodeQsQuery; never decode URI.query and then pass it to qs, because encoded delimiters and percent signs can be decoded too early.
• OkHttp and Ktor URL helpers preserve qs-kotlin output by using encoded query parameter APIs; normal decoded query APIs can double-encode bracket notation.
• Ktor server code should parse ApplicationRequest.queryString(), not queryParameters, when qs-style nested or repeated values matter.
• Spring Web queryQs must be followed by build(true) and rejects EncodeOptions(encode = false).

Response Shape

For code-generation requests, answer with:

1. A short statement of assumptions, especially language, artifact coordinate, list format, null handling, charset, prefix handling, and whether input is trusted.
2. One concrete Kotlin or Java snippet using decode, encode, toQueryMap, toQueryString, decodeQsQuery, QS, or the relevant framework helper.
3. A brief explanation of only the options used.
4. A small verification example, such as an expected map, expected query string, JUnit assertion, Kotest assertion, or check(...).

Keep snippets application-oriented. Prefer public API imports from io.github.techouse.qskotlin; do not ask users to import from io.github.techouse.qskotlin.internal.