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Reputation: 1582

How is String class in Kotlin Standard library implemented?

File can be found here - https://github.com/JetBrains/kotlin/blob/master/core/builtins/native/kotlin/String.kt

Adding String.kt code for reference

package kotlin

/**
* The `String` class represents character strings. All string literals in 
* Kotlin programs, such as `"abc"`, are
* implemented as instances of this class.
*/

public class String : Comparable<String>, CharSequence {
companion object {}

/**
 * Returns a string obtained by concatenating this string with the string representation of the given [other] object.
 */
public operator fun plus(other: Any?): String

public override val length: Int

/**
 * Returns the character of this string at the specified [index].
 *
 * If the [index] is out of bounds of this string, throws an [IndexOutOfBoundsException] except in Kotlin/JS
 * where the behavior is unspecified.
 */
public override fun get(index: Int): Char

public override fun subSequence(startIndex: Int, endIndex: Int): CharSequence

public override fun compareTo(other: String): Int
}

I want to understand how the methods shown doesn't have any implementation? Also please brief on how this code could possibly work?

If there is native code being used. Please link the source code of the same..

Upvotes: 2

Views: 1046

Answers (1)

Iaroslav Postovalov
Iaroslav Postovalov

Reputation: 2463

Kotlin's standard library is shared between all the platforms supported by Kotlin, so it contains only signatures of methods. Several classes are implemented identically to platform-specific ones: for example, in Kotlin/JVM kotlin.text.StringBuilder is implemented as a type-alias to java.lang.StringBuilder.

However, some types are mapped by the compiler specifically to platform types (some methods are removed, some methods are renamed). These types include primitive types (Int, Byte, etc.), String and collections.

As for JVM the rough mapping is:

kotlin.String.length => java.lang.String.length()
kotlin.String.compareTo(other: String) => java.lang.String.compareTo(java.langString anotherString)
kotlin.String.get(index: Int) => java.lang.String.charAt(int index)
kotlin.String.plus(other: Any?) is implemented as plain Java concatentation (it is implemented on compiler level with StringConcatFactory magic)
kotlin.String.subSequence(startIndex: Int, endIndex: Int) => java.lang.String.subSequence(int beginIndex, int endIndex)

Other java.lang.String methods like intern are not available in kotlin.String (however, it is actually possible to reference java.lang.String directly).

In Kotlin/Native kotlin.String is implemented with C++: https://github.com/JetBrains/kotlin-native/blob/045b20a36b8a1fe86716a99cb25f03021e583592/runtime/src/main/cpp/KString.cpp.

JavaScript implementation involves builtin JavaScript String too.

Upvotes: 3

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