How is Kotlin able to call Java 11 code using JDK …

I was playing around with a Kotlin project which has some Java 11 calls and noticed that it seems to build and run fine using Java 8.

The call in particular is to java.lang.String#isBlank(), which was introduced in Java 11.

I’m using Gradle to build and run tests. Build JDK is set to JDK 1.8 and Kotlin compile target is also 1.8:

tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
    kotlinOptions {
        freeCompilerArgs += listOf("-Xjsr305=strict")
        jvmTarget = "1.8"
    }
}

gradle -v

------------------------------------------------------------
Gradle 6.6
------------------------------------------------------------

Build time:   2020-08-10 22:06:19 UTC
Revision:     d119144684a0c301aea027b79857815659e431b9

Kotlin:       1.3.72
Groovy:       2.5.12
Ant:          Apache Ant(TM) version 1.10.8 compiled on May 10 2020
JVM:          1.8.0_265 (AdoptOpenJDK 25.265-b01)
OS:           Mac OS X 10.15.6 x86_64

This is a unit test which calls Java 11 but still compiles and runs:

import org.junit.jupiter.api.Test

class BlankTest {

    @Test
    fun testBlank() {
        val s = java.lang.String("test string")
        val blank = s.isBlank()
        println("Input string blank? : $blank")
    }
}

How is this possible?

Answer

The answer is that you’re not actually using Java 11’s isBlank() method, you’re calling Kotlin’s isBlank extension function that can be applied to java.lang.String.

How is Kotlin able to call Java 11 code using JDK 1.8?

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Answer