Android java 实现 kotlin 含有默认方法的接口

1. 报错

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public class TestJava implements TestKotlin {

}

interface TestKotlin {

    fun testKotlin() {
        println("TestKotlin#testKotlin()")
    }

}

提示需要实现 testKotlin() 方法.png

2. 简单解决

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实现 testKotlin() 方法即可

public class TestJava implements TestKotlin {

    @Override
    public void testKotlin() {
        // do nothing.
    }

}

3. 追求卓越

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如果你提供了一个 kotlin 的库给公众使用，那么公众就需要在每个地方都去实现一个空方法，是不是隐约听到 SB 作者 的骂声一片一片传来。

android {
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = "1.8"
        freeCompilerArgs += [
                '-Xjvm-default', 'compatibility'
        ]
    }
}

interface TestKotlin {

    @JvmDefault
    fun testKotlin() {
        println("TestKotlin#testKotlin()")
    }

}

public class TestJava implements TestKotlin {

}

使用 @JvmDefault 注解完美解决，前提是需要配置下 build.gradle 文件

参考

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1. JvmDefault - Kotlin Programming Language (kotlinlang.org)
2. JvmDefault 源码

/**
 *
 * Specifies that a JVM default method should be generated for non-abstract Kotlin interface member.
 *
 * Usages of this annotation require an explicit compilation argument to be specified:
 * either `-Xjvm-default=enable` or `-Xjvm-default=compatibility`.
 *
 * * with `-Xjvm-default=enable`, only default method in interface is generated for each @[JvmDefault] method.
 *   In this mode, annotating an existing method with @[JvmDefault] can break binary compatibility, because it will effectively
 *   remove the method from the `DefaultImpls` class.
 * * with `-Xjvm-default=compatibility`, in addition to the default interface method, a compatibility accessor is generated
 *   in the `DefaultImpls` class, that calls the default interface method via a synthetic accessor.
 *   In this mode, annotating an existing method with @[JvmDefault] is binary compatible, but results in more methods in bytecode.
 *
 * Removing this annotation from an interface member is a binary incompatible change in both modes.
 *
 * Generation of default methods is only possible with JVM target bytecode version 1.8 (`-jvm-target 1.8`) or higher.
 *
 * @[JvmDefault] methods are excluded from interface delegation.
 */
@SinceKotlin("1.2")
@RequireKotlin("1.2.40", versionKind = RequireKotlinVersionKind.COMPILER_VERSION)
@Target(AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY)
@Deprecated("Switch to new -Xjvm-default modes: `all` or `all-compatibility`")
annotation class JvmDefault

附录

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根据作者本地代码提示（kotlin 1.5.30 版本），该注解已被废弃，建议使用 -Xjvm-default all 或者 Xjvm-default all-compatibility 代替

android {
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = "1.8"
        freeCompilerArgs += [
                '-Xjvm-default', 'all-compatibility'
        ]
    }
}

interface TestKotlin {

    fun testKotlin() {
        println("TestKotlin#testKotlin()")
    }

}

public class TestJava implements TestKotlin {

}

这样就不用每个方法上都去加 @JvmDefault 注解了

最后

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鼓掌.png