Kotlin-40.kotlin调用Java之二(Call Java from Kotlin)

本文探讨了Kotlin与Java互操作的关键概念,包括空安全、平台类型、类型映射等,介绍了如何在Kotlin中处理Java代码的可空性和类型转换。

摘要生成于 C知道 ,由 DeepSeek-R1 满血版支持, 前往体验 >

官方文档: http://kotlinlang.org/docs/reference/java-interop.html

继续上一章

5.Java空安全和平台类型(Null-Safety and Platform Type)

在Java中任何引用都可能是null,这使Kotlin要求Java对象严格空安全是不现实的!
Java声明的类型在Kotlin中会被特别对待,称为平台类型(platform types)!
kotlin对平台类型的空检查(Null-checks)会放宽,因此它们在kotlin中的安全性与在Java中一样!
示例:
    val list = ArrayList<String>() // 非空(构造函数结果)
    list.add("Item")
    val size = list.size() // 非空(原生 int)
    val item = list[0] // 推断为平台类型(普通Java对象)

当调用平台类型的方法时,Kotlin不会在编译时检查可空性null错误, 
但在运行时调用可能报错,出现空指针异常NullPointerException或者Kotlin断言阻止null传播!
    item.substring(1) // 允许,如果item == null,可能会抛出异常

平台类型(platform types)是不可标示(non-denotable),意味着不能在kotlin语言中明确地写下指定!
当把平台类型值赋值给Kotlin变量时,可以依赖类型推断,或者选择期望的类型(可空null或非空null类型均可):
    val nullable: String? = item // 允许可空(null)
    val notNull: String = item // 允许非空(notNull),运行时可能失败(null)

如果选择非空类型(kotlin),编译器在赋值时会触发断言assertion,防止Kotlin的非空变量保存空值(null)!
当把平台值传递给期待非空的Kotlin函数时,也会触发断言!
总而言之,编译器尽力阻止空值(null)通过程序传播(鉴于泛型原因,有时不能完全消除)

1.平台类型表示法(Notation for Platform Type)
    如上所述,平台类型不能在程序中显式表示,因此在kotlin语言中没有相应语法! 
    但是有时编译器或IDE要在错误/参数信息中显示平台类型,所以可用助记符标记:
        T!                       表示T 或者 T?
        (Mutable)Collection<T>!  表示T的Java集合 可变或不可变, 可空或不可空
        Array<(out) T>!          表示T(或T子类)的Java数组 可空或者不可空

2.可空性注解(Nullability annotation)
    具有可空性注解的Java类型并不表示为平台类型,而表示为实际可空或非空的Kotlin类型,
    编译器支持多种可空性注解:
        JetBrains (@Nullable and @NotNull from the org.jetbrains.annotations package)
        Android (com.android.annotations and android.support.annotations)
        JSR-305 (javax.annotation)
        FindBugs (edu.umd.cs.findbugs.annotations)
        Eclipse (org.eclipse.jdt.annotation)
        Lombok (lombok.NonNull).

6.Java类型映射(Mapped type)

Kotlin会特殊处理部分的Java类型,映射到相应的Kotlin类型,映射只发生在编译期间,运行时表示保持不变!

1.Java基本类型映射到相应Kotlin类型:
    Java类型      Kotlin类型
    byte         kotlin.Byte
    short        kotlin.Short
    int          kotlin.Int
    long         kotlin.Long
    char         kotlin.Char
    float        kotlin.Float
    double       kotlin.Double
    boolean      kotlin.Boolean

2.Java包装类(基本类型)映射成可空Kotlin类:
      Java类型                  Kotlin类型
    java.lang.Byte             kotlin.Byte?
    java.lang.Short            kotlin.Short?
    java.lang.Integer          kotlin.Int?
    java.lang.Long             kotlin.Long?
    java.lang.Character        kotlin.Char?
    java.lang.Float            kotlin.Float?
    java.lang.Double           kotlin.Double?
    java.lang.Boolean          kotlin.Boolean?
注意: 当java包装类作为类型参数,会被映射成平台类型,例如,List<java.lang.Integer>在Kotlin中会变成List<kotlin.Int!>

3.Java一些非基本类型也会映射:
      Java类型                  Kotlin类型
    java.lang.Object           kotlin.Any!
    java.lang.Cloneable        kotlin.Cloneable!
    java.lang.Comparable       kotlin.Comparable!
    java.lang.Enum             kotlin.Enum!
    java.lang.Annotation       kotlin.Annotation!
    java.lang.Deprecated       kotlin.Deprecated!
    java.lang.CharSequence     kotlin.CharSequence!
    java.lang.String           kotlin.String!
    java.lang.Number           kotlin.Number!
    java.lang.Throwable        kotlin.Throwable!

4.Java集合类型在Kotlin中既能只读,也能可变,因此有以下映射(Kotlin集合在kotlin.collections包)    
    Java类型          Kotlin只读类型     Kotlin可变类型               平台类型(platform type)
    Iterator<T>       Iterator<T>       MutableIterator<T>           (Mutable)Iterator<T>!
    Iterable<T>       Iterable<T>       MutableIterable<T>           (Mutable)Iterable<T>!
    Collection<T>     Collection<T>     MutableCollection<T>         (Mutable)Collection<T>!
    Set<T>            Set<T>            MutableSet<T>                (Mutable)Set<T>!
    List<T>           List<T>           MutableList<T>               (Mutable)List<T>!
    ListIterator<T>   ListIterator<T>   MutableListIterator<T>       (Mutable)ListIterator<T>!
    Map<K, V>         Map<K, V>         MutableMap<K, V>             (Mutable)Map<K, V>!
    Map.Entry<K, V>   Map.Entry<K, V>   MutableMap.MutableEntry<K,V> (Mutable)Map.(Mutable)Entry<K, V>!

5.Java数组在Kotlin中映射:
    Java类型    Kotlin类型
    int[]       kotlin.IntArray!
    String[]    kotlin.Array<(out) String>!

7.在Kotlin中的Java泛型(Java generics in Kotlin)

Kotlin泛型与Java有些不同,当在Kotlin中使用Java泛型时,会执行一些转换:
    1.Java通配符(wildcard)转换成类型投影(type projection)
        Foo<? extends Bar> 转换成 Foo<out Bar!>!
        Foo<? super Bar> 转换成 Foo<in Bar!>!

    2.Java原始类型转换成星投影(star projection)
        List 转换成 List<*>! 即List<out Any?>!

和Java一样,Kotlin在运行时不保留泛型,即对象不会把类型参数传递到构造器!
即不能区分ArrayList<Integer>()和ArrayList<Character>() 
这使得is操作符不能检测泛型,Kotlin只允许is检测星投影的泛型类型:
    if (a is List<Int>) // 错误: 无法检查a是否为Int列表       
    if (a is List<*>) // OK: 不保证列表内容

简书:http://www.jianshu.com/p/bc65be16a489
优快云博客: http://blog.youkuaiyun.com/qq_32115439/article/details/75093931
GitHub博客:http://lioil.win/2017/07/13/Kotlin-javaInKotlin2.html
Coding博客:http://c.lioil.win/2017/07/13/Kotlin-javaInKotlin2.html

[ 95% 474/495] //frameworks/base/services/permission:services.permission kotlinc [common] FAILED: out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlin/services.permission.jar out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlin_headers/services.permission.jar rm -rf "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/classes" "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/header_classes" "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/srcJars" "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc-build.xml" "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/empty" && mkdir -p "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/classes" "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/header_classes" "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/srcJars" "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/empty" && out_system/host/linux-x86/bin/zipsync -d out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/srcJars -l out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/srcJars/list -f "*.java" out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kapt/kapt-sources.jar && out_system/host/linux-x86/bin/gen-kotlin-build-file --classpath "out_system/soong/.intermediates/build/soong/java/core-libraries/stable.core.platform.api.stubs/android_common/turbine-combined/stable.core.platform.api.stubs.jar:out_system/soong/.intermediates/libcore/core-lambda-stubs/android_common/turbine-combined/core-lambda-stubs.jar:out_system/soong/.intermediates/frameworks/base/ext/android_common/turbine-combined/ext.jar:out_system/soong/.intermediates/frameworks/base/framework/android_common/turbine-combined/framework.jar:out_system/soong/.intermediates/frameworks/base/services/core/services.core/android_common/turbine-combined/services.core.jar:out_system/soong/.intermediates/external/kotlinc/kotlin-annotations/android_common/combined/kotlin-annotations.jar:out_system/soong/.intermediates/external/kotlinc/kotlin-stdlib/android_common/combined/kotlin-stdlib.jar:out_system/soong/.intermediates/external/kotlinc/kotlin-stdlib-jdk7/android_common/combined/kotlin-stdlib-jdk7.jar" --name "frameworks__base__services__permission__android_common__services.permission" --out_dir "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/classes" --srcs "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlin/services.permission.jar.rsp" --srcs "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/srcJars/list" --out "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc-build.xml" && external/kotlinc/bin/kotlinc -Didea.plugins.compatible.build=999.SNAPSHOT -J--add-opens=java.base/java.util=ALL-UNNAMED -J-Xmx4096M -Xjvm-default=all -Xno-call-assertions -Xno-param-assertions -Xno-receiver-assertions -Xsam-conversions=class -no-stdlib -no-jdk -jvm-target 17 -Xbuild-file=out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc-build.xml -kotlin-home out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/empty -Xplugin=external/kotlinc/lib/jvm-abi-gen.jar -P plugin:org.jetbrains.kotlin.jvm.abi:outputDir=out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/header_classes && out_system/host/linux-x86/bin/soong_zip -jar -o out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlin/services.permission.jar -C out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/classes -D out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/classes -write_if_changed && out_system/host/linux-x86/bin/soong_zip -jar -o out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlin_headers/services.permission.jar -C out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/header_classes -D out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/header_classes -write_if_changed && rm -rf "out_system/soong/.intermediates/frameworks/base/services/permission/services.permission/android_common/kotlinc/srcJars" exception: org.jetbrains.kotlin.backend.common.BackendException: Backend Internal error: Exception during IR lowering File being compiled: /home/user256/work/TVE1086/system/frameworks/base/services/permission/java/com/android/server/permission/access/AccessCheckingService.kt The root cause java.lang.RuntimeException was thrown at: org.jetbrains.kotlin.backend.jvm.codegen.FunctionCodegen.generate(FunctionCodegen.kt:49) at org.jetbrains.kotlin.backend.common.CodegenUtil.reportBackendException(CodegenUtil.kt:241) at org.jetbrains.kotlin.backend.common.CodegenUtil.reportBackendException$default(CodegenUtil.kt:236) at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invokeSequential(performByIrFile.kt:68) at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invoke(performByIrFile.kt:55) at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invoke(performByIrFile.kt:41) at org.jetbrains.kotlin.backend.common.phaser.NamedCompilerPhase.invoke(CompilerPhase.kt:96) at org.jetbrains.kotlin.backend.common.phaser.CompositePhase.invoke(PhaseBuilders.kt:29) at org.jetbrains.kotlin.backend.common.phaser.NamedCompilerPhase.invoke(CompilerPhase.kt:96) at org.jetbrains.kotlin.backend.common.phaser.CompilerPhaseKt.invokeToplevel(CompilerPhase.kt:43) at org.jetbrains.kotlin.backend.jvm.JvmIrCodegenFactory.invokeCodegen(JvmIrCodegenFactory.kt:312) at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.runCodegen(KotlinToJVMBytecodeCompiler.kt:348) at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules$cli(KotlinToJVMBytecodeCompiler.kt:123) at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules$cli$default(KotlinToJVMBytecodeCompiler.kt:47) at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:167) at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:53) at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.kt:101) at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.kt:47) at org.jetbrains.kotlin.cli.common.CLITool.exec(CLITool.kt:101) at org.jetbrains.kotlin.cli.common.CLITool.exec(CLITool.kt:79) at org.jetbrains.kotlin.cli.common.CLITool.exec(CLITool.kt:43) at org.jetbrains.kotlin.cli.common.CLITool$Companion.doMainNoExit(CLITool.kt:179) at org.jetbrains.kotlin.cli.common.CLITool$Companion.doMainNoExit$default(CLITool.kt:177) at org.jetbrains.kotlin.cli.common.CLITool$Companion.doMain(CLITool.kt:166) at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler$Companion.main(K2JVMCompiler.kt:250) at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.main(K2JVMCompiler.kt) at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.base/java.lang.reflect.Method.invoke(Method.java:568) at org.jetbrains.kotlin.preloading.Preloader.run(Preloader.java:87) at org.jetbrains.kotlin.preloading.Preloader.main(Preloader.java:44) Caused by: java.lang.RuntimeException: Exception while generating code for: FUN name:initialize visibility:public modality:FINAL <> ($this:com.android.server.permission.access.AccessCheckingService) returnType:kotlin.Unit $this: VALUE_PARAMETER name:<this> type:com.android.server.permission.access.AccessCheckingService这个报错是什么原因怎么修复
最新发布
08-09
评论
添加红包

请填写红包祝福语或标题

红包个数最小为10个

红包金额最低5元

当前余额3.43前往充值 >
需支付:10.00
成就一亿技术人!
领取后你会自动成为博主和红包主的粉丝 规则
hope_wisdom
发出的红包
实付
使用余额支付
点击重新获取
扫码支付
钱包余额 0

抵扣说明:

1.余额是钱包充值的虚拟货币,按照1:1的比例进行支付金额的抵扣。
2.余额无法直接购买下载,可以购买VIP、付费专栏及课程。

余额充值