【问题标题】:What is the difference between my variable name and 'it' when using 'let' in kotlin?在 kotlin 中使用“let”时,我的变量名和“it”有什么区别?
【发布时间】:2020-08-26 18:00:17
【问题描述】:

在下面的代码sn-p中,两个Codeblocks有什么区别?
如果我检查println(name==it),则返回 true,所以它们必须引用同一个对象,对吧?

但是,当 name 变量有值时,一切正常,但是一旦我将其设置为 null,代码块 1 就可以工作(这意味着 let 块根本不会被执行),但代码块 2 会抛出错误。当name为空时,为什么 kotlin 不只是跳过/忽略代码块 2?

fun main() {
    
    var name:String? = "Cedric"
    //name = null
    
    //Codeblock 1
    name?.let{
        println("The length of name is ${name.length}")
    }
    
    //Codeblock 1
    name?.let{
        println("The length of name is ${it.length}")
    }
    
}

非常感谢,感谢您的帮助。

【问题讨论】:

    标签: android kotlin null let kotlin-null-safety


    【解决方案1】:

    代码块 2 实际上给出了编译时错误,而代码块 1 没有。我认为这只是一个奇怪的 linting bug,因为这两种情况应该被同等对待。

    【讨论】:

    • 作为另一个 Kotlin 学习者,当 name 为 null 时,整个 let 块不应该首先执行,我是否正确?那样的话,还真是挺奇怪的。
    • 为什么 2 个代码块会出现编译时错误?他不应该,也作为第一个。
    • 看起来如果 name = null 未注释,那么编译器会将 name 替换为 null,因为类型提示显示 it 的类型是 Nothing
    • @Fantailed 在这两种情况下都不会执行。但是 linter 仍然会检查块内的代码在语法上是否正确。如果 name = null 未注释,它会在案例 2 中给出错误,但在案例 1 中不会。即使在案例 1 中 name 被智能转换为 kotlin.Nothing,它仍然可以让您从中获取 String.length,这我认为不应该这样做。或者也许我错了,智能投射出于某种原因应该像这样工作。
    • @Tenfour04 不适合我:i.imgur.com/30bXHmb.png
    【解决方案2】:

    据我了解,您在编译器中发现了错误。我在 Android Studio 和 IntelliJ 中有两种不同的行为。 万一在编译的时候,编译后的代码被优化了,反编译的时候是这样的:

       public final void main() {
          Object name = "Cedric";
          name = (String)null;
       }
    

    如果它没有被编译我得到一个错误:

    org.jetbrains.kotlin.codegen.CompilationException: Back-end (JVM) Internal error: Failed to generate expression: KtNameReferenceExpression
    File being compiled: (12,45) in D:/Projekti/kotlin playground/src/Main.kt
    The root cause java.lang.AssertionError was thrown at: org.jetbrains.kotlin.codegen.ExpressionCodegen.visitSimpleNameExpression(ExpressionCodegen.java:1797)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genQualified(ExpressionCodegen.java:326)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genQualified(ExpressionCodegen.java:286)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.visitDotQualifiedExpression(ExpressionCodegen.java:3258)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.visitDotQualifiedExpression(ExpressionCodegen.java:112)
        at org.jetbrains.kotlin.psi.KtDotQualifiedExpression.accept(KtDotQualifiedExpression.kt:32)
        at org.jetbrains.kotlin.codegen.CodegenStatementVisitor.visitKtElement(CodegenStatementVisitor.java:31)
        at org.jetbrains.kotlin.codegen.CodegenStatementVisitor.visitKtElement(CodegenStatementVisitor.java:22)
        at org.jetbrains.kotlin.psi.KtVisitor.visitExpression(KtVisitor.java:182)
        at org.jetbrains.kotlin.psi.KtVisitor.visitQualifiedExpression(KtVisitor.java:286)
        at org.jetbrains.kotlin.psi.KtVisitor.visitDotQualifiedExpression(KtVisitor.java:302)
        at org.jetbrains.kotlin.psi.KtDotQualifiedExpression.accept(KtDotQualifiedExpression.kt:32)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genQualified(ExpressionCodegen.java:304)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genStatement(ExpressionCodegen.java:409)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.gen(ExpressionCodegen.java:366)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.invokeAppend(ExpressionCodegen.java:4108)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.invokeAppendForEntries(ExpressionCodegen.java:874)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.lambda$visitStringTemplateExpression$6(ExpressionCodegen.java:864)
        at org.jetbrains.kotlin.codegen.OperationStackValue.putSelector(StackValue.kt:79)
        at org.jetbrains.kotlin.codegen.StackValue.put(StackValue.java:122)
        at org.jetbrains.kotlin.codegen.StackValue.put(StackValue.java:115)
        at org.jetbrains.kotlin.codegen.CallGenerator$DefaultCallGenerator.genValueAndPut(CallGenerator.kt:71)
        at org.jetbrains.kotlin.codegen.CallBasedArgumentGenerator.generateExpression(CallBasedArgumentGenerator.kt:42)
        at org.jetbrains.kotlin.codegen.ArgumentGenerator.generate(ArgumentGenerator.kt:68)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.invokeMethodWithArguments(ExpressionCodegen.java:2534)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.invokeMethodWithArguments(ExpressionCodegen.java:2506)
        at org.jetbrains.kotlin.codegen.Callable$invokeMethodWithArguments$1.invoke(Callable.kt:41)
        at org.jetbrains.kotlin.codegen.Callable$invokeMethodWithArguments$1.invoke(Callable.kt:13)
        at org.jetbrains.kotlin.codegen.OperationStackValue.putSelector(StackValue.kt:79)
        at org.jetbrains.kotlin.codegen.StackValueWithLeaveTask.putSelector(StackValue.kt:67)
        at org.jetbrains.kotlin.codegen.StackValue.put(StackValue.java:122)
        at org.jetbrains.kotlin.codegen.StackValue.put(StackValue.java:115)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.putStackValue(ExpressionCodegen.java:386)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.gen(ExpressionCodegen.java:367)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.returnExpression(ExpressionCodegen.java:1731)
        at org.jetbrains.kotlin.codegen.FunctionGenerationStrategy$FunctionDefault.doGenerateBody(FunctionGenerationStrategy.java:64)
        at org.jetbrains.kotlin.codegen.ClosureGenerationStrategy.doGenerateBody(ClosureGenerationStrategy.kt:35)
        at org.jetbrains.kotlin.codegen.FunctionGenerationStrategy$CodegenBased.generateBody(FunctionGenerationStrategy.java:84)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.generateMethodBody(FunctionCodegen.java:654)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.generateMethodBody(FunctionCodegen.java:479)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.generateMethod(FunctionCodegen.java:261)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.generateMethod(FunctionCodegen.java:177)
        at org.jetbrains.kotlin.codegen.ClosureCodegen.generateClosureBody(ClosureCodegen.java:181)
        at org.jetbrains.kotlin.codegen.ClosureCodegen.generateBody(ClosureCodegen.java:169)
        at org.jetbrains.kotlin.codegen.MemberCodegen.generate(MemberCodegen.java:129)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genClosure(ExpressionCodegen.java:1038)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genClosure(ExpressionCodegen.java:1008)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.visitLambdaExpression(ExpressionCodegen.java:999)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.visitLambdaExpression(ExpressionCodegen.java:112)
        at org.jetbrains.kotlin.psi.KtLambdaExpression.accept(KtLambdaExpression.java:40)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genQualified(ExpressionCodegen.java:304)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genQualified(ExpressionCodegen.java:286)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.gen(ExpressionCodegen.java:358)
        at org.jetbrains.kotlin.codegen.CallGenerator$DefaultCallGenerator.genValueAndPut(CallGenerator.kt:70)
        at org.jetbrains.kotlin.codegen.CallBasedArgumentGenerator.generateExpression(CallBasedArgumentGenerator.kt:42)
        at org.jetbrains.kotlin.codegen.ArgumentGenerator.generate(ArgumentGenerator.kt:68)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.invokeMethodWithArguments(ExpressionCodegen.java:2534)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.invokeMethodWithArguments(ExpressionCodegen.java:2506)
        at org.jetbrains.kotlin.codegen.Callable$invokeMethodWithArguments$1.invoke(Callable.kt:41)
        at org.jetbrains.kotlin.codegen.Callable$invokeMethodWithArguments$1.invoke(Callable.kt:13)
        at org.jetbrains.kotlin.codegen.OperationStackValue.putSelector(StackValue.kt:79)
        at org.jetbrains.kotlin.codegen.CoercionValue.putSelector(StackValue.kt:31)
        at org.jetbrains.kotlin.codegen.StackValue.put(StackValue.java:122)
        at org.jetbrains.kotlin.codegen.StackValue.put(StackValue.java:115)
        at org.jetbrains.kotlin.codegen.StackValue$StackValueWithSimpleReceiver.putReceiver(StackValue.java:2089)
        at org.jetbrains.kotlin.codegen.StackValueWithLeaveTask.putReceiver(StackValue.kt:63)
        at org.jetbrains.kotlin.codegen.StackValue.put(StackValue.java:120)
        at org.jetbrains.kotlin.codegen.StackValue.put(StackValue.java:115)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.putStackValue(ExpressionCodegen.java:386)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.gen(ExpressionCodegen.java:367)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.returnExpression(ExpressionCodegen.java:1731)
        at org.jetbrains.kotlin.codegen.FunctionGenerationStrategy$FunctionDefault.doGenerateBody(FunctionGenerationStrategy.java:64)
        at org.jetbrains.kotlin.codegen.FunctionGenerationStrategy$CodegenBased.generateBody(FunctionGenerationStrategy.java:84)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.generateMethodBody(FunctionCodegen.java:654)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.generateMethodBody(FunctionCodegen.java:479)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.generateMethod(FunctionCodegen.java:261)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.generateMethod(FunctionCodegen.java:177)
        at org.jetbrains.kotlin.codegen.FunctionCodegen.gen(FunctionCodegen.java:148)
        at org.jetbrains.kotlin.codegen.MemberCodegen.genSimpleMember(MemberCodegen.java:197)
        at org.jetbrains.kotlin.codegen.PackagePartCodegen.generateBody(PackagePartCodegen.java:95)
        at org.jetbrains.kotlin.codegen.MemberCodegen.generate(MemberCodegen.java:129)
        at org.jetbrains.kotlin.codegen.PackageCodegenImpl.generateFile(PackageCodegenImpl.java:145)
        at org.jetbrains.kotlin.codegen.PackageCodegenImpl.generate(PackageCodegenImpl.java:70)
        at org.jetbrains.kotlin.codegen.DefaultCodegenFactory.generatePackage(CodegenFactory.kt:88)
        at org.jetbrains.kotlin.codegen.DefaultCodegenFactory.generateModule(CodegenFactory.kt:67)
        at org.jetbrains.kotlin.codegen.KotlinCodegenFacade.compileCorrectFiles(KotlinCodegenFacade.java:35)
        at org.jetbrains.kotlin.idea.internal.KotlinBytecodeToolWindow$Companion.compileSingleFile(KotlinBytecodeToolWindow.kt:335)
        at org.jetbrains.kotlin.idea.internal.KotlinBytecodeToolWindow$Companion.getBytecodeForFile(KotlinBytecodeToolWindow.kt:248)
        at org.jetbrains.kotlin.idea.internal.KotlinBytecodeToolWindow$UpdateBytecodeToolWindowTask.processRequest(KotlinBytecodeToolWindow.kt:127)
        at org.jetbrains.kotlin.idea.internal.KotlinBytecodeToolWindow$UpdateBytecodeToolWindowTask.processRequest(KotlinBytecodeToolWindow.kt:74)
        at org.jetbrains.kotlin.idea.util.LongRunningReadTask$1$1.run(LongRunningReadTask.java:115)
        at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:869)
        at org.jetbrains.kotlin.idea.util.LongRunningReadTask.lambda$runWithWriteActionPriority$0(LongRunningReadTask.java:235)
        at com.intellij.openapi.progress.impl.CoreProgressManager.lambda$runProcess$2(CoreProgressManager.java:166)
        at com.intellij.openapi.progress.impl.CoreProgressManager.registerIndicatorAndRun(CoreProgressManager.java:627)
        at com.intellij.openapi.progress.impl.CoreProgressManager.executeProcessUnderProgress(CoreProgressManager.java:572)
        at com.intellij.openapi.progress.impl.ProgressManagerImpl.executeProcessUnderProgress(ProgressManagerImpl.java:61)
        at com.intellij.openapi.progress.impl.CoreProgressManager.runProcess(CoreProgressManager.java:153)
        at org.jetbrains.kotlin.idea.util.LongRunningReadTask.runWithWriteActionPriority(LongRunningReadTask.java:235)
        at org.jetbrains.kotlin.idea.util.LongRunningReadTask$1.run(LongRunningReadTask.java:110)
        at com.intellij.util.RunnableCallable.call(RunnableCallable.java:20)
        at com.intellij.util.RunnableCallable.call(RunnableCallable.java:11)
        at com.intellij.openapi.application.impl.ApplicationImpl$1.call(ApplicationImpl.java:255)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
        at java.base/java.lang.Thread.run(Thread.java:834)
    Caused by: java.lang.AssertionError: Couldn't find descriptor for 'length'
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.visitSimpleNameExpression(ExpressionCodegen.java:1797)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.visitSimpleNameExpression(ExpressionCodegen.java:112)
        at org.jetbrains.kotlin.psi.KtNameReferenceExpression.accept(KtNameReferenceExpression.kt:59)
        at org.jetbrains.kotlin.codegen.ExpressionCodegen.genQualified(ExpressionCodegen.java:304)
        ... 105 more
    

    编译器似乎没有遵循代码优化。因此,即使它应该或不应该编译,也有两种不同的行为,这意味着其中一种是不正确的。

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