Kotlin implement a method from an interface that's already present in super class - kotlin

Question

Let's say I have this interface:
interface Things {
fun size(): Int
}
And I want to subclass a List and implement this interface.
class Cars : ArrayList<String>, Things {}
I get a compilation error:
Inherited platform declarations clash: The following declarations have
the same JVM signature (size()I): fun (): Int defined in
Things fun size(): Int defined in Things
I can get around the compilation error by changing the size contract to a var implicit getter:
interface Things {
var size: Int
}
But then I get a long and complicated error that looks like a runtime error (IllegalStateException) but seems to happen when Kotlin is compiling :shrug:
I suspect I know why this is happening - ArrayList already has int size() and Kotlin now tries to add another of the same signature, but even if I'm right (10% chance?) it doesn't help much.
I'll also mention that it allows me to have get as part of the Things interface, which is satisfied by the list. I'm guessing the operator operator let's this be just different enough to slide?
So anyway, no IDE errors, but when I build I get this:
java.lang.IllegalStateException: Backend Internal error: Exception during code generation
Cause: Concrete fake override public open fun <set-size>(<set-?>: kotlin.Int): kotlin.Unit defined in org.blah.Stuff[PropertySetterDescriptorImpl#673f2280] should have exactly one concrete super-declaration: []
File being compiled at position: file:///blah/Stuff.kt
The root cause was thrown at: bridges.kt:122
at org.jetbrains.kotlin.codegen.CompilationErrorHandler.lambda$static$0(CompilationErrorHandler.java:24)
at org.jetbrains.kotlin.codegen.PackageCodegenImpl.generate(PackageCodegenImpl.java:76)
at org.jetbrains.kotlin.codegen.DefaultCodegenFactory.generatePackage(CodegenFactory.kt:96)
at org.jetbrains.kotlin.codegen.DefaultCodegenFactory.generateModule(CodegenFactory.kt:67)
at org.jetbrains.kotlin.codegen.KotlinCodegenFacade.doGenerateFiles(KotlinCodegenFacade.java:47)
at org.jetbrains.kotlin.codegen.KotlinCodegenFacade.compileCorrectFiles(KotlinCodegenFacade.java:39)
at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.generate(KotlinToJVMBytecodeCompiler.kt:476)
at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules$cli(KotlinToJVMBytecodeCompiler.kt:164)
at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:166)
at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:56)
at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.kt:84)
at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.kt:42)
at org.jetbrains.kotlin.cli.common.CLITool.exec(CLITool.kt:104)
at org.jetbrains.kotlin.incremental.IncrementalJvmCompilerRunner.runCompiler(IncrementalJvmCompilerRunner.kt:349)
at org.jetbrains.kotlin.incremental.IncrementalJvmCompilerRunner.runCompiler(IncrementalJvmCompilerRunner.kt:105)
at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compileIncrementally(IncrementalCompilerRunner.kt:237)
at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.access$compileIncrementally(IncrementalCompilerRunner.kt:37)
at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner$compile$2.invoke(IncrementalCompilerRunner.kt:79)
at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compile(IncrementalCompilerRunner.kt:91)
at org.jetbrains.kotlin.daemon.CompileServiceImpl.execIncrementalCompiler(CompileServiceImpl.kt:579)
at org.jetbrains.kotlin.daemon.CompileServiceImpl.access$execIncrementalCompiler(CompileServiceImpl.kt:102)
at org.jetbrains.kotlin.daemon.CompileServiceImpl$compile$$inlined$ifAlive$lambda$2.invoke(CompileServiceImpl.kt:455)
at org.jetbrains.kotlin.daemon.CompileServiceImpl$compile$$inlined$ifAlive$lambda$2.invoke(CompileServiceImpl.kt:102)
at org.jetbrains.kotlin.daemon.CompileServiceImpl$doCompile$$inlined$ifAlive$lambda$2.invoke(CompileServiceImpl.kt:1005)
at org.jetbrains.kotlin.daemon.CompileServiceImpl$doCompile$$inlined$ifAlive$lambda$2.invoke(CompileServiceImpl.kt:102)
at org.jetbrains.kotlin.daemon.common.DummyProfiler.withMeasure(PerfUtils.kt:138)
at org.jetbrains.kotlin.daemon.CompileServiceImpl.checkedCompile(CompileServiceImpl.kt:1047)
at org.jetbrains.kotlin.daemon.CompileServiceImpl.doCompile(CompileServiceImpl.kt:1004)
at org.jetbrains.kotlin.daemon.CompileServiceImpl.compile(CompileServiceImpl.kt:454)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:346)
at sun.rmi.transport.Transport$1.run(Transport.java:200)
at sun.rmi.transport.Transport$1.run(Transport.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.Transport.serviceCall(Transport.java:196)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:568)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:826)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:683)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:682)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalStateException: Concrete fake override public open fun <set-size>(<set-?>: kotlin.Int): kotlin.Unit defined in org.blah.Stuff[PropertySetterDescriptorImpl#673f2280] should have exactly one concrete super-declaration: []
at org.jetbrains.kotlin.backend.common.bridges.BridgesKt.findConcreteSuperDeclaration(bridges.kt:122)
at org.jetbrains.kotlin.backend.common.bridges.BridgesKt.generateBridges(bridges.kt:59)
at org.jetbrains.kotlin.codegen.JvmBridgesImplKt.generateBridgesForFunctionDescriptorForJvm(JvmBridgesImpl.kt:92)
at org.jetbrains.kotlin.codegen.FunctionCodegen.generateBridges(FunctionCodegen.java:1041)
at org.jetbrains.kotlin.codegen.ClassBodyCodegen.generateBridges(ClassBodyCodegen.java:138)
at org.jetbrains.kotlin.codegen.ClassBodyCodegen.generateBody(ClassBodyCodegen.java:116)
at org.jetbrains.kotlin.codegen.MemberCodegen.generate(MemberCodegen.java:129)
at org.jetbrains.kotlin.codegen.MemberCodegen.genClassOrObject(MemberCodegen.java:302)
at org.jetbrains.kotlin.codegen.MemberCodegen.genClassOrObject(MemberCodegen.java:286)
at org.jetbrains.kotlin.codegen.PackageCodegenImpl.generateClassesAndObjectsInFile(PackageCodegenImpl.java:118)
at org.jetbrains.kotlin.codegen.PackageCodegenImpl.generateFile(PackageCodegenImpl.java:137)
at org.jetbrains.kotlin.codegen.PackageCodegenImpl.generate(PackageCodegenImpl.java:68)
... 44 more
> Task :app:buildInfoGeneratorDebug
Anything I'm missing. I've spent most of my life in Java and am late to the Kotlin party. Any workarounds, alternatives... or is this a known thing?
TYIA

Answer 1

The problem is because kotlin have extension val property for List: size.
The first error says that you have two different kotlin "things" (method and property) which in jvm are the same. And the second error is because you have val implementation in ArrayList and var field in interface so your class Cars needs to implement both setter and getter but implements only getter (from List val size).
The solution is simple: just change your interface property to val size: Int