CODEWITHSUNDEEP
kotlinkotlin-reflect
ericn
ericn

Reputation: 13113

Kotlin Class should have a single no-arg constructor

I instantiate my object using:

val currency = Currency::class.createInstance()
            .copy(code = "GBP")

but I'm getting an exception at that line:

java.lang.IllegalArgumentException: Class should have a single no-arg constructor: class com.abc.Currency
    at kotlin.reflect.full.KClasses.createInstance(KClasses.kt:281)
    at com.abc.MyInteractorTest.setUp(MyInteractorTest.kt:55)
    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 org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.mockito.internal.runners.DefaultInternalRunner$1.run(DefaultInternalRunner.java:79)
    at org.mockito.internal.runners.DefaultInternalRunner.run(DefaultInternalRunner.java:85)
    at org.mockito.internal.runners.StrictRunner.run(StrictRunner.java:39)
    at org.mockito.junit.MockitoJUnitRunner.run(MockitoJUnitRunner.java:163)
    at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.runTestClass(JUnitTestClassExecutor.java:106)
    at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:58)
    at org.gradle.api.internal.tasks.testing.junit.JUnitTestClassExecutor.execute(JUnitTestClassExecutor.java:38)
    at org.gradle.api.internal.tasks.testing.junit.AbstractJUnitTestClassProcessor.processTestClass(AbstractJUnitTestClassProcessor.java:66)
    at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.processTestClass(SuiteTestClassProcessor.java:51)
    at sun.reflect.GeneratedMethodAccessor35.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:35)
    at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
    at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:32)
    at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:93)
    at com.sun.proxy.$Proxy2.processTestClass(Unknown Source)
    at org.gradle.api.internal.tasks.testing.worker.TestWorker.processTestClass(TestWorker.java:117)
    at sun.reflect.GeneratedMethodAccessor34.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:35)
    at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
    at org.gradle.internal.remote.internal.hub.MessageHubBackedObjectConnection$DispatchWrapper.dispatch(MessageHubBackedObjectConnection.java:155)
    at org.gradle.internal.remote.internal.hub.MessageHubBackedObjectConnection$DispatchWrapper.dispatch(MessageHubBackedObjectConnection.java:137)
    at org.gradle.internal.remote.internal.hub.MessageHub$Handler.run(MessageHub.java:404)
    at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:63)
    at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:46)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:55)
    at java.lang.Thread.run(Thread.java:748)

Upvotes: 3

Views: 12998

Answers (3)

ericn
ericn

Reputation: 13113

Sorry for the confusion and thanks for the help.
Indeed, it's obvious that KClasses.createInstance() requires classes to have a "no-arg constructor".

So 1 solution is to change my classes to introduce a "no-arg constructor".

Another solution is to use my own createInstance() which accepts classes without "no-arg constructor". Turned out my team already have our own createInstance() and I was using the wrong method (KClasses.createInstance())

Upvotes: 1

Roland
Roland

Reputation: 23312

Either ensure that your data class initializes all values (therefore a default constructor will be available) or supply an appropriate default constructor yourself:

// setting default values on all properties:
data class Currency(val code : String = "GBP" / * all other properties need to have a default value assigned too */)

// or add a default constructor:
data class Currency(val code : String /* other properties */) {
  constructor() : this("GBP" /* other properties default values */)
}

Note that createInstance uses singleOrNull internally and throws an exception if there exists more than 1 (or none) constructor meeting the criteria of having just optional parameters. So if you have several such constructors or you do not want to change the Currency-class, then iterate over the available constructors, use the one that fits and fill up the parameters instead. You can then probably even skip that copy-call.

Why do you even use this approach to copy the data? copy only makes sense if there is already an object where you only want to change some of the properties but otherwise keep the rest as is and your sample could probably even be written as just: 

val payInCurrency = Currency(code = "GBP")

You already know that you want a Currency and you also know that you want the code to be "GBP".

Upvotes: 6

Geoffrey Wiseman
Geoffrey Wiseman

Reputation: 5637

You're calling createInstance(), and the documentation for that method says:

Creates a new instance of the class, calling a constructor which either has no parameters or all parameters of which are optional (see KParameter.isOptional). If there are no or many such constructors, an exception is thrown.

You're calling a createInstance on class and it's throwing the exception specified, the class either:

  • has no constructor with no parameters (or where all parameters are optional)
  • has multiple constructors meeting that definition

Upvotes: 4

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