Kotlin类型推断失败-类型不匹配“找到的数组* ?，必需的数组*?". [英] Kotlin type inference failed - type mismatch "Found Array<*?>, Required Array<*>?"

问题描述

我遇到Kotlin的字型系统问题.我在类范围内声明了以下变量:

var planets: ArrayList<Planet>? = null

在构造函数中，我尝试初始化数组，但是遇到类型不匹配错误:

planets = arrayListOf(earth, mars, saturn, jupiter, uranus, neptune, pluto)

错误:

Required: ArrayList<Planet>?
Found: ArrayList<Planet?>

为什么会出现此错误以及如何解决?

解决方案

至少有一个行星(earth, mars, saturn, jupiter, uranus, neptune, pluto)具有可为空的类型Planet?，因此推断出的arrayListOf(earth, ...)类型为ArrayList<Planet?>.

由于ArrayList<Planet>与类型Planet不一致，因此不能安全地为其分配值ArrayList<Planet?>.

要解决此问题，您可以:

• 确保所有行星的类型都不为空Planet

• 如果上述不可行，请更改

  var planets: ArrayList<Planet>? = null
  

  到

  var planets = arrayListOf<Planet?>()
  

• 过滤出null个行星，然后将结果集合分配给planets:

  planets = arrayListOf(*arrayListOf(earth, ...).filterNotNull().toTypedArray())
  

让编译器满意的另一种方法是使planets contravariant 像这样:

var planets: ArrayList<in Planet>? = null

PS.使用 kotlin集合类型 List<T>，Set<T>和相应的listOf，setOf，而不是Java的对应类型只要有可能.

I am encountering an issue with Kotlin's type system. I declared the variable as below at class scope:

var planets: ArrayList<Planet>? = null

and in the constructor I try to initialize the array but I am encountering a type mismatch error:

planets = arrayListOf(earth, mars, saturn, jupiter, uranus, neptune, pluto)

error:

Required: ArrayList<Planet>?
Found: ArrayList<Planet?>

Why am I getting this error and how do I fix it?

解决方案

At least one of the planets (earth, mars, saturn, jupiter, uranus, neptune, pluto) is of nullable type Planet? hence the inferred type of arrayListOf(earth, ...) is ArrayList<Planet?>.

Since ArrayList<Planet> is not contravariant on type Planet it cannot be safely to assigned with value ArrayList<Planet?>.

To resolve the problem you can:

• make sure all planets are of not nullable type Planet

• if the above is not feasible change

  var planets: ArrayList<Planet>? = null
  

  to

  var planets = arrayListOf<Planet?>()
  

• filter out null planets and then assign the result collections to planets:

  planets = arrayListOf(*arrayListOf(earth, ...).filterNotNull().toTypedArray())
  

Another way to make the compiler happy is to make the planets contravariant like so:

var planets: ArrayList<in Planet>? = null

PS. Use kotlin collection types List<T>, Set<T> and corresponding listOf, setOf instead of Java's counterparts whenever possible.

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