blocks|key|210892|text|问题是类型参数被擦除了，所以您无法检查完整的类型映射，因为在运行时没有关于这些字符串和任何字符串的信息。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|210893|要解决此问题，请使用通配符：|210894|if+(value+is+Map<*,+*>)+{...}|code-block|syntax|javascript|210895|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

The problem is that type arguments are erased, so you can't check against the full type Map, because at runtime there's no information about those String and Any.

To work around this, use wildcards:

<pre><code>if (value is Map&lt;*, *&gt;) {...}
</code></pre>

blocks|key|3060633|text|JVM删除泛型类型信息。但是Kotlin将泛型具体化了。如果你有一个泛型类型T，你可以将内联函数的类型参数T标记为实例化，这样它就能够在运行时检查它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3060634|所以你可以这样做：|3060635|inline+fun+<reified+T>+checkType(obj:+Object,+contract:+T)+{
++if+(obj+is+T)+{
++++//+object+implements+the+contract+type+T
++}
}|code-block|syntax|javascript|3060636|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

JVM removes the generic type information. But Kotlin has reified generics. If you have a generic type T, you can mark type parameter T of an inline function as reified so it will be able to check it at runtime.

So you can do:

<pre><code>inline fun &lt;reified T&gt; checkType(obj: Object, contract: T) {
 if (obj is T) {
 // object implements the contract type T
 }
}
</code></pre>

blocks|key|2847777|text|我认为这是更合适的方式|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2847778|inline+fun+<reified+T>+tryCast(instance:+Any?,+block:+T.()+->+Unit)+{
++++if+(instance+is+T)+{
++++++++block(instance)
++++}
}|code-block|syntax|javascript|2847779|用法|2847780|//+myVar+is+nullable
tryCast<MyType>(myVar)+{
++++//+todo+with+this+e.g.
++++this.canDoSomething()
}|2847781|另一种更短的方法|offset|length|style|BOLD|2847782|inline+fun+<reified+T>+Any?.tryCast(block:+T.()+->+Unit)+{
++++if+(this+is+T)+{
++++++++block()
++++}
}|2847783|2847784|//+myVar+is+nullable
myVar.tryCast<MyType>+{
++++//+todo+with+this+e.g.
++++this.canDoSomething()
}|2847785|entityMap^0|0|0|0|0|0|8|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Z|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|10|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|11|8|@$M|12|N|13|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|D|7|14|8|@]|9|@]|A|$E|F]]|$1|S|3|H|5|6|7|15|8|@]|9|@]|A|$]]|$1|T|3|U|5|D|7|16|8|@]|9|@]|A|$E|F]]|$1|V|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|W|$]]

I think this is more appropriate way

<pre><code>inline fun &lt;reified T&gt; tryCast(instance: Any?, block: T.() -&gt; Unit) {
 if (instance is T) {
 block(instance)
 }
}
</code></pre>

Usage

<pre><code>// myVar is nullable
tryCast&lt;MyType&gt;(myVar) {
 // todo with this e.g.
 this.canDoSomething()
}
</code></pre>

Another shorter approach

<pre><code>inline fun &lt;reified T&gt; Any?.tryCast(block: T.() -&gt; Unit) {
 if (this is T) {
 block()
 }
}
</code></pre>

Usage

<pre><code>// myVar is nullable
myVar.tryCast&lt;MyType&gt; {
 // todo with this e.g.
 this.canDoSomething()
}
</code></pre>

blocks|key|3060683|text|我已经用tryCast<Array<String?>>尝试了上面的解决方案，我想，在我列出的具体任务中，由于涉及到许多类型，这不是一个好主意，因为它会极大地降低性能。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3060684|这就是我最终实现的解决方案--手动检查条目并调用方法，如下所示：|3060685|+fun+foo()+{
++++val+map:+Map<String?,+Any?>+=+mapOf()
++++map.forEach+{+entry+->
++++++++when+(entry.value)+{
++++++++++++is+String+->+{
++++++++++++++++doSomeWork(entry.key,+entry.value+as+String)
++++++++++++}
++++++++++++is+Array<*>+->+{
++++++++++++++++doSomeWork(entry.key,+(entry.value+as?+Array<*>)?.map+{
++++++++++++++++++++if+(it+is+String)+{
++++++++++++++++++++++++it
++++++++++++++++++++}+else+null
++++++++++++++++}?.toList())
++++++++++++}
++++++++}
++++}
}


private+fun+doSomeWork(key:+String?,+value:+String)+{

}
private+fun+doSomeWork(key:+String?,+values:+List<String?>?)+{

}|code-block|syntax|javascript|3060686|entityMap^0|4|N|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|R|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|S|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

I have tried the solution above with <code>tryCast&lt;Array&lt;String?&gt;&gt;</code> and, I guess, in my specific task in listing with many castings involved it was no so great idea, because it was slowing the performance drastically.
This is the solution I did finally - manually check the entries and call methods, like this:
<pre><code> fun foo() {
 val map: Map&lt;String?, Any?&gt; = mapOf()
 map.forEach { entry -&gt;
 when (entry.value) {
 is String -&gt; {
 doSomeWork(entry.key, entry.value as String)
 }
 is Array&lt;*&gt; -&gt; {
 doSomeWork(entry.key, (entry.value as? Array&lt;*&gt;)?.map {
 if (it is String) {
 it
 } else null
 }?.toList())
 }
 }
 }
}


private fun doSomeWork(key: String?, value: String) {

}
private fun doSomeWork(key: String?, values: List&lt;String?&gt;?) {

}
</code></pre>

blocks|key|2847898|text|我将给出一个变通的解决方案，但我认为它是干净的，有点|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2847899|try{
++(value+as+Map<String,Any>?)?.let+{+castedValue+->
+++++doYourStuffHere()+//using+castedValue
++}
}catch(e:+Exception){
++valueIsNotOfType()+//Map<String,Any>
}|code-block|syntax|javascript|2847900|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I'm gonna give a workaround solution but I think its clean, kind of
<pre><code>try{
 (value as Map&lt;String,Any&gt;?)?.let { castedValue -&gt;
 doYourStuffHere() //using castedValue
 }
}catch(e: Exception){
 valueIsNotOfType() //Map&lt;String,Any&gt;
}
</code></pre>

I'm trying to test for a generic type in Kotlin.

<pre><code>if (value is Map&lt;String, Any&gt;) { ... }
</code></pre>

But the compiler complains with

<blockquote>
 Cannot check for instance of erased type: jet.Map
</blockquote>

The check with a normal type works well.

<pre><code>if (value is String) { ... }
</code></pre>

Kotlin 0.4.68 is used.

What am I missing here?

How can I check for generic type in Kotlin

编译器

Kotlin

我正在尝试测试Kotlin中的泛型类型。if (value is Map<String, Any>) { ... }但是编译器抱怨说无法检查已擦除类型的实例: jet.Map 使用普通类型的检查效果很好。if (value is String) { ... }使用Kotlin 0.4.68。这里我漏掉了什么？

问如何在Kotlin中检查泛型类型
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何在Kotlin中检查泛型类型EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问如何在Kotlin中检查泛型类型
EN