使用scala调用java.util.Hashtable#put
我在调用老式哈希表时遇到了意外的麻烦。这是怎么回事？
Welcome to Scala version 2.8.0.final (Java HotSpot(TM) Client VM, Java 1.6.0_21).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import com.ibm.mq._
import com.ibm.mq._
scala> MQEnvironment.properties
res1: java.util.Hashtable[_, _] = {}
scala> res1.put("transport", "MQSeries")
<console>:10: error: type mismatch;
found : java.lang.String("transport")
required: ?0 where type ?0
res1.put("transport", "MQSeries")
^
PS，问题仍然存在，但我有一个令人讨厌的解决方法：
scala> new java.util.Hashtable[String, String]
res10: java.util.Hashtable[String,String] = {}
scala> res10.put("transport", "MQSeries")
res11: String = null
scala> MQEnvironment.properties = res10
scala> MQEnvironment.properties
res13: java.util.Hashtable[_, _] = {transport=MQSeries}
I've an unexpected trouble calling put on an old-school hashtable. What's going on here?
Welcome to Scala version 2.8.0.final (Java HotSpot(TM) Client VM, Java 1.6.0_21).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import com.ibm.mq._
import com.ibm.mq._
scala> MQEnvironment.properties
res1: java.util.Hashtable[_, _] = {}
scala> res1.put("transport", "MQSeries")
<console>:10: error: type mismatch;
found : java.lang.String("transport")
required: ?0 where type ?0
res1.put("transport", "MQSeries")
^
PS, the question still stands as is, but I have a nasty workaround:
scala> new java.util.Hashtable[String, String]
res10: java.util.Hashtable[String,String] = {}
scala> res10.put("transport", "MQSeries")
res11: String = null
scala> MQEnvironment.properties = res10
scala> MQEnvironment.properties
res13: java.util.Hashtable[_, _] = {transport=MQSeries}
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properties接口似乎是早于 Java 泛型的老式 API 之一。java.util.HashTable[_, _]中的下划线是存在类型的简写，其中第一个（键类型）对应于出现在诊断。这些旧的 Java“原始”类型是 Scala 的 Java 互操作性中的一个不幸的、可见的接缝，尽管这种接缝通常只出现在非常旧的 API 中。That
propertiesinterface appears to be one of those old-school APIs that pre-date Java generics. Those underscores injava.util.HashTable[_, _]are shorthands for existential types, the first of which (the key type) corresponds to the?0appearing in the diagnostic. These old Java "raw" types are an unfortunate, visible seam in Scala's Java interoperability, albeit one that usually shows up only in very old APIs.