如何从Scala中的泛型函数返回null? [英] How to return null from a generic function in Scala?
问题描述
我写自己的简单的 javax.sql.DataSource
实现,我需要的唯一方法是 getConnection:Connection code>,但接口从
javax.sql.CommonDataSource
和 java.sql中继承了许多其他方法(我不需要) .Wrapper
。所以,我想实施那些不需要的方法,这样他们就不会真正起作用,但在被调用时会表现出合适的方式。例如,我将 boolean isWrapperFor(Class <?> iface)
作为
def isWrapperFor(iface:Class [_]):Boolean = false
我想实现< T> T解包(Class< T> iface)
为
def unwrap [T](iface:Class [T]):T = null
但最后不起作用:编译器报告类型不匹配。
使用 null.asInstanceOf [T]
还是有更好的方法吗?当然,我认为只是抛出 UnsupportedOperationException
,而不是在这个特定的情况下,但恕我直言,这个问题仍然可以是有趣的。
这是因为 T
可能是一个不可为空的类型。当你强制执行 T
为可空类型时:
def unwrap [T>:Null](iface:Class [T]):T = null
unwrap(classOf [String])//编译
unwrap(classOf [ Int])//不能编译,因为Int不能为空
I am writing my own simple javax.sql.DataSource
implementation, the only method of it I need to work is getConnection: Connection
, but the interface inherits many other methods (which I don't need) from javax.sql.CommonDataSource
and java.sql.Wrapper
. So, I would like to "implement" those unneeded methods a way they wouldn't actually work but would behave an adequate way when called. For example I implement boolean isWrapperFor(Class<?> iface)
as
def isWrapperFor(iface: Class[_]): Boolean = false
and I'd like to implement <T> T unwrap(Class<T> iface)
as
def unwrap[T](iface: Class[T]): T = null
But the last doesn't work: the compiler reports type mismatch.
Will it be correct to use null.asInstanceOf[T]
or is there a better way? Of course I consider just throwing UnsupportedOperationException
instead in this particular case, but IMHO the question can still be interesting.
This is because T
could be a non-nullable type. It works when you enforce T
to be a nullable type:
def unwrap[T >: Null](iface: Class[T]): T = null
unwrap(classOf[String]) // compiles
unwrap(classOf[Int]) // does not compile, because Int is not nullable
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