Why is this reference ambiguous?

问题:

import swing._

object PeerTest extends SimpleSwingApplication {
  def top = new MainFrame {
    val p = peer.getMousePosition 
  }
}

gives

error: ambiguous reference to overloaded definition,
both method getMousePosition in class Container of type (x$1: Boolean)java.awt.Point
and  method getMousePosition in class Component of type ()java.awt.Point
match expected type ?
val p = peer.getMousePosition

but adding the type

val p: Point = peer.getMousePosition 

makes it ok. Why?

edit: causes problem:

class A {
  def value() = 123
}

class B extends A {
  def value(b: Boolean) = 42  
}

object Main extends App {
  println ((new B).value) 
}

doesn't cause problem:

class A {
  def value() = 123
  def value(b: Boolean) = 42  
}

class B extends A {}

object Main extends App {
  println ((new B).value) 
}

So I think the answer has to explain why it only occurs when the methods are in different classes.

回答1:

There are two methods getMousePosition one without and one with a boolean parameter.

Without a type annotation Scala does not know if you want a reference to the method in one parameter (a Function1 object) or if you want to invoke the one without parameters (resulting in a Point).

Specifying the expected type clarifies your intend.

Using getMousePosition() should work as well.

回答2:

A more direct way to refer to the desired overloaded alternative is by including the empty argument list.

peer.getMousePosition()