Scala中为何可无参调用接收Any参数的方法？且为何返回true？

Hey folks, let's unpack your two Scala questions clearly—they're linked by a neat little language quirk that trips up a lot of new Scala devs!

问题1：为何调用接收Any类型参数的方法时不传参是合法的？ #

This boils down to a Scala language feature tied to method calls and the Unit type:

• Your method hello takes a single parameter of type Any.
• When you call it with empty parentheses (hello()), Scala doesn't throw an error—it automatically passes the only instance of the Unit type: ().
• Since Any is the root of Scala's entire type hierarchy, every type (including Unit, which is an AnyVal) extends Any. That means () is a valid value for an Any parameter, so the call is completely legal.

问题2：为什么无参调用hello方法返回true？ #

Let's walk through the code step by step with the actual parameter being passed:

class Bla { def hello(x: Any): Boolean = x.toString.length == 2 }
// When you call new Bla().hello(), you're really doing this under the hood:
new Bla().hello(())

Now, what's the result of ().toString? If you test this in the Scala REPL, you'll see it returns the string "()"—which has a length of exactly 2.

Your method checks if the length of x.toString equals 2, so this condition evaluates to true—hence the output you see.

内容的提问来源于stack exchange，提问作者Jagat