What does -= operator mean in scala def context

Question

I discovered the below odd def definition syntax in Kafka source code . Can somebody help me understand this syntax as I am unable to find in scala documents. The first def definition works but the second one does not work

  private def test_=(x: Int) {

  }

  private def test=(x: Int) {

  }

Answer 1

It isn't an operator. There is just a special rule that if you have a method with a name ending in _= and a single argument, and a no-argument method with the same name without _= (in this case def test = ... ), then Scala will understand foo.test = ... as the foo.test_=(...) call.