Question

Why does scala complains for below code?

scala> class Http(var status: Int)
defined class Http

scala> case class Post(url: String, data: String, status: Int) extends Http(status)
<console>:8: error: overriding variable status in class Http of type Int;
 value status needs `override' modifier
       case class Post(url: String, data: String, status: Int) extends Http(status)
                                                  ^

scala> case class Post(url: String, data: String, sta: Int) extends Http(sta)
defined class Post

But this runs fine.

scala> class C(boo:Int)
defined class C

scala> case class D(far:Int, boo:Int) extends C(boo)
defined class D

Answer 1

var status: Int will create accessor methods, something like

def status() = this.status
def status(status: Int) {this.status = status}

case class creates these methods by default, so you have two methods with same signature, and scala requires you to add override keyword when overriding methods.

without var you only create class fields and they are not overridden.