private final member missing

[ghost]

Assuming you compiled the following Scala code like (on a Mac with Scala 2.12.1 and Java 1.8.0_112):

$ echo > Dog.scala <<EOF
class Dog(name:String, age:Int) {
  println("Dog is created!")
  def sayHello() = println(s"My name is $name, I am $age years old")
}
EOF
$ scalac Dog.scala

Then a Dog.class file is created. Decompiling it with the JD-GUI version 1.4.0 brings the following output:

import scala.Predef.;
import scala.StringContext;
import scala.reflect.ScalaSignature;
import scala.runtime.BoxesRunTime;

@ScalaSignature(bytes="\006\001-2A!\001\002\001\013\t\031Ai\\4\013\003\r\tq\001P3naRLhh\001\001\024\005\0011\001CA\004\013\033\005A!\"A\005\002\013M\034\027\r\\1\n\005-A!AB!osJ+g\r\003\005\016\001\t\005\t\025!\003\017\003\021q\027-\\3\021\005=1bB\001\t\025!\t\t\002\"D\001\023\025\t\031B!\001\004=e>|GOP\005\003+!\ta\001\025:fI\0264\027BA\f\031\005\031\031FO]5oO*\021Q\003\003\005\t5\001\021\t\021)A\0057\005\031\021mZ3\021\005\035a\022BA\017\t\005\rIe\016\036\005\006?\001!\t\001I\001\007y%t\027\016\036 \025\007\005\032C\005\005\002#\0015\t!\001C\003\016=\001\007a\002C\003\033=\001\0071\004C\003'\001\021\005q%\001\005tCfDU\r\0347p)\005A\003CA\004*\023\tQ\003B\001\003V]&$\b")
public class Dog
{
  private final String name;
  
  public Dog(String name, int age)
  {
    Predef..MODULE$.println("Dog is created!");
  }
  
  public void sayHello()
  {
    Predef..MODULE$.println(new StringContext(Predef..MODULE$.wrapRefArray((Object[])new String[] { "My name is ", ", I am ", " years old" })).s(Predef..MODULE$.genericWrapArray(new Object[] { this.name, BoxesRunTime.boxToInteger(this.age) })));
  }
}

There is clearly a private final int age field missing. You can validate that easily by using javap like:

$javap -private Dog.class
Compiled from "Dog.scala"
public class Dog {
  private final java.lang.String name;
  private final int age;
  public void sayHello();
  public Dog(java.lang.String, int);
}

See also: http://stackoverflow.com/questions/41561061/scala-primary-constructor-does-not-generate-field