Scala compile error: member of method parameter not visible to class method

Question

I am getting a compile error of:

value txn is not a member of Charge
new Charge(this.txn + that.txn)
                           ^

with the following Scala class definition:

class Charge(txn: Double = 0){
  def combine(that:Charge): Charge = 
    new Charge(this.txn + that.txn)
}

Explicitly declaring txn as a val allows it to work:

class Charge(val txn: Double = 0){
  def combine(that:Charge): Charge = 
    new Charge(this.txn + that.txn)
}

I thought val was assumed? Can somebody explain this? Is it a problem with my understanding of the default constructor or the scope of the method?

Answer 1

In scala, you can define classes in two forms, for ex.

• class Charge(txn: Double) -> In this case scala compiler compiles it to java like below

public class Charge {
  ....
  public Charge combine(Charge);
  ....
  public Charge(double);
  ....
}

As you can notice in compiled java code, there is no public accessor for txn Let's look at another variation of Charge class, If you define like this class Charge(val txn: String), it gets compiled to below

public class Charge {

  ...
  public double txn();
  ...  
  public Charge combine(Charge);
  ...
  public Charge(double);
  ...
}

As you can see, in this case compiler generates public accessor for txn hence you are able to access that.txn when you mark it as val

• case class Charge(txn: Double): This is data class for which scala generates getters, equals and toString for you. You can compile this class

• scalac Charge.scala

• javap -c Charge.class And then see what it generates