Scala Cookbook

4.7. Preventing Getter and Setter Methods from Being Generated

Problem

When you define a class field as a var, Scala automatically generates getter and setter methods for the field, and defining a field as a val automatically generates a getter method, but you don’t want either a getter or setter.

Solution

Define the field with the private or private[this] access modifiers, as shown with the currentPrice field in this example:

class Stock {

  // getter and setter methods are generated
  var delayedPrice: Double = _

  // keep this field hidden from other classes
  private var currentPrice: Double = _
}

When you compile this class with scalac, and then disassemble it with javap, you’ll see this interface:

// Compiled from "Stock.scala"
public class Stock extends java.lang.Object implements scala.ScalaObject{
  public double delayedPrice();
  public void delayedPrice_$eq(double);
  public Stock();
}

This shows that getter and setter methods are defined for the delayedPrice field, and there are no getter or setter methods for the currentPrice field, as desired.

Discussion

Defining a field as private limits the field so it’s only available to instances of the same class, in this case instances of the Stock class. To be clear, any instance of a Stock class can access a private field of any other Stock instance.

As an example, the following code yields true when the Driver object is run, because the isHigher method in the Stock class can access the price field both (a) in its object, and (b) in the other Stock ...