5.1. Controlling Method Scope

Problem

Scala methods are public by default, and you want to control their scope in ways similar to Java.

Solution

Scala lets you control method visibility in a more granular and powerful way than Java. In order from “most restrictive” to “most open,” Scala provides these scope options:

  • Object-private scope

  • Private

  • Package

  • Package-specific

  • Public

These scopes are demonstrated in the examples that follow.

Object-private scope

The most restrictive access is to mark a method as object-private. When you do this, the method is available only to the current instance of the current object. Other instances of the same class cannot access the method.

You mark a method as object-private by placing the access modifier private[this] before the method declaration:

  private[this] def isFoo = true

In the following example, the method doFoo takes an instance of a Foo object, but because the isFoo method is declared as an object-private method, the code won’t compile:

class Foo {

  private[this] def isFoo = true

  def doFoo(other: Foo) {
    if (other.isFoo) {  // this line won't compile
      // ...
    }
  }

}

The code won’t compile because the current Foo instance can’t access the isFoo method of the other instance, because isFoo is declared as private[this]. As you can see, the object-private scope is extremely restrictive.

Private scope

A slightly less restrictive access is to mark a method private, which makes the method available to (a) the current class and (b) other instances of the current class. ...

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