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4.2.8. Public, Package-Scope, and Private Methods

A method can be public, package-scope, or private.

Any object can send a message that runs a public method.

A package-scope method can only be invoked from methods or routines defined in the same package as the package-scope method.

A private method can only be invoked from specific calling contexts. These contexts are:

  1. From within a method owned by the same class as the target. This is frequently the same object, accessed via the special variable SELF. Private methods of an object can also be accessed from other instances of the same class (or subclass instances).

  2. From within a method defined at the same class scope as the method. For example:
    Example 4.10. Referencing methods
    ::class Savings
::method newCheckingAccount CLASS
  instance = self~new
  instance~makeChecking
  return instance

::method makeChecking private
  expose checking
  checking = .true

    The newCheckingAccount CLASS method is able to invoke the makeChecking method because the scope of the makeChecking method is .Savings.

  3. From within an instance (or subclass instance) of a class to a private class method of its class. For example:
    Example 4.11. Referencing methods
    ::class Savings
::method init class
  expose counter
  counter = 0

::method allocateAccountNumber private class
  expose counter
  counter = counter + 1
  return counter

::method init
  expose accountNumber
  accountNumber = self~class~allocateAccountNumber

    The instance init method of the Savings class is able to invoke the allocateAccountNumber private method of the .Savings class object because it is owned by an instance of the .Savings class.

Private methods include methods at different scopes within the same object. This allows superclasses to make methods available to their subclasses while hiding those methods from other objects. A private method is like an internal subroutine. It shields the internal information of an object to outsiders, but allowing objects to share information with each other and their defining classes.