4.7 Private Fields and Methods🔗ℹ

Sometimes, a class needs extra fields in its objects that are not visible outside the class’s implementation. The class form supports two ways of declaring private fields. One way is to use private field in the class body:

Another way is to use the private modifier for a field listed in parentheses after the class name. Also including a default-value expression with = avoids the need for a custom constructor, since it is automatically omitted from the default constructor.

Both approaches work in this example, because the field is mutable. If a private field is immutable, then it needs to be written with other fields in parentheses, because a field declaration in a class body always creates a mutable field. If the initial value of a private depends on values supplied when an object is created, then a custom constructor may be needed. When a private field is declared within parentheses after a class name, then the underlying constructor accessed with super—as used in a custom constructor—accepts values for private fields as well as public ones.

Private fields are visible only within methods of the same class when accessed directly or through the class’s annotation, within a constructor through the class’s annotation, or through an internal name’s static annotation. For example, declaring an internal name _Car makes a priviate gas field accessible outside the class’s implementation:

Methods can be private too, following essentially the same rules as private fields. Private methods can be useful as helpers within a class’s implementation, or via an internal name that is selectively exported, they can be used to limit access to some methods.