Access Control

Java has four levels of controlling access to fields, methods, and classes:

• public access

Can be accessed by methods of all classes

• private access

Can be accessed only by the methods of their own class

• protected access

Protected features can be accessed by all subclasses and all classes in the same package.

• package access

  • The default, when no access modifier is given

  • Can be accessed by all classes in the same package

  • Good default for classes, but extremely unfortunate for fields

Recommended Access Levels

• Instance and static fields: Always private. Exceptions:

  • public static final constants are useful and safe

  • Some objects, such as System.out, need to be accessible to all programs (public)

  • Occasionally, classes in a package must collaborate very closely (give some fields package access); inner classes are usually better

• Methods: public or private

• Classes and interfaces: public or package

  • Better alternative to package access: inner classes

    • In general, inner classes should not be public (some exceptions exist, e.g., Ellipse2D.Double)

• Beware of accidental package access (forgetting public or private)

What is a common reason for defining package-visible instance fields?

   Answer: Accidentally forgetting the private modifier.

If a class with a public constructor has package access, who can construct objects of it?

   Answer: Any methods of classes in the same package.