Packages and Permissions

Packages

• A package is a collection of classes (types) and, possibly, subpackages.
  java is a package that contains the subpackage java.lang.
  java.lang contains about 50 classes and several subpackages.
• A package hierarchy corresponds to a file system hierarchy on disk. In Windows, substitute '\' for '.' in the package name.
• You can name the package that the classes in a compilation unit belong to using the package statement at the beginning of the file.
• Use import statements to name classes whose members you want to reference without having to specify the fully-qualified member name.
• All the compilation units in the same directory are part of the same unnamed package unless you use a package statement in some (or all) of them.

Permissions

• A public class or member can be referenced outside its package.
• A protected class or member can be referenced only inside its own class or from within a subclass.
• A private class or member can be referenced only from inside the class that it is defined in.