Language Fundamentals - Package Declarations

Syntax

    package packageName;

• packages provide a naming context and an organizational structure for Java compilation units
• package names are hierarchical with component names separated by dots (JPL pg 25)
• the standard convention for package naming is to use the reversed internet domain name of whoever's creating the package. For example:

        com.sun.java.awt        // Sun packages
        com.ibm.utils           // IBM packages
        com.acme.tools          // Acme company packages

• the package naming structure directly maps to a directory structure. For example, if you were developing the Acme company packages your compilation units for the com.acme.tools package would be in:

    directory.............. com
      subdirectory...........acme
        subdirectory..........tools

• the Java compiler uses a combination of the CLASSPATH and package name to locate the source file
• host systems may store packages in databases (JLS §7.2.2)
• if used, it must be the first statement in the source code file
• you can not declare more than one
• the package naming structure is for ease of organization only, it does not confer a special relationship (JLS §7.1) ie

    There is no special relationship between the packages:
    
            com.acme.tools,         and
            com.acme.utils
            
    The fact that they share a common subpackage, acme,
    has no meaning in terms of a types scope.

Unnamed packages (JLS §7.4.2)

• if no package declaration is found, the class or interface is made part of an unnamed package
• every implementation of Java must provide for at least one unnamed package
• most systems allow for one unnamed package per directory