Packages

Packages are used in Java in order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier, etc.

A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.

Some of the existing packages in Java are::

• java.lang - bundles the fundamental classes
• java.io - classes for input , output functions are bundled in this package

Programmers can define their own packages to bundle group of classes/interfaces, etc. It is a good practice to group related classes implemented by you so that a programmer can easily determine that the classes, interfaces, enumerations, annotations are related.

Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classes.

Creating a package:

When creating a package, you should choose a name for the package and put a package statement with that name at the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package.

The package statement should be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.

If a package statement is not used then the class, interfaces, enumerations, and annotation types will be put into an unnamed package.

Example:

Let us look at an example that creates a package called animals. It is common practice to use lowercased names of packages to avoid any conflicts with the names of classes, interfaces.

Put an interface in the package animals:

/* File name : Animal.java */package animals;interface Animal {
   public void eat();
   public void travel();}

The Directory Structure of Packages:

Two major results occur when a class is placed in a package:

• The name of the package becomes a part of the name of the class, as we just discussed in the previous section.
• The name of the package must match the directory structure where the corresponding bytecode resides.

Here is simple way of managing your files in Java:

Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java. For example:

// File Name :  Car.javapackage vehicle;public class Car {
   // Class implementation.   }

Now, put the source file in a directory whose name reflects the name of the package to which the class belongs:

....\vehicle\Car.java

Now, the qualified class name and pathname would be as below:

• Class name -> vehicle.Car
• Path name -> vehicle\Car.java (in windows)