Packages

A java package is a group of similar types of classes, interfaces and sub-packages. Package in java can be categorized in two form, built-in package and user-defined package. There are many built-in packages such as java, lang, awt, javax, swing, net, io, util, sql etc. Here, we will have the detailed learning of creating and using user-defined packages.

Advantage of Java Package:

Java package is used to categorize the classes and interfaces so that they can be easily maintained.
Java package provides access protection.
Java package removes naming collision.

The package keyword is used to create a package in java.

$\begin{lstlisting} //save as Simple.java package mypack; public class Simple{ ... ... args[]){ System.out.println(''Welcome to package''); } } \end{lstlisting}$

If you are not using any IDE, you need to follow the syntax given below:

$\begin{lstlisting} javac -d directory javafilename \end{lstlisting}$

For example

$\begin{lstlisting} javac -d . Simple.java \end{lstlisting}$

The -d switch specifies the destination where to put the generated class file. You can use any directory name like /home (in case of Linux), d:/abc (in case of windows) etc. If you want to keep the package within the same directory, you can use . (dot). You need to use fully qualified name e.g. mypack.Simple etc to run the class.

To Compile: javac -d . Simple.java
To Run: java mypack.Simple

This will make Welcome to package output. The -d is a switch that tells the compiler where to put the class file i.e. it represents destination. The . represents the current folder. There are three ways to access the package from outside the package.

import package.*;
import package.classname;
fully qualified name.

If you use package.* then all the classes and interfaces of this package will be accessible but not subpackages. The import keyword is used to make the classes and interface of another package accessible to the current package. The following code snippet serves as an example of package that import the packagename.*

$\begin{lstlisting} //save by A.java package pack; public class A{ public voi... ...id main(String args[]){ A obj = new A(); obj.msg(); } } \end{lstlisting}$

The estimated output will be Hello. If you import package.classname then only declared class of this package will be accessible.

Example:

$\begin{lstlisting} //save by A.java \par package pack; public class A{ publi... ...id main(String args[]){ A obj = new A(); obj.msg(); } } \end{lstlisting}$

Output

$\begin{lstlisting} Hello \end{lstlisting}$

If you use fully qualified name then only declared class of this package will be accessible. Now there is no need to import. But you need to use fully qualified name every time when you are accessing the class or interface. It is generally used when two packages have same class name e.g. java.util and java.sql packages contain Date class.

Example:

$\begin{lstlisting} //save by A.java package pack; public class A{ public voi... ... new pack.A();//using fully qualified name obj.msg(); } } \end{lstlisting}$
Output

$\begin{lstlisting} Hello \end{lstlisting}$

If you import a package, all the classes and interface of that package will be imported excluding the classes and interfaces of the subpackages. Hence, you need to import the subpackage as well.

Subsections

Subpackage in java
- Example of Subpackage