Packages

Package

• A package is a group of similar types of classes, interfaces and sub-packages.
• It helps Organize your classes into a folder structure and make it easy to locate and use them.More importantly,it helps improve re-usability.
• To avoid naming conflicts packages are given names of the domain name of the company in reverse Ex :  com.guru99, com.microsoft, com.infosys etc.
• When a package name is not specified , a class is into the default package (the current working directory) and the package itself is given no name.
• While creating a package, care should be taken that the statement for creating  package must be written before any other import statements.

Package 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.

Advantage of Package

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

package in java

Simple example of package

The package keyword is used to create a package.

    //save as Simple.java  

      

    package mypack;  

    public class Simple{  

     public static void main(String args[]){  

        System.out.println("Welcome to package");  

       }  

    }  

How to compile the Package (if not using IDE)

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

    javac -d directory javafilename  

For example

    javac -d . Simple.java  

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).

How to run the Package (if not using IDE)

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

Output:Welcome to package

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.

How to access package from another package?

There are three ways to access the package from outside the package.

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

Using packagename.*

If you use package.* then all the classes and interfaces of this package will be accessible but not sub-packages.

The import keyword is used to make the classes and interface of another package accessible to the current package.

Example of package that import the packagename.*

    //save by A.java (java class one) 

      

    package pack;  

    public class A{  

      public void msg(){System.out.println("Hello");}  

    }  

    //save by B.java (java class two) 

      

    package mypack;  

    import pack.*;  

      

    class B{  

      public static void main(String args[]){  

       A obj = new A();  

       obj.msg();  

      }  

    }  

Output:Hello

Using packagename.classname

If you import package.classname then only declared class of this package will be accessible.

Example of package by import package.classname

    //save by A.java  (java class one) 

      

    package pack;  

    public class A{  

      public void msg(){System.out.println("Hello");}  

    }  

    //save by B.java  (java class two) 

      

    package mypack;  

    import pack.A;  

      

    class B{  

      public static void main(String args[]){  

       A obj = new A();  

       obj.msg();  

      }  

    }  

Output:Hello

Using fully qualified name

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 of package by import fully qualified name

    //save by A.java  (java class one) 

      

    package pack;  

    public class A{  

      public void msg(){System.out.println("Hello");}  

    }  

    //save by B.java  (java class two) 

      

    package mypack;  

    class B{  

      public static void main(String args[]){  

       pack.A obj = new pack.A();//using fully qualified name  

       obj.msg();  

      }  

    }  

Output:Hello

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

How to send the class file to another directory or drive?

This can be done using classpath. 

Classpath is a parameter—set either on the command-line, or through an environment variable—that tells the Java Virtual Machine or the Java compiler where to look for user-defined classes and packages.

For example:

how to put class file in another package

    //save as Simple.java  

      

    package mypack;  

    public class Simple{  

     public static void main(String args[]){  

        System.out.println("Welcome to package");  

       }  

    }  

To Compile:

e:\sources> javac -d c:\classes Simple.java

To Run:

To run this program from e:\source directory, you need to set classpath of the directory where the class file resides.

e:\sources> set classpath=c:\classes;.;

e:\sources> java mypack.Simple

Another way to run this program by -classpath switch of java:

The -classpath switch can be used with javac and java tool.

To run this program from e:\source directory, you can use -classpath switch of java that tells where to look for class file. For example:

e:\sources> java -classpath c:\classes mypack.Simple

Output:Welcome to package