Packages – Creating Packages, Accessing package, writing package

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

1) Java package is used to categorize the classes and interfaces so that they can be easily maintained.

2) Java package provides access protection.

3) Java package removes naming collision.

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Simple example of java package

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

1. //save as Simple.java  
2. package mypack;  
3. public class Simple{  
4.  public static void main(String args[]){  
5.     System.out.println("Welcome to package");  
6.    }  
7. }  

How to compile java package

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

1. javac -d directory javafilename  

For example

1. 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).

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How to run java package program

You need to use fully qualified name e.g. mypack.Simple etc to run the class.

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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.

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How to access package from another package?

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

1. import package.*;
2. import package.classname;
3. fully qualified name.

1) Using packagename.*

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.

Example of package that import the packagename.*

1. //save by A.java  
2. package pack;  
3. public class A{  
4.   public void msg(){System.out.println("Hello");}  
5. }  

1. //save by B.java  
2. package mypack;  
3. import pack.*;  
4.   
5. class B{  
6.   public static void main(String args[]){  
7.    A obj = new A();  
8.    obj.msg();  
9.   }  
10. }  

Output:Hello

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2) 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

1. //save by A.java  
2.   
3. package pack;  
4. public class A{  
5.   public void msg(){System.out.println("Hello");}  
6. }  

1. //save by B.java  
2. package mypack;  
3. import pack.A;  
4.   
5. class B{  
6.   public static void main(String args[]){  
7.    A obj = new A();  
8.    obj.msg();  
9.   }  
10. }  

Output:Hello

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3) 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

1. //save by A.java  
2. package pack;  
3. public class A{  
4.   public void msg(){System.out.println("Hello");}  
5. }  

1. //save by B.java  
2. package mypack;  
3. class B{  
4.   public static void main(String args[]){  
5.    pack.A obj = new pack.A();//using fully qualified name  
6.    obj.msg();  
7.   }  
8. }  

Output:Hello

Rule: There can be only one public class in a java source file and it must be saved by the public class name.

1. //save as C.java otherwise Compilte Time Error  
2.   
3. class A{}  
4. class B{}  
5. public class C{}  

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How to put two public classes in a package?

If you want to put two public classes in a package, have two java source files containing one public class, but keep the package name same. For example:

1. //save as A.java  
2.   
3. package javatpoint;  
4. public class A{}  

1. //save as B.java  
2.   
3. package javatpoint;  
4. public class B{}