A class packages is a group of similar of classes ,interface and sub packages in java can be categorized in 2 forms , built in packages and user defends packages . there are many built in packages as java, LANG ,AWT,JAVA-X,SWING ,NET,IO,SQL etc.

Many implementations of Java use a hierarchical file system to manage source and class files. It is easy to organize class files into packages. All we need to do is put related class files in the same directory, give the directory a name that relates to the purpose of the classes, and add a line to the top of each class file that declares the package name, which is the same as the directory name where they reside.

In java there are already many predefined packages that we use while programming.For example: java.lang, java.io, java.util etc. However one of the most useful feature of java is that we can define our own packages


ADVANTAGES OF PACKAGES :

  1. Java package is used to categorizes the and interfaces so that they can be easily maintain.
  2. Java packages providers as protection .
  3. Java package remove the naming collisions .

Reusability: Reusability of code is one of the most important requirements in the software industry. Reusability saves time, effort and also ensures consistency. A class once developed can be reused by any number of programs wishing to incorporate the class in that particular program.

In real life situation there may arise scenarios where we need to define files of the same name. This may lead to “name-space collisions”. Packages are a way of avoiding “name-space collisions”.


Types of package:

  1. User defined package: The package we create is called user-defined package.
  2. Built-in package: The already defined package like java.io.*, java.lang.* etc are known as built-in packages.
Example :

//save  as simple.java
Packages  my pack;
Public class  simple
{
Public  static void main(String args[])
{
System.out.println(“Welcome to packages “);
}
}
//save  as  simple1 . java
 Import  mypack.*;
Class  simple1
{
Public  static void main(String  args[])
{
Simple obj=new simple();
Obj.msg();
}
}

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:

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 java package program

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
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.

  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.*

//save by A.java  	  
package mypack;  
public class A{  
public void msg(){System.out.println("Hello");}  
}  
//save by B.java    
import mypack.*;  	  
class B{  
public static void main(String args[]){  
A obj = new A();  
obj.msg();  
}  
}  

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

//save by A.java  	  
package mypack;  
public class A{  
public void msg(){System.out.println("Hello");}  
}  
//save by B.java  	  
import mypack.A;  	  
class B
{  
public static void main(String args[]){  
A obj = new A();  
obj.msg();  
}  
}  

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

//save by A.java  	  
package mypack;  
public class A{  
public void msg(){System.out.println("Hello");}  
}  

//save by B.java  	 	
class B
{  
public static void main(String args[]){  
pack.A obj = new pack.A();//using fully qualified name  
obj.msg();  
}  
}  

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.

Note: Sequence of the program must be package then import then class.


Subpackage in java

Package inside the package is called the subpackage. It should be created to categorize the package further.

Let's take an example, Sun Microsystem has definded a package named java that contains many classes like System, String, Reader, Writer, Socket etc. These classes represent a particular group e.g. Reader and Writer classes are for Input/Output operation, Socket and ServerSocket classes are for networking etc and so on. So, Sun has subcategorized the java package into subpackages such as lang, net, io etc. and put the Input/Output related classes in io package, Server and ServerSocket classes in net packages and so on.


Example of Subpackage

package com.javatpoint.core;  
class Simple{  
public static void main(String args[]){  
System.out.println("Hello subpackage");  
}  
}  

To Compile: javac -d . Simple.java
To Run: java com.javatpoint.core.Simple How to send the class file to another directory or drive?
There is a scenario, I want to put the class file of A.java source file in classes folder of c: drive. For example:

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

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:

	//save as A.java  	  
	package javatpoint;  
	public class A{}  

	//save as B.java  	  
	package javatpoint;  
	public class B{}  

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