You have completed java course.
• 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.
Simple example of java package
//save as Simple.java
package mypack;
public class Simple{
public static void main(String args[]){
System.out.println("Welcome to package");
}
} The package keyword is used to create a package in java.
How to compile java package
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 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
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.
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 pack;
public class A{
public void msg(){System.out.println("Hello");}
}
//save by B.java
package mypack;
import pack.*;
class B{
public static void main(String args[]){
A obj = new A();
obj.msg();
}
}
javac -d . A.java
javac -d . B.java
java mypack.B
Hello
2) Using packagename.classname
If you import package.classname then only declared class of this package will be accessible.
//save by A.java
package pack;
public class A{
public void msg(){System.out.println("Hello");}
}
//save by B.java
package mypack;
import pack.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.
//save by A.java
package pack;
public class A{
public void msg(){System.out.println("Hello");}
}
//save by B.java
package mypack;
class B{
public static void main(String args[]){
pack.A obj = new pack.A();//using fully qualified name
obj.msg();
}
}
Note: If you import a package, subpackages will not be imported.
