A java package is a group of similar types of classes, interfaces, and sub-packages.
Package in java can be categorized in two forms, 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.
Example of java package
The package keyword is used to create a package in java.
//save as Simple.java
package mypack;
public class Simple{
public static void main(String args[]){
System.out.println("Welcome to package");
}
}
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 imports 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();
}
}
2) Using packagename.classname
If you import package.classname then only declared class of this package will be accessible.
Example of a package by import package.classname
//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 a fully qualified name then only the declared class of this package will be accessible. Now there is no need to import. But you need to use a fully qualified name every time when you are accessing the class or interface.
It is generally used when two packages have the same class name e.g. java.util and java.sql packages contain the Date class.
Example of package by import fully qualified name
//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();
}
}
Subpackage in java
The package inside the package is called the subpackage. It should be created to categorize the package further.
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