coreJava : packages in java

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.

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.

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 java packages");  

   }  

}  

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 ex:  javac -d . Simple.java  

How to run java package program?

We 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 java packages

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 pack;  

public class A{  

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

}  

//save by B.java  

package mypack;  

import pack.*;  

  

public class B{  

  public static void main(String args[]){  

   A obj = new A();  

   obj.msg();  

  }  

}  

Output:Hello

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 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(); 

  } 

} 

Output:Hello 

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 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();  

  }  

}  

Output:Hello

Note: If you import a package, subpackages will not be imported.

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.

save as E.java

package com.mypack1;
public class E {
public void methodE(){
System.out.println("from E");
}

public static void main(String[] args) {
// TODO Auto-generated method stub

}

}

save as X.java

package com.mypack1;
public class X {
public void methodX(){
System.out.println("from X");
}
public static void main(String[] args) {
// TODO Auto-generated method stub

}


}

//save by F.java

package com.mypack1.subpack1;
public class F {
public void methodF(){
System.out.println("from F");
}
public static void main(String[] args) {
// TODO Auto-generated method stub

}
}

//save by Y.java

package com.mypack2;
import com.mypack1.*;
import com.mypack1.subpack1.F;
public class Y {
public void methodY(){
System.out.println("from Y");
}
public static void main(String[] args) {
X x1 = new X();
x1.methodX();
F f1 = new F();
}

}

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

//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

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 java9m;  

public class A{}  

//save as B.java  

  

package java9m;  

public class B{}  

package class in java

The package class provides methods to get information about the specification and implementation of a package. It provides methods such as getName(), getImplementationTitle(), getImplementationVendor(), getImplementationVersion() etc.

 Example of Package class

 In this example, we are printing the details of java.lang package by invoking the methods of  package class.

 public class PackageInfo{

 public static void main(String args[]){

 Package p=Package.getPackage("java.lang");

 System.out.println("package name: "+p.getName());

 System.out.println("Specification Title: "+p.getSpecificationTitle());

 System.out.println("Specification Vendor: "+p.getSpecificationVendor());

 System.out.println("Specification Version: "+p.getSpecificationVersion());

 System.out.println("Implementaion Title: "+p.getImplementationTitle());

 System.out.println("Implementation Vendor: "+p.getImplementationVendor());

 System.out.println("Implementation Version: "+p.getImplementationVersion());

 System.out.println("Is sealed: "+p.isSealed());

 }

 }

Output:

package name: java.lang

Specification Title: Java Platform API Specification

Specification Vendor: Oracle Corporation

Specification Version: 1.8

Implementaion Title: Java Runtime Environment

Implementation Vendor: Oracle Corporation

Implementation Version: 1.8.0_102

Is sealed: false