Packages in Java

In this article, you’ll learn about Packages in Java, Advantages of Packages , Types of Packages, Creating a Package and The “import” keyword .

• A package is a mechanism to group the similar type of classes, interfaces and sub-packages and provide access control.  
• It organizes classes into single unit.
• In Java already many predefined packages are available, used while programming.

For example: java.lang, java.io, java.util etc.

Advantages of Packages

• Packages provide code reusability, because a package has group of classes.
• It helps in resolving naming collision when multiple packages have classes with the same name.
• Package also provides the hiding of class facility. Thus other programs cannot use the classes from hidden package.
• Access limitation can be applied with the help of packages.
• One package can be defined in another package.

Types of Packages

There are two types of packages available in Java.

1. Built-in packages
Built-in packages are already defined in java API. For example: java.util, java.io, java,lang, java.awt, java.applet, java.net, etc.
2. User defined packages
The package we create according to our need is called user defined package.

Creating a Package

We can create our own package by creating our own classes and interfaces together. The package statement should be declared at the beginning of the program.
Syntax:

package <packagename>;
class ClassName
{
……..
……..
}

Example: Creating a Package

// Demo.java
package p1;
class Demo
{
   public void m1()
   {
       System.out.println("Method m1..");
   }
}

How to compile?

Syntax:  javac –d directory javafilename
For Example: javac –d . Demo.java

How to run?

To run: java p1.Demo

Example: Program to create and use a user defined ackage in Java.

// Vehicle.java
package vehicles;
interface Vehicle
{
   public void run();
   public void speed();
}

//Bike.java
package vehicles;
public class Bike implements Vehicle
{
    public void run()
   {
      System.out.println("Bike is running.");
   }
   public void speed()
   {
      System.out.println("Speed of Bike: 50 Km/h");
   }
   public static void main(String args[])
   {
      Bike bike = new Bike();
      bike.run();
      bike.speed();
   }
}

Compile

javac –d . Vehicle.java
javac –d . Bike.java

Run

java vehicles.Bike

Output

Bike is running
Speed of Bike: 50 Km/h

The “import” keyword

• The import keyword provides the access to other package classes and interfaces in current packages.
• “import” keyword is used to import built-in and user defined packages in java program.

There are different 3 ways to access a package from other packages.

1. Using full qualified name

Example

class Demo extends java.util.Scanner
{
    //statements
}

2. import only single class

Example

import java.util.Scanner;
class Demo
{
      // statements
}

3. import all classes

Example

import java.util.*;
class Demo
{
    // statements
}