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A package is a collection of related classes. It helps Organize your classes into a folder structure and make it easy to locate and use them. More importantly, it helps improve re-usability.

Each package in Java has its unique name and organizes its classes and interfaces into a separate namespace, or name group.

Although interfaces and classes with the same name cannot appear in the same package, they can appear in different packages. This is possible by assigning a separate namespace to each package.

Syntax:-

package;

The following video takes you through the steps of creating a package.

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Let's see this with an example. We define a class and object and later compile this it in our package p1. After compilation, we execute the code as a java package .

Step 1) Consider the following code,

Creating and Using package in Java

Here,

  1. To put a class into a package, at the first line of code define package p1
  2. Create a class c1
  3. Defining a method m1 which prints a line.
  4. Defining main method
  5. Creating an object of class c1
  6. Calling method m1

Step 2) In next step, save this file as demo.java

Creating and Using package in Java

Creating and Using package in Java

Step 3) In this step, we compile the file.

Creating and Using package in Java

The compilation is completed. A class file c1 is created. However, no package is created? Let's do that in next step

Creating and Using package in Java

Step 4) Now we have to create a package, use the command

javac –d . demo.java

This command forces the compiler to create a package.

The "." operator represents the current working directory.

Creating and Using package in Java

When you execute the code, it creates a package p1. When you open the java package p1 inside you will see c1.class file.

Creating and Using package in Java

Step 7) Compile the same file using the following code

  • javac –d .. demo.java
  • Here ".." indicates the parent directory. In our case file will be saved in parent directory which is C Drive

Creating and Using package in Java

File saved in parent directory when above code is executed.

Creating and Using package in Java

Step 8) Now let's say you want to create a sub package p2 within our existing java package p1. Then we will modify our code as

package p1.p2

Creating and Using package in Java

Step 9) Compile the file

Creating and Using package in Java

As seen in below screenshot, it creates a sub-package p2 having class c1 inside the package.

Creating and Using package in Java

Step 10) To execute the code mention the fully qualified name of the class i.e. the package name followed by the sub-package name followed by the class name -

java p1.p2.c1

Creating and Using package in Java

This is how the package is executed and gives the output as "m1 of c1" from the code file.

Creating and Using package in Java

Importing packages

To create an object of a class (bundled in a package), in your code, you have to use its fully qualified name.

Ex.

java.awt.event.actionListner object = new java.awt.event.actionListner();

But , it could become tedious to type in the long dot-separated package path name for every class you want to use. Instead it is recommended you use the import statement.

Syntax

import;			

Once imported , you can use the class without mentioning its fully qualified name.

import java.awt.event.*; // * signifies all classes in this package
import javax.swing.JFrame // here only the JFrame class is imported
//Usage
JFrame f = new JFrame; // without fully qualified name.

Assignment 3:

To import package

Step 1) Copy the code into an editor.

// Using packages created in earlier assignment
package p3;
import p1.p2.*; //imports classes only in package p2 and NOT  in the sub-package p1
class c3{
  public   void m3(){
     System.out.println("Method m3 of Class c3");
  }
  public static void main(String args[]){
    c1 obj1 = new c1();
    obj1.m1();
    p1.p2.c1 obj2 = new p1.p2.c1();
    obj2.m1();
  }
}

Step 2) Save the file as Demo2.java . Compile the file using the command javac –d .Demo2.java

Step3) Execute the code using the command java p3.c3

Packages - points to note:

  • To avoid naming conflicts packages are given names of the domain name of the company in reverse Ex : com.guru99. com.microsoft, com.infosys etc.
  • When a package name is not specified , a class is into the default package (the current working directory) and the package itself is given no name. Hence you were able to execute assignments earlier.
  • While creating a package, care should be taken that the statement for creating package must be written before any other import statements
// not allowed
import package p1.*;
package p3;
//correct syntax
package p3;
import package p1.*;

the java.lang package is imported by default for any class that you create in Java. You can study it more in detail here.

The java API is very extensive , contains classes which can perform almost all your programming tasks right from Data Structure Manipulation til Networking. More often than not , you will be using API files in your code. You can see the API documentation here.

 

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