Packages Introduction

• A package is a namespace that organizes a set of related classes, interfaces, enums, and annotations.
• A package indicates the directory or folder in your file system where you can place your project files.
• Package can be defined as a namespace to identify classes uniquely.
• Package can be used to specify the access scope of your class or class members.

3.6.1 Package Declaration

Syntax:

package <package Name>;

Examples:

package p1;
package p2;

Example Programs

Lab450.java (Without Package)

class Lab450 {

  public static void main(String str[]) {
    System.out.println("Hello Guys!");
  }
}

Lab451.java (With Package)

package p1;

class Lab451 {

  public static void main(String str[]) {
    System.out.println("Hello Guys!");
  }
}

Steps to Compile WITHOUT Package

Syntax:

javac <sourceFileName>.java

Example:

javac Lab450.java

Steps to Run WITHOUT Package

Syntax:

java <class Name>

Example:

java Lab450

Steps to Compile WITH Package

Syntax:

javac -d <location> <sourceFileName>.java

Example:

javac -d . Lab451.java

Steps to Run WITH Package

Syntax:

java <package Name>.<class Name>

Example:

java p1.Lab451

Important Notes

• In the above examples:
  • Lab450 is written without package declaration.
  • Lab451 is written with package declaration (package p1).
• When you write classes without any package declaration, those classes are placed in the default package.
• Default package has no name.
• When you compile files in default package, .class files are placed in the current folder only.

🔥 Quick Understanding

• Package = folder structure + namespace

• Helps avoid naming conflicts

• Controls accessibility

• Required for large applications

• When you are writing classes with a package declaration statement, then those classes will be placed in the specified package. → When you compile source files with a package declaration, the compiled .class files will be placed in the specified package.

• When a class is specified inside a package, then that class must always be referred using the package name, which is also called the Fully Qualified Class Name (FQCN).

Example:

p1.Lab451

Directory Structure Example

3.6.2 Organizing Package Names

• Package name is a combination of multiple words separated by dot (.) symbol.
• Each word in the package name represents one folder/directory in the file system.

Examples:

package com.javaworld.sms.struts;
package com.javaworld.sms.hibernate;
package com.javaworld.sms.ejb;

Package Structure Representation

🔥 Quick Understanding

• . in package → folder separation
• Package = directory structure + namespace
• FQCN → package Name.class Name
• Compilation with package → creates folders automatically

Lab452.java

package com.javaworld.p1;

class Lab452 {

  public static void main(String str[]) {
    System.out.println("Hello Guys!");
  }
}

Lab453.java

package com.javaworld.p2;

class Lab453 {

  public static void main(String str[]) {
    System.out.println("Hello Guys!");
  }
}

Directory / Package Structure

3.6.3 Exploring Compilation

Syntax

javac -d <Location> <SourceFileName>.java

Example

javac -d . Lab.java

Important Notes

• Here . (dot) is used as Location. → Package will be created in the current working directory and compiled classes will be placed inside that package.
• If you want to create the package in a specific directory instead of current directory, then you must specify the location explicitly.
• Assume the current working directory is:

D:\JavaWorld\D1\pack

🔥 Quick Understanding

• javac -d → creates package folders automatically
• . means current directory
• Package structure mirrors folder structure
• .class files go inside package folders

Hello.java

package com.JavaWorldIndia;

class Hello {

  public static void main(String str[]) {
    System.out.println("Hello Guys!");
  }
}

Case 1: Creating Package in CURRENT DIRECTORY

Relative Location

javac -d . Hello.java

Absolute Location

javac -d D:\JavaWorld\D1\pack Hello.java

Directory Structure (Case 1)

graph TD
    D["D:\"] --> JavaWorld["JavaWorld"]
    JavaWorld --> D1["D1"]
    D1 --> PACK["PACK"]

    PACK --> SRC["Hello.java"]

    PACK --> com["com"]
    com --> JavaWorldIndia["JavaWorldIndia"]
    JavaWorldIndia --> CLASS["Hello.class"]

Case 2: Creating Package in OTHER DIRECTORY IN SAME DRIVE

Relative Location

javac -d class es Hello.java

Absolute Location

javac -d D:\JavaWorld\D1\pack\class es Hello.java

Directory Structure (Case 2)

Important Notes

• javac -d creates the package directory structure automatically.
• d . → creates package folders in the current directory.
• d classes → creates package folders inside the classes directory.
• Relative path → relative to current working directory.
• Absolute path → full folder path.

🔥 Quick Understanding

Example:

If package is:

package com.JavaWorldIndia;

Compiler creates:

com/JavaWorldIndia/

and places:

Hello.class

inside it.

Compilation Flow