Packages in Java

Package names map directly to directory structures.

Example:

package university.department.cs;

Directory structure:

university/
 └── department/
     └── cs/

Java uses the CLASSPATH to locate packages and classes at runtime.

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Java package names usually follow reverse domain naming:

com.company.project.module
org.organization.application
university.department.math

This guarantees global uniqueness.

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To add a class to a package:

1. Declare the package at the top of the file
2. Save the file in the corresponding directory
3. Compile the file

package mypack;

public class MyClass {
    public void display() {
        System.out.println("Hello from MyClass in mypack.");
    }
}

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A subpackage is a package inside another package.

java.util
java.util.concurrent

⚠️ Subpackages are not automatically imported with parent packages.

import java.util.*;          // Does NOT import java.util.concurrent

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Import a Specific Class

import java.util.ArrayList;

Import All Classes from a Package

import java.util.*;

Subpackages are excluded.

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import java.util.List;

public class DemoImport {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();

        java.util.LinkedList<String> items =
                new java.util.LinkedList<>();
    }
}

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1. Built-in Packages

Provided by Java API.

Common examples:

• java.lang
• java.util
• java.io
• java.net
• java.awt

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2. User-Defined Packages

// File: mypack/MyClass.java
package mypack;

public class MyClass {
    public void display() {
        System.out.println("Hello from MyClass in mypack.");
    }
}

Usage:

import mypack.MyClass;

public class TestPackage {
    public static void main(String[] args) {
        MyClass obj = new MyClass();
        obj.display();
    }
}

Output

Hello from MyClass in mypack.

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package myPackage;

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello from myPackage!");
    }
}

Compile

javac -d . HelloWorld.java

Run

java myPackage.HelloWorld

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Static import allows direct access to static members without class name.

import static java.lang.Math.*;

public class StaticImportExample {
    public static void main(String[] args) {
        System.out.println(PI);
        System.out.println(sqrt(16));
    }
}

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When two packages contain classes with the same name, use fully qualified names.

import java.util.Date;
import java.sql.*;

public class ConflictExample {
    public static void main(String[] args) {
        java.util.Date utilDate = new java.util.Date();
        java.sql.Date sqlDate =
                new java.sql.Date(System.currentTimeMillis());

        System.out.println(utilDate);
        System.out.println(sqlDate);
    }
}

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com.example.shapes.Circle
↓
BASE_DIR/com/example/shapes/Circle.class

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Provides utility classes for:

• Collections
• Date & time
• Random numbers
• Locale & formatting
• Timers

Example

import java.util.ArrayList;

public class Example {
    public static void main(String[] args) {
        ArrayList<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");

        System.out.println(fruits);
    }
}

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Automatically imported in every Java program.

Key classes:

• Object
• String
• Math
• System
• Thread
• Wrapper classes (Integer, Double, etc.)

Eg:

public class Example {
    public static void main(String[] args) {
        String msg = "Hello Java";
        System.out.println(msg.length());
    }
}

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Handles:

• File input/output
• Streams
• Serialization
• Buffered I/O

Example: File Copy

import java.io.*;

public class FileCopyExample {
    public static void main(String[] args) {
        try (FileInputStream in = new FileInputStream("source.txt");
             FileOutputStream out = new FileOutputStream("dest.txt")) {

            int data;
            while ((data = in.read()) != -1) {
                out.write(data);
            }
            System.out.println("File copied successfully");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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import java.io.*;

public class BufferedExample {
    public static void main(String[] args) {
        try (BufferedReader reader =
                     new BufferedReader(new FileReader("input.txt"));
             BufferedWriter writer =
                     new BufferedWriter(new FileWriter("output.txt"))) {

            String line;
            while ((line = reader.readLine()) != null) {
                writer.write(line);
                writer.newLine();
            }
            System.out.println("File processed successfully.");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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• Packages organize Java code logically
• Prevent naming conflicts
• Improve encapsulation and security
• Support modular, reusable development
• Java provides rich built-in packages
• Developers can create custom packages easily