Packages

What is a Package?

A package is a namespace that organises a set of related classes and interfaces — similar to a folder on your file system. Packages prevent naming conflicts and provide access control.

Benefits

• Avoids naming conflicts (two classes can have same name in different packages)
• Provides access protection
• Makes code easier to locate and maintain

Types of Packages

Creating Packages

The package statement must be the first line in a Java source file (before any imports or class declarations).

// File: in/anits/cse/Student.java
package in.anits.cse;

public class Student {
    private int    rollNo;
    private String name;

    public Student(int r, String n) { rollNo = r; name = n; }

    public void show() {
        System.out.println("Roll: " + rollNo + " | Name: " + name);
    }
}

# Compile with package structure
javac -d . in/anits/cse/Student.java

# This creates:  ./in/anits/cse/Student.class
# Run:
java in.anits.cse.Student

Importing Packages

import java.util.Scanner;
import java.util.ArrayList;
import in.anits.cse.Student;

public class Main {
    public static void main(String[] args) {
        Student s = new Student(101, "Arjun");
        s.show();
    }
}

Access Modifiers

Built-in Packages

Static Import

Static import allows you to use static members of a class directly without the class prefix.

import static java.lang.Math.*;
import static java.lang.System.out;

public class MathDemo {
    public static void main(String[] args) {
        // Without static import: Math.sqrt(16), System.out.println
        out.println(sqrt(16));   // 4.0
        out.println(pow(2, 10)); // 1024.0
        out.println(PI);         // 3.141592653589793
    }
}