Perfect! Let’s move on to Packages in Java — a fundamental concept to organize classes and interfaces. I’ll explain it in full detail.

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Packages in Java

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1. What is a Package?

• Definition: A package is a collection of related classes, interfaces, and sub-packages.

• Purpose:

  1. Organize code into logical groups.

  2. Avoid naming conflicts (classes with same names).

  3. Control access using access modifiers (public, protected, etc.).

  4. Reusable and easier to maintain.

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2. Types of Packages

1. Built-in / Predefined Packages: Provided by Java SDK. Examples:

   • java.lang – automatically imported, contains fundamental classes (String, Math, Object).

   • java.util – collection framework, Date, Random.

   • java.io – file handling classes (File, BufferedReader).

   • java.sql – database connectivity.

2. User-defined Packages: Created by developers to organize their own classes.

3. Unnamed / Default Package: Classes written without package declaration automatically go here. Usually for small programs.

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3. How to Create a User-defined Package

Step 1: Declare a package

// File: Car.java
package vehicles; // package declaration

public class Car {
    public void display() {
        System.out.println("This is a car");
    }
}

Step 2: Compile the package

javac -d . Car.java

• -d . tells compiler to create folder structure for the package (vehicles folder).

Step 3: Use the package in another class

// File: Main.java
import vehicles.Car; // import package

public class Main {
    public static void main(String[] args) {
        Car c = new Car();
        c.display();
    }
}

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4. Access Modifiers in Packages

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5. Advantages of Packages

1. Code Reusability – Organize classes to reuse them in different projects.

2. Better Maintainability – Easier to manage large projects.

3. Namespace Management – Avoids class name conflicts.

4. Access Control – Control which classes/members are visible outside the package.

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6. Built-in Package Example

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        System.out.println(list);
    }
}

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✅ Summary Table – Package Types

Perfect! Let’s cover Non-Access Modifiers in Java in full detail. These are modifiers that do not control access, but instead change behavior or properties of classes, methods, or variables.

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Non-Access Modifiers in Java

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1. What is a Non-Access Modifier?

• Definition: Modifiers that modify behavior of classes, methods, or variables without affecting access control.

• Used for: class, method, variable

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2. Types of Non-Access Modifiers

- Method: Cannot be overridden
- Variable: Constant value | final int x = 10;
final class Car {}
final void start(){} | | static | Method, variable, block, nested class | Belongs to class, shared by all objects | static int count = 0;
static void display(){} | | abstract | Class, method | - Class: Cannot create objects
- Method: Must be implemented by subclass | abstract class Vehicle { abstract void start(); } | | synchronized | Method, block | Thread-safe; only one thread can execute at a time | synchronized void increment() {} | | native | Method | Method implemented in other language (C/C++) | native void display(); | | transient | Variable | Not serialized during object serialization | transient String password; | | volatile | Variable | Ensures visibility of changes across threads | volatile int counter; | | strictfp | Class, method | Forces floating-point calculations to be platform-independent | strictfp class Calculator {} |

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3. Detailed Examples

final

final class Car {} // cannot be extended
final int speed = 100; // constant
final void start() { System.out.println("Start"); } // cannot override

static

class Car {
    static int wheels = 4; // shared by all objects
    static void display() { System.out.println("Static method"); }
}

abstract

abstract class Vehicle {
    abstract void start(); // must be implemented in subclass
}
class Bike extends Vehicle {
    void start() { System.out.println("Bike started"); }
}

synchronized

class Counter {
    private int count = 0;
    synchronized void increment() { count++; } // thread-safe
}

native

class Example {
    native void display(); // implemented in C/C++
}

transient

class User implements java.io.Serializable {
    transient String password; // not saved during serialization
}

volatile

class Counter {
    volatile int count; // ensures thread visibility
}

strictfp

strictfp class Calculator {
    float divide(float a, float b) { return a / b; }
}

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4. Quick Notes

1. final, static, abstract are the most commonly used.

2. transient, volatile, synchronized, native, strictfp are mostly advanced and used in multithreading, serialization, and system-level programming.

3. Cannot combine some modifiers:

   • abstract and final cannot be used together.

   • abstract and private cannot be used together.

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✅ Summary Table