DEV Community: Mohamed Ajmal

Package in Java.

Mohamed Ajmal — Tue, 21 Apr 2026 06:17:40 +0000

Package is the Primary way to organize the related classes, Interface and sub packages in to shared name space. That helps to improve organization, maintainability, and re-usability.

Defining a package is the primary way to organize related classes, interfaces, and sub-packages into a shared namespace.

Key purposes

Organization: Structures large projects into smaller, manageable, related units, similar to folders in a file directory.

Prevent Naming Conflicts: Allows different parts of a program to use classes with the same name without conflict.

Access Control: Manages visibility of classes, methods, or variables using modifiers like public, protected, or package-private.

Re-usability & Distribution: Packaged code is easier to import, reuse across projects, or distribute as a library.

Types of Packages

Built-in Packages: Provided by the Java API (Standard Library). (TBD)

    java.lang: Core classes like String and Math; automatically

imported into every program.

    java.util: Utility classes for collections, dates, and scanners.

    java.io: Support for input and output operations.


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

Constructor in Java.

Mohamed Ajmal — Fri, 17 Apr 2026 23:07:02 +0000

A specialized block of code that similar to method that is automatically invoked when an instance of class is created. It's primary purpose is to initialize the state of the new object, often by assigning initial values to instance variables. It is used to set default or user-defined values for the object's attributes

Key Rules

Must match the exact name of the class,
Must not have an explicit return type, not even void,
Automatically called using the new keyword,
Can use access modifiers (public, private, protected) but cannot be abstract, static, final, or synchronized. (TBD)

Types

Default Constructor: Automatically provided by the Java compiler if no constructors are explicitly defined in the class. It initializes numeric fields to 0, boolean to false, and object references to null.
No-Arg Constructor: A constructor explicitly written by the programmer that accepts no parameters.
Parameterized Constructor: Takes one or more parameters to allow objects to be initialized with custom values at creation. (TBD)
Copy Constructor: Used to create a new object by copying the data from an existing instance of the same class. (TBD)

Java variables and Objects.

Mohamed Ajmal — Sat, 04 Apr 2026 05:57:31 +0000

Static variables: These are variables that are shared among all the instances of a class.

Non-static variables: These are variables that belong to each individual instance of a class.

Java Static Variables : When a variable is declared as static, then a single copy of the variable is created and shared among all objects at a class level. Static variables are essentially, global variables.

Java Non-static variables : Non-static variables are variables that belongs to a specified object of a class, it is also known as instance variable.
These variables are declared outside of a method, constructor or block. Each object created from the class gets its own separate copy of these variables.

Ref : https://www.geeksforgeeks.org/java/difference-between-static-and-non-static-variables-in-java/

Difference between static and non-static:

OBJECTS

In Java, an object is an instance of a class, created using the new keyword. The class acts as a blueprint, defining the object's attributes (state) and methods (behavior), while the object is the concrete entity with specific values in memory.

State: Represented by attributes (variables) that store data.

Behavior: Represented by methods (functions) that define the actions an object can perform.

Identity: A unique name or identifier that distinguishes one object from another, even if they have the same state and behavior.

OOPS Concepts in Java.

Mohamed Ajmal — Wed, 01 Apr 2026 05:57:28 +0000

Object-Oriented Programming (OOPS) is a paradigm that organize the code using Classes and objects.

The seven core concepts of Object-Oriented Programming (OOPS) include the four main pillars—Encapsulation, Abstraction, Inheritance, and Polymorphism—along with Classes, Objects, and Association. These concepts allow for modular, reusable, and secure code by organizing software around data rather than functions.

1.Class - Class is the Blueprint or Templates that defines data member (Attributes) and member functions (Behaviors).

2.Object - An instance of a class. It is a real-world entity that holds data and has a specific state and behavior.

3.Encapsulation- Wrapping data (variables) and code (methods) together as a single unit, hiding the internal state from the outside world to protect integrity.

4.Abstraction- Representing essential features while hiding unnecessary background details, simplifying complex systems.

5.Inheritance- The mechanism by which one class (subclass/child) acquires the properties and behaviors of another class (super class/parent), promoting code re-usability.

6.Polymorphism- The ability of an entity (method or object) to take on many forms, allowing one interface to be used for a general class of actions.

Java Basic Errors.

Mohamed Ajmal — Mon, 30 Mar 2026 05:55:20 +0000

public must be start with small letter.

Home.java:1: error: class, interface, enum, or record expected
Public class Home {
^
1 error

2.File name and Class name must be same.

Home.java:1: error: class home is public, should be declared in a file named home.java
public class home {
^
1 error

*In Java, text must always be inside double quotes *

Home.java:3: error: ')' or ',' expected
System.out.println(Hello, World!");
^
Home.java:3: error: unclosed string literal
System.out.println(Hello, World!");
^
2 errors

4.**Java is case-sensitive

You wrote: system ❌
Correct is: System ✅

Java thinks system is a package**

Home.java:3: error: package system does not exist
system.out.println("Hello, World!");
^
1 error

**missed the method name

Java expected an identifier (method name) after void, but it’s missing.**

Home.java:2: error: expected
public static void (String[] args) {
^
1 error

Server provider companies.

Mohamed Ajmal — Sat, 28 Mar 2026 06:00:36 +0000

Popular server providers

1.AWS - Amazon web service.

Market leader in cloud computing
Services: EC2 (servers), S3 (storage), RDS (databases)
Used by: Netflix, Airbnb
Best for: Scalability & enterprise apps.

2.Microsoft Azure.

Strong integration with Microsoft tools
Services: Virtual Machines, Azure SQL, AI services
Best for: .NET applications & enterprise solutions.

3.Google Cloud Platform (GCP).

Known for performance and data analytics
Services: Compute Engine, BigQuery
Used for AI & machine learning projects.

4.IBM Cloud:

Focus on enterprise and hybrid cloud
Strong in AI with Watson
Used in banking and large organizations.

Oracle Cloud:

Best for database-heavy applications
Strong integration with Oracle DB.

Popular in India:

Zoho Cloud:

Indian-based cloud services
Focus on business applications.

Tata Communications:

Enterprise cloud and hosting
Strong network infrastructure.

Java Architecture.

Mohamed Ajmal — Sat, 28 Mar 2026 05:51:10 +0000

Java architecture is designed to make programs platform-independent It's run anywhere.

Source code -> Byte code -> JVM -> Machine code.

Java Source Code- Written by developers in .java files
Example: Hello.java

Java Compiler - Converts source code into bytecode
Output file: .class

Bytecode - Intermediate code generated after compilation
Can run on any system with JVM.

Java Virtual Machine (JVM) - Core of Java architecture Converts bytecode into machine code.

Java Runtime Environment (JRE) - Provides environment to run Java programs
Includes: JVM, Libraries.

Java Development Kit (JDK)- Full package for developers
Includes: - JRE, Compiler (javac) Development tools

JDK → Java Development tools,
JRE → Java Runtime environment,
JVM → Java Virtual Machine.

ref:

History of JAVA.

Mohamed Ajmal — Sat, 28 Mar 2026 05:37:51 +0000

HISTORY OF JAVA

Java was developed by James Gosling and his team at Sun Micro-systems.

The project began in 1991 and was initially called Oak (named after an oak tree outside Gosling’s office).

The goal was to create a language for embedded systems.

Java officially launched in 1995, It became popular due to its "Write Once, Run Anywhere" (WORA) concept. This was possible because of the Java Virtual Machine (JVM), which allows Java programs to run on any system.

In late 90s, Java gained huge popularity during the rise of the web.
Java evolved into a powerful back-end language.
Introduction of platforms: J2SE (Standard Edition), J2EE (Enterprise Edition), J2ME (Micro Edition).

It became widely used for: Web applications, Banking systems, Android development. In 2010, Oracle Corporation acquired Sun Microsystems.
Since then, Oracle has been responsible for Java’s development and updates. Java continues to evolve with regular updates (Java 8, 11, 17, etc.).

Key Features of Java: Platform-independent, Object-oriented, Secure and robust, Multithreaded, High performance (with JIT compiler).

React JS.

Mohamed Ajmal — Thu, 12 Mar 2026 08:03:13 +0000

React is a JavaScript library used to build user interfaces (UI), especially for web applications.

It helps developers create interactive and dynamic websites.
It's Developed by Meta platform.

React is used to build fast, interactive, and reusable web interfaces using components.

Why we are using React:

1) Component-Based Architecture - React allows you to create small reusable components. Ex: Navbar, Button, Footer, Card.

2) Fast Performance (Virtual DOM) - React uses Virtual DOM, so it updates only the changed parts of the page instead of reloading the whole page.

3) Reusable Code - Components can be reused in many places, which reduces code repetition.

4) Easy State Management - React provides hooks like useState and useEffect to manage data and updates.

5) Single Page Applications (SPA) - React helps build fast single-page applications where pages update without full reload.

6) Large Community - Many developers use React, so there are many libraries, tutorials, and support available.

Events and Event Listener in JS.

Mohamed Ajmal — Thu, 12 Mar 2026 07:57:30 +0000

HTML events are things that happen to HTML elements. Event is an action performed by the user or browser.

Event Listener is a function that listens to that event and executes code.

Examples of Events:

User clicks a button → click

User types → keydown

Page loads → load

Form submits → submit

Event = something that happens.

Event listener

An event listener is a function that waits for an event and responds when the event occurs.

Event listener = code that reacts to an event.

example code:

Click Me

let btn = document.getElementById("btn");

btn.addEventListener("click", () => {
console.log("Button clicked");
});

DOM in JS.

Mohamed Ajmal — Tue, 20 Jan 2026 06:34:18 +0000

DOM is a programming interface that represent an HTML Document as a tree structure. JavaScript uses the DOM to access, change, add, or remove elements on a web page dynamically.

Note : DOM is provided by the browser, not JavaScript. JavaScript only uses the DOM to interact with HTML.

Accessing DOM Elements in the several ways:

getElementById - document.getElementById("heading");

getElementsByClassName - document.getElementsByClassName("banner");

getElementsByTagName - document.getElementsByTagName("p");

querySelector - document.querySelector("#heading"); // id
document.querySelector(".banner"); // class

querySelectorAll - document.querySelectorAll("p");

Changing content using DOM :

Change text - document.getElementById("heading").innerText = "Hello world!";

Change HTML - document.getElementById("heading").innerHTML ="

welcome

Changing Styles - document.getElementById("heading").style.color = "red";
document.getElementById("heading").style.fontSize = "30px";

Creating Elements -
let p = document.createElement("p");
p.innerText = "This is new paragraph";
document.body.appendChild(p);

Removing Elements -
let elem = document.getElementById("heading");
elem.remove();

For ref: geeks for geeks - https://www.geeksforgeeks.org/javascript/dom-document-object-model/

Function and Objects In JS.

Mohamed Ajmal — Tue, 20 Jan 2026 06:12:52 +0000

FUNCTION :

A function is a block of code that performs a specific task and can be reused. There are three ways!

1.Function Declaration,
2.Function Expression,
3.Arrow Function (ES6) (Short syntax, Commonly used in modern JS).

Examples:

1.
function greet(name) {
return "Hello " + name;
}

console.log(greet("Ajmal"));

const add = function (a, b) { return a + b; };

console.log(add(5, 3));

3.const multiply = (a, b) => a * b;

console.log(multiply(4, 2));

OBJECTS: An object stores data in key-value pairs. It's a combination of state and behavior.

ex:
const person = {
firstName: "John", // Property
lastName: "Doe", // Property
age: 50, // Property
fullName: function() { // Method
return this.firstName + " " + this.lastName;
}
};

OBJECT CON