DEV Community: Hashim HB

Java Fundamentals , My Day 1–4 Learning Journey

Hashim HB — Sun, 26 Jul 2026 19:13:30 +0000

🚀After deciding to strengthen my programming fundamentals as part of my DevOps learning journey, I started learning Java.

I know these are basic concepts, but every experienced developer once started here. Building strong fundamentals makes learning advanced topics much easier later.

"Starting from scratch is never a weakness. Whether the concepts are basic or advanced, every lesson learned today becomes the building block for tomorrow's expertise."

In this blog, I'll summarize everything I learned during my first four days of Java.

Day 1 – Introduction to Java

Java is a high-level, object-oriented programming language designed to be platform-independent.

The famous Java principle is:

Write Once, Run Anywhere (WORA)

This is possible because Java programs are first compiled into bytecode, which can run on any operating system that has a Java Virtual Machine (JVM).

How Java Works

Java Source Code (.java)
          │
          ▼
      Java Compiler
        (javac)
          │
          ▼
      Bytecode (.class)
          │
          ▼
Java Virtual Machine (JVM)
          │
          ▼
 Machine Code
          │
          ▼
 Operating System
(Windows/Linux/macOS)

The JVM converts bytecode into machine code that the operating system understands.

JDK vs JRE vs JVM

JVM (Java Virtual Machine)

Executes Java bytecode
Converts bytecode into machine code
Makes Java platform independent

JRE (Java Runtime Environment)

The JRE contains:

JVM
Core Java Libraries
Runtime files

Its purpose is simply to run Java applications.

JDK (Java Development Kit)

The JDK contains:

JRE
Java Compiler (javac)
Debugger
Documentation
Development tools

The JDK is used to develop Java applications.

IDE

IDE stands for Integrated Development Environment.

Popular Java IDEs include:

IntelliJ IDEA
Eclipse
VS Code
NetBeans

An IDE provides:

Code completion
Debugging
Syntax highlighting
Project management

First Java Program

public class Hello {

    public static void main(String[] args) {

        System.out.println("Hello World!");

    }

}

Understanding the code

public

Accessible from anywhere.

class

Defines a class.

Hello

Class name.

main()

Entry point of every Java application.

System.out.println()

Prints output to the console.

Java Keywords

Keywords are reserved words already defined by Java.

Examples:

class
public
private
static
void
if
else
switch
while
for
return

Keywords cannot be used as variable names.

Variables

A variable stores data in memory.

Example:

int age = 22;

Rules for naming variables:

Case-sensitive
Must begin with a letter, _, or $
Cannot begin with a number
Cannot be a Java keyword
Prefer camelCase naming

Example:

int employeeAge;
String firstName;
double accountBalance;

Primitive Data Types

Java has eight primitive data types.

Data Type	Size	Example
byte	1 byte	100
short	2 bytes	2000
int	4 bytes	100000
long	8 bytes	100000000L
float	4 bytes	12.5f
double	8 bytes	12.56
char	2 bytes	'A'
boolean	1 bit (logical)	true

Example:

int age = 22;

float cgpa = 8.75f;

double salary = 25000.55;

char grade = 'A';

boolean isPassed = true;

Notice the f after float values.

Without it, Java treats decimal numbers as double.

Type Conversion

Implicit Conversion (Widening)

Smaller data types are automatically converted into larger data types.

int age = 25;

long number = age;

No data loss occurs.

Explicit Conversion (Casting)

Large data types can be converted into smaller ones.

int age = 150;

byte newAge = (byte) age;

Output:

-106

Since byte ranges only from -128 to 127, overflow occurs.

Comments

Single-line comment

// This is a comment

Multi-line comment

/********************
This is
a multi-line comment
********************/

Day 2 – Binary Number System

Computers understand only:

0
1

Every value stored in memory is represented in binary.

Example:

int age = 12;

Internally:

00000000 00000000 00000000 00001100

Bytes and Bits

1 Byte = 8 Bits

short = 16 bits

int = 32 bits

long = 64 bits

MSB and LSB

MSB

Most Significant Bit

In signed integers, the MSB represents the sign bit.

0 → Positive
1 → Negative (using two's complement representation)

LSB

Least Significant Bit

Represents the smallest place value.

Two's Complement

Negative numbers are represented using Two's Complement.

Steps:

Invert all bits
Add 1

Example:

5

Binary

00000101

Invert

11111010

Add 1

11111011

= -5

Java Operators

Arithmetic Operators

+

-

*

/

%

Example:

int a = 10;
int b = 3;

System.out.println(a+b);

System.out.println(a-b);

System.out.println(a*b);

System.out.println(a/b);

System.out.println(a%b);

Assignment Operators

=

+=

-=

*=

/=

%=

Relational Operators

==

!=

>

<

>=

<=

Example:

System.out.println(5>3);

System.out.println(5==5);

System.out.println(5!=3);

Logical Operators

&&

||

!

Example

boolean a = true;

boolean b = false;

System.out.println(a && b);

System.out.println(a || b);

System.out.println(!a);

Bitwise Operators

&

|

^

~

<<

>>

>>>

Increment and Decrement

i++;

i--;

Ternary Operator

condition ? value1 : value2;

Example

int age = 18;

String result = age >= 18 ? "Adult" : "Minor";

System.out.println(result);

Taking Input from User

Java provides the Scanner class.

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        System.out.print("Enter your age: ");

        int age = sc.nextInt();

        System.out.println(age);

        sc.close();

    }

}

Useful methods:

nextInt()

nextFloat()

nextDouble()

next()

nextLine()

nextBoolean()

Calling close() releases the underlying input resource when you're done using the scanner.

Day 3 – Conditional Statements

Java executes code depending on conditions.

if Statement

int age = 20;

if(age >= 18){

    System.out.println("Eligible");

}

if-else

if(age >=18){

    System.out.println("Adult");

}

else{

    System.out.println("Minor");

}

if-else-if

int marks = 85;

if(marks >=90){

    System.out.println("A");

}

else if(marks>=75){

    System.out.println("B");

}

else{

    System.out.println("C");

}

Nested if

if(age>=18){

    if(age<60){

        System.out.println("Working");

    }

}

Switch Statement

int day = 3;

switch(day){

case 1:

    System.out.println("Monday");

    break;

case 2:

    System.out.println("Tuesday");

    break;

case 3:

    System.out.println("Wednesday");

    break;

default:

    System.out.println("Invalid");

}

Switch statements improve readability when checking many fixed values.

Day 4 – Loops

Loops execute a block of code repeatedly.

for Loop

for(int i = 1; i <= 5; i++){

    System.out.println(i);

}

Structure:

Initialization

Condition

Update

Loop Body

Example:

Sum of first n numbers

int n = 10;

int sum = n * (n + 1) / 2;

System.out.println(sum);

while Loop

Used when the number of iterations is unknown.

int i = 1;

while(i <= 5){

    System.out.println(i);

    i++;

}

do-while Loop

Executes at least once.

int i = 1;

do{

    System.out.println(i);

    i++;

}while(i <= 5);

break

Stops the loop immediately.

for(int i=1;i<=10;i++){

    if(i==5)

        break;

    System.out.println(i);

}

continue

Skips the current iteration.

for(int i=1;i<=5;i++){

    if(i==3)

        continue;

    System.out.println(i);

}

Nested Loops

for(int i=1;i<=3;i++){

    for(int j=1;j<=3;j++){

        System.out.print("* ");

    }

    System.out.println();

}

Output

* * *
* * *
* * *

Labeled Break

outer:

for(int i=1;i<=3;i++){

    for(int j=1;j<=3;j++){

        if(i==2)

            break outer;

    }

}

A labeled break exits multiple nested loops at once.

What's Next?

The next topic in my learning journey is Arrays in Java, where I'll explore:

Creating arrays
Accessing elements
Iterating using loops
Array operations
Multidimensional arrays

Stay tuned!

Final Thoughts

These first four days reminded me that programming isn't about memorizing syntax—it's about understanding how the language works behind the scenes.

From learning how Java compiles code into bytecode, to understanding primitive data types, operators, conditions, and loops, every concept lays the groundwork for more advanced topics like Object-Oriented Programming, Collections, and Multithreading.

I'm documenting this journey publicly to reinforce my own understanding and hopefully help others who are just getting started.

If you're also learning Java, feel free to connect and share your learning journey. Happy coding! 🚀

Introduction to Computer Networks for Beginners

Hashim HB — Sun, 19 Jul 2026 16:52:38 +0000

WHAT IS A NETWORK?

Computers connected with each other are called a Network.

WHAT IS INTERNET?

The collection of interconnected Computers is called as Internet.

HOW IT ALL STARTED?

In an alternate history, the U.S. and the Soviet Union were engaged in a major war, with both sides competing intensely in technology and space. The Soviet Union gained a significant advantage by launching a powerful rocket, Sputnik Omega, which showcased their advanced technology and disrupted American plans.

In response, the U.S. formed ARPA- Advanced Research Projects Agency, a new agency dedicated to rapidly developing cutting-edge technologies. One of ARPA's key achievements was creating ARPANET, a network that would eventually become the Internet. Despite the initial advantage of the Soviets, this technological breakthrough helped the U.S. regain its footing.

The war eventually ended in a stalemate, leading both nations to focus on peace and collaboration. The intense competition and innovation from this period laid the foundation for many modern technological advancements.

There were 4 Places where the computers were located in USA

-MIT

-UCLA

-UNIVERSITY OF UTAH

-STANFORD

They used TCP - Transmission control protocol for transferring messages or whatever required for them

Everything was functioning smoothly with ARPANET, but a problem soon arose. As a research project, ARPANET was primarily designed for sharing information within the scientific community. However, researchers began using it to exchange their papers and data across different institutions and locations.

For instance, if Stanford sent information about computers to MIT, including a link to detailed research, the link itself often couldn't be accessed or properly interpreted by the receiving end. This limitation became apparent as the network grew and the need for a more user-friendly system to manage and retrieve information became clear.

To address this issue, the World Wide Web (WWW) was developed. The WWW provided a more intuitive way to link and access documents across the network, making it easier for researchers to share and find information. With its creation, the WWW significantly improved the functionality of ARPANET, transforming it into a more effective tool for global information exchange.

TIM BERNERS-LEE developed WWW

Click to Know The first www is a collection of pages or documents identified by URL- Uniform Resource Locator and interlinked by hyperlinks and accessed over the internet.

But how do we search we require search engines so this starts the development of search engines. YAHOO was the first to develop search engines

What are Protocols?

Protocols are like a set of rules that one needs to follow

Who sets these rules?

THE INTERNET SOCIETY is the one that sets these rules you can check it out -> https://www.internetsociety.org/

Check out how we are connected with the whole world using cables under the sea -> https://www.submarinecablemap.com/

WHAT IS A SERVER?

A server is a computer system that provides resources, programs or data to other computers known as clients over the network.

image

Some of List of Protocols

-TCP -> Transmission Control Protocol

-UDP -> User Datagram Protocol

-HTTP -> Hyper text transfer protocol

-HTTPS -> Hyper text transfer protocol secure

IP-ADDRESS -> Internet Protocol Address

$curl ifconfig.me -s -> to check IP-Address of your computer

IP-Address -> Its basically an address of your computer connected to the internet.

A port is a 16-bit numbers. So the total number of port numbers we can have is 2^16

A port or port number is a number assigned to uniquely identify a connection endpoint and to direct data to a specific service.

There are certain port numbers that are fixed they are :

HTTP - port 80
SQL - port 1433
MongoDB - port 27017
Reserved Ports -[0-1023]
Registered for Applications -[1024-49152]
Remaining - Public use

1Mbps -> mega bits per second
1Gbps -> Giga bits per second

when we are sending data to someone its called UPLOAD

when we are accessing data from someone its called DOWNLOAD

WHAT ARE THE TYPES OF NETWORK

LAN- (LOCAL AREA NETWORK)

It's a network that covers a small area/office

It is used to connect to computers in the same building or campus

MAN -(METROPOLITAN AREA NETWORK)

It's a network used to connect across Cities

WAN -(WIDE AREA NETWORK)

It's a network used to connect across countries

SONET - (SYNCHRONOUS OPTICAL NETWORK)

FRAME RELAY -(Connecting LAN to the internet)

HOW ARE COMPUTERS CONNECTED?

TOPOLOGIES

SOME OF THE TOPOLOGIES ARE:

BUS
RING
STAR
MESH
TREE

BUS TOPOLOGY :

All computers are connected in a single line.
If one computer fails, the entire network connection is lost.
Only one computer can send data at a time.

RING TOPOLOGY:

Computers are connected in a circular loop.
If one computer fails, the whole network can be disrupted.
Data can take longer routes because it must pass through several computers to reach its destination.

STAR TOPOLOGY:

All computers are connected to a central hub or switch.
If the central hub fails, the entire network is lost.
It is easy to add new computers and manage the network.

MESH TOPOLOGY:

Every computer is connected to every other computer.
It is expensive to set up because of the many connections required.
Adding a new computer involves connecting it to every other computer.
There can be stability issues if not all connections are reliable.

TREE TOPOLOGY

Combines elements of Bus and Star topologies.

It has fault tolerance, meaning the network can continue to operate even if some parts fail.
It’s organized hierarchically, which helps in managing and expanding the network efficiently.

STRUCTURE OF NETWORK

THE MOST IMPORTANT CONCEPT ASKED IN INTERVIEWS OSI MODEL

OSI MODEL - Open System Interconnection

There are 7 layers

Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data Link layer
Physical layer

The easier way to remember is:

ALL - APPLICATION LAYER

PEOPLE - PRESENTATION LAYER

SHOULD - SESSION LAYER

TRY - TRANSPORT LAYER

NEW - NETWORK LAYER

DOMINOS - DATALINK

PIZZA - PHYSICAL LAYER

Application Layer - It provides services to the user

Presentation Layer - Takes care of the syntax and semantics of the information exchange between two communication systems.

Session Layer - It establishes, maintain , synchronize and terminate interaction between the sender and reciever.

Transport Layer - It responsible for process to process delivery

Network Layer - Responsible for delivery of individual packet from source to destination.

Data-Link Layer - Responsible for moving from one hop to next hop.

Physical Layer - Responsible for moving individual bits from one device to another.

The transport layer will break down the data into small segments and each segment will have its own ip-address and a sequencce number - sequence number is necessary in order to keep a track of the order of data.

The network layer will asign ip-address of sender and the reciever to every segments and it forms an ip packet . so that every data packet will reach the correct destination

IP-ADDRESS VS MAC ADDRESS

IP-ADDRESS -> it helps us to identify the device globally

MAC ADDRESS -> it helps us to identify the device locally

ANOTHER MODEL IS TCP/IP MODEL

It is known as Transmission Control Protocol / Internet Protocol also known as Internet Protocol Suite.

It consist of 5 layers

The presentation layer and session layer is combined into one

Layers are :

APPLICATION LAYER
TRANSPORT LAYER
NETWORK LAYER
DATA-LINK LAYER
PHYSICAL LAYER

It provides services to the user example: Applications such as watsup, facebook etc:

There are two types of Architecture:

Client-Server Architecture
Peer-to-Peer Architecture

Client-Server Architecture:

SOME OF THE IMP CONCEPTS:

Repeaters
Bridges
Hub
Gateway
Routers
Brouters

Some of Protocols:

HTTP -> hyper text transfer protocol

DHCP -> dynamic host control protocol

FTP -> File transfer Protocol

SMTP -> Simple mail Transfer Protocol

POP3 -> post-office-protocol

SSH -> Secure shell

VNC -> Virtual network control

telnet -> it uses port 23

UDP -> user Data gram protocol

Process vs thread

Process-> It's an instance of a program that is being execited.

Thread-> It's a segment of proccess.

Example: Opening a new browser (say Chrome) is an example of creating a process. At this point, a new process will start to execute. opening multiple tabs in the browser is an example of creating the thread.

HTTP METHODS :

GET -> Used for fetching data, appending parameters in the URL, ideal for searches.

POST-> Used for updates, sends data securely in the request body, perfect for forms

PUT -> to put the data at specific location

DELETE -> to delete the data from the server

STATUS CODE: It helps us to know whether our request is accepted or not

1xx -> Information related

2xx-> Successful

3xx->Redirection purpose

4xx-> client error

5xx-> Server error

Cookies - > Cookies are small pieces of text sent to your browser by a website you visit.

HOW EMAIL WORKS?

It works with the help of SMTP protocol

WHAT IS DNS?

DNS -> Domain Name System

DNS is like the phone book of the internet, making it easier to search for websites by using words instead of numbers.

You can check it out from the following sites:

rootservers.org
icann.org

TRANSPORT LAYER :

The transport layer is responsible for reliable data transfer between devices on the network. It ensures that the data packets are delivered error-free, in the correct sequence, and without loss or duplication. There are two main protocols at the transport layer

Transmission Control Protocol (TCP): Provides reliable, connection-oriented communication. It ensures error recovery, flow control, and retransmission of lost packets.

User Datagram Protocol (UDP): Provides unreliable, connectionless communication. It does not guarantee the delivery of packets or their order but is faster due to the lack of overhead for reliability.

Key Functions of the Transport Layer:

Segmentation and Reassembly: The transport layer divides large messages into smaller packets and puts them back together at the destination.

Error Control: It detects and handles errors that occur during transmission.

Flow Control: It regulates the rate of data transmission to prevent congestion.

Connection Establishment and Termination: In protocols like TCP, the transport layer sets up, maintains, and ends connections.

Linux Commands for Transport Layer:

netstat: Displays the network connections, routing tables, and network interface statistics. Use it to see active TCP connections.

ss: A more modern tool than netstat, which provides detailed information about TCP, UDP sockets.

telnet / nc (Netcat): Used to test connectivity on specific ports. For instance, testing if a TCP connection to a server is available.

1. netstat -tuln
2. ss -tuln
3. telnet <hostname> <port>

NETWORK LAYER:

Responsible for packet forwarding, routing, and addressing, including the use of IP addresses.

Linux Command: ping, traceroute, route, and ip commands are helpful for network diagnostics.


1. ping 192.168.1.1
2. traceroute www.google.com
3. ip route show

DATA LINK LAYER:

Ensures data transfer between adjacent network nodes and handles error detection and correction.

Linux Command: arp, ethtool, and iwconfig help in managing network interfaces.

1. arp -a
2. ethtool eth0

PHYSICAL LAYER:

Deals with the physical medium of data transmission like cables and network devices.

Linux Command: ifconfig or ip link can show the status of the network interfaces.

1. ifconfig
2. ip link show

Mastering Jenkins: A Step-by-Step Guide to Setting Up and Supercharging Your CI/CD Workflow

Hashim HB — Tue, 07 Jan 2025 20:13:05 +0000

What is Jenkins?

→ Jenkins is an open-source tool used to automate tasks in software development, mainly focusing on Continuous Integration (CI) and Continuous Delivery (CD).

→ It also helps developers automatically build, test, and deploy their code changes whenever they make updates.

Features of Jenkins:

CI/CD INTEGRATIONS
PIPELINES
EASY INSTALLATION
EASY CONFIGURATION
DISTRIBUTED
EXTENSIBLE
OPEN-SOURCE

How to Set Up Jenkins in your Local Machine?

☆ There are many ways in which can set up you can choose which suits you the best

✦ You can set up in Windows also but I am going to teach you how we can setup Jenkins locally with Oracle VirtualBox VM.

✦ First create a VM (Virtual Machine) using a Virtual Box and install the Linux (Ubuntu/CentOS) as your wish

✦ Open Terminal in any one of the following Linux Distros it might be different for UBUNTU you can check Jenkins documentation for that I will guide you for CentOS.

sudo wget -O /etc/yum.repos.d/jenkins.repo \ https://pkg.jenkins.io/redhat-stable/jenkins.repo

sudo rpm --import https://pkg.jenkins.io/redhat-stable/jenkins.io-2023.key
sudo yum upgrade

# Add required dependencies for the jenkins package
sudo yum install fontconfig java-17-openjdk
sudo yum install jenkins
sudo systemctl daemon-reload

✦ Start Jenkins by using systemctl

sudo systemctl enable jenkins
sudo systemctl start jenkins # to start jenkins
sudo systemctl status jenkins # to check the status if its running or not

✦ If everything went well, the beginning of the status output shows that the service is active and configured to start at boot(enabled)

✦ Now that Jenkins is up and running, let’s adjust our firewall rules so that we can reach it from a web browser to complete the initial setup.

IMPORTANT STEP :

✦ check if firewall is installed in the system

sudo yum install firewalld
sudo systemctl enable firewalld
sudo reboot

✦ Now everything is done now we can start using Jenkins

ifconfig

✦ By running this command we get the IP-Address of Linux system use this IP-Address to access the Jenkins dashboard in the web-browser by typing ipaddress:8080 and enter

✦ This will successfully Land you to this page:

✦ Use the terminal in your VM and run

sudo cat /var/lib/jenkins/secrets/initialAdminPassword

✦ Copy the 32-character alphanumeric password from the terminal and paste it into the Administrator password field on the Jenkins page. Then, click Continue.

✦ Click on Install suggested plugins

✦ It will Install the required plugins

✦ Once all the plugins are installed then:

✦ Fill in the required fields and proceed

✦ URL will be provided i have removed for security purposes just click on save and finish

✦ Now you have successfully done the setup for Jenkins

✦ This is what the Jenkins Dashboard Looks like

Now this is for the Linux system it will only run in Linux environment if we want to access in our local machine (Windows) you need to follow certain steps:

In Linux environment run the following commands:

sudo firewall-cmd --permanent --zone=public --add-port=8080/tcp
sudo firewall-cmd --reload

✦ Both the commands should give SUCCESS or else you have done something wrong

Setting Up Port Forwarding

Open VirtualBox VM settings.
Navigate to the NAT network adapter settings.
Click Advanced and then Port Forwarding.
Add a new rule named "Jenkins".
Set Protocol to TCP.
Enter Host IP as 127.0.0.1 and Host Port as 8080.
Guest Port should also be set to 8080.
Click OK to save the settings.

After this step→ Open your browser and go to 127.0.0.1:8080 to access Jenkins, you will land on the getting started Jenkins page:

✦ Enter the Username and Password which you created Before and there you go

✦ You have Successfully setup Jenkins in your Local Machine

How to create Jenkins first Job?

✦ Click on → Create a Job

✦ Then give a name for the job and select freestyle Project as we are just starting off.

✦ You will get a page something like this:

✦Scroll down to Build Steps:

✦Choose the option what you like

✦We will choose Execute Shell and write the command that you want to execute and save

✦ Now just click on Build Now and that it you have successfully created your first job in Jenkins

✦ After the job is created click on the job and get to see the following:

✦ This was the output and we successfully created and saw the output of the command in Jenkins

✦ We can even see the number of jobs that we have created

✦ This was all about Jenkins Hope You guys like it.