DEV Community: rohit juyal

# 📘 Java Basics – More of theory part.

rohit juyal — Sun, 28 Sep 2025 07:54:51 +0000

Date: 28 Sept 2025

Welcome back to my Java learning journey!
I am not counting days for blogging as it was creating a unnecessary pressure on me to blog daily. I know it's good but with job it's hard to do coding and most of my time was going into blogging which I could have diverted into coding. Blogging is not waste of time, I learn a lot when I have to explain the topic in my own words. I am just limiting it to once in a week or so.

🔹 Variables in Java

A variable is a name given to a memory location that stores data.

Stored in RAM during execution.
We must define its data type to indicate what kind of value it will hold.
First, we declare a variable, then we may initialise it.

int a;      // declaration
a = 10;     // initialisation
int b = 20; // declaration + initialisation

✅ A variable can only be declared once, but its value can be changed anytime.

🔹 Data Types in Java

Java has two categories of data types:

Primitive (8 types)
Non-Primitive (String, Arrays, Classes, etc.)

🟢 1. Primitive Data Types

These are the most basic types in Java, predefined by the language.

A. Whole Numbers

byte
short
int
long

B. Real Numbers (Decimals)

float
double

C. Single Character

char

D. Logical Values

boolean

⚠️ Each primitive data type has a predefined range. For example:

byte b = 127;   // max value
byte c = -128;  // min value

🟢 2. Non-Primitive Data Types

These are built using primitive types. Examples:

String
Array
Class

📌 Note:

"" → String literal
'' → Char literal (cannot be empty)

🔹 Literals

A literal is simply a fixed value that appears exactly as written.

Example: 10, "Hello", 'A', true.

👉 String is a data type, and it just means text.

🔹 Statement vs Expression

Expression → Anything that produces a value.
Statement → A complete command that can be executed (usually ends with ;).

Examples:

3 + 4        // Expression → produces 7
int x = 3+4; // Statement → assigns 7 to variable x
"Hello"      // String literal → an expression

🔹 Keywords

Keywords are reserved words in Java with predefined meaning.
Examples: class, public, static, void, int, if, else.

❌ You cannot use keywords as variable names.

🔹 Output in Java

Using System.out.println()

println("Hello") → prints text and moves to the next line.
println() → prints a blank line.

Example:

System.out.println("Java is fun!");
System.out.println(); // blank line
System.out.println("Let's learn more...");

🔹 Playing with JShell

JShell is an interactive tool for running Java code without creating a full class.

Some useful commands:

/list → shows commands entered so far
/var → lists variables created
ctrl + c → cancels a process
/exit → exits JShell

📌 In JShell, each line is treated as a statement, so ; is optional.

📝 Wrap-up

Today I learned:
✔ What variables are and how to declare/initialize them
✔ Primitive vs Non-primitive data types
✔ Statements vs Expressions
✔ What literals and keywords mean
✔ How to use System.out.println() for output
✔ Some handy JShell commands

Self Notes

Coding is a serious thing, in last two days I wasted much of my time in other activities. I have realised my mistake and now I will code daily. I am already short of time and wasting time isn't good. I won't be scrolling social media now on.

Hello Java

rohit juyal — Thu, 25 Sep 2025 09:32:11 +0000

Java Learning Journey: Day 02 ✨

Date: 25 Sept, 2025

Hello everyone 👋 Today I wrote my very first Java program:

public class Hello {
    public static void main(String[] args) {
        System.out.print("Hello java");
    }
}

🔎 Breaking Down the Code

Semicolon `;`

Tells the compiler (javac) that one instruction has ended.

Methods (Functions)

A method performs a specific action.
Methods can be predefined (like System.out.print()) or user-defined.
We can pass data into methods called as arguments.

👉 Example here:

System.out.print("Hello java");

Prints the string "Hello java".
"Hello java" is a string literal (just text).

Curly Braces `{ }`

Used for grouping code together into a block.

Classes

In Java, everything lives inside a class.
Hello is our class name.
Convention: class names start with a capital letter.

💡 Important:

If a class is declared public, the filename must match the class name.
Public class → Hello.java ✅
Non-public class → filename can be anything (but best practice is to match).
No matter what it will name bytecode file same as class name.
Remember:
We compile files (javac Hello.java)
We run classes (java Hello)

The `main` Method

public static void main(String[] args) { }

This is special in Java:

Entry point of the program.
Execution starts here.
Must be public → JVM can access it.
Must be static → JVM can call it without creating an object.
void → no return value.
String[] args → command-line arguments (will learn later).

👉 Without the main method, the program compiles fine, but the JVM can’t run it.

`public`, `static`, `void` — Quick Notes

public → anyone can access it.
static → belongs to the class, not an object (deep dive later).
void → no return value.

🖥️ Running the Program

Compile the program

javac Hello.java
ls

Output:

Hello.java  Hello.class

Run the program

java Hello

Output:

Hello java

⚡ Bonus: JShell

I also explored JShell, a REPL (Read–Eval–Print–Loop) included in JDK.

Read → takes your input
Evaluate → runs it
Print → shows the result
Loop → waits for more commands

👉 JShell is not an IDE. Its main purpose is quick testing, not for full development.

Example in JShell

jshell> System.out.println("Hello from JShell!");

Output:

Hello from JShell!

✨ Takeaway

Wrote and ran my first Java program 🎉
Got basic introduction to some keywords.
Learned how to compile/run from terminal.
Got introduced to JShell for quick testing.

Next up: For tomorrow, I will be doing front-end basics.🚀

Self notes

I am happy that I am better than yesterday but it is long jouney ahead. Today day 2 I did my best with what time I got, as I was busy in some personal stuff. I wasted much time in choosing ide, later realising it doesn't matter. Somethings are better learned in hard way. I will be careful in future. Above notes are what I understand about the topics, they may be wrong. Thanks for reading.

Day 01 Theory behind java

rohit juyal — Wed, 24 Sep 2025 18:56:23 +0000

My Java Learning Journey: Day 01 🚀

Hi everyone! 👋 I’m Rohit, currently preparing to become a full-stack developer. My plan is to learn Java for backend and JavaScript for frontend, alternating between the two on different days. On holidays, I’ll dedicate time to building projects.

This post marks Day 01 of my Java journey. Today I focused more on the theory — understanding the big picture of how Java works.

How a Java Program Runs ⚙️

When we write and run a Java program, it undergoes following steps:

a. Writing source code

We write the program in Java and save it with a .java extension.
A project can have multiple .java files.

b. Compiling with javac

Java source code is compiled using the Java compiler (javac).
It checks for errors and converts the code into bytecode.
This bytecode is saved in a .class file.
The bytecode is platform-independent, meaning it can run on any system.

c. Running with JVM

The Java Virtual Machine (JVM) takes the bytecode and executes it.
The JVM is platform-dependent (different for different OS and hardware) but the bytecode remains the same.
This makes Java portable.

JDK vs JRE vs JVM 🔑

This part was a little tricky, but here’s how I understood it:

JDK (Java Development Kit)
Contains everything needed to develop Java applications: compiler (javac), JRE, and other tools.
👉 We developers install this.
JRE (Java Runtime Environment)
Provides the environment to run Java applications. Includes JVM, libraries, and other runtime tools.
👉 End-users typically need this.
JVM (Java Virtual Machine)
Executes the bytecode. It’s like the “engine” that makes Java programs run.
👉 Different for each OS, but invisible to us as developers.

Key Features of Java 🌟

Here are some features that stood out to me:

a. Write Once, Run Anywhere

Compile once, run on any platform with JVM.

b. Backward Compatibility

New updates don’t break older code.

c. Security

We share bytecode, not the source code.
JVM restricts resource access (like RAM, storage) for safety.
If a program crashes, only the JVM crashes — not the whole system.

d. Multi-threading

Supports running multiple tasks at the same time.
Threading means running multiple tasks concurrently. Java implements software multi-threading, creating many threads in your program.
The CPU can only execute as many tasks truly in parallel as it has hardware threads (physical cores × threads per core).
If there are more software threads than hardware threads, the OS time-slices them so all threads appear to run at the same time.

e. Object-Oriented Programming

Data is stored in form of object and classes.
This helps in many things like reusability which I will learn later.

Final Thoughts ✨

Today was all about understanding the ecosystem — how Java runs and what makes it powerful. I did write the code but time was limited to share it here. Kindly pardon for any error.

Thanks for reading! Please do suggest any improvement 💻🚀

DEV Community: rohit juyal

# 📘 Java Basics – More of theory part.

🔹 Variables in Java

🔹 Data Types in Java

🟢 1. Primitive Data Types

A. Whole Numbers

B. Real Numbers (Decimals)

C. Single Character

D. Logical Values

🟢 2. Non-Primitive Data Types

🔹 Literals

🔹 Statement vs Expression

🔹 Keywords

🔹 Output in Java

🔹 Playing with JShell

📝 Wrap-up

Self Notes

Hello Java

Java Learning Journey: Day 02 ✨

🔎 Breaking Down the Code

Semicolon ;

Methods (Functions)

Curly Braces { }

Classes

The main Method

public, static, void — Quick Notes

🖥️ Running the Program

Compile the program

Run the program

⚡ Bonus: JShell

Example in JShell

✨ Takeaway

Self notes

Day 01 Theory behind java

My Java Learning Journey: Day 01 🚀

How a Java Program Runs ⚙️

JDK vs JRE vs JVM 🔑

Key Features of Java 🌟

Final Thoughts ✨

Semicolon `;`

Curly Braces `{ }`

The `main` Method

`public`, `static`, `void` — Quick Notes