rohit juyal

Posted on Sep 28

# 📘 Java Basics – More of theory part.

#beginners #devjournal #java

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.

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