Module 3 — Java Basics: Syntax, Data Types & Variables

🎯 Learning Objectives

By the end of this module, you will be able to:

• Understand Java’s strict syntax rules and why they exist
• Declare variables using the correct data types
• Distinguish between primitive types and reference types
• Understand why Java requires you to declare types explicitly
• Know the default values of primitive types
• Apply implicit widening and explicit narrowing type casting
• Recognise common pitfalls: integer overflow and floating point precision

1. Java is a Statically Typed Language

In JavaScript, you can write:

let message = "Hello";
message = 42; // perfectly fine in JS

In Java, every variable has a fixed type decided at compile time. Once you declare a variable as a String, it stays a String forever.

String message = "Hello";
message = 42; // ❌ Compile error — type mismatch

This strictness feels like extra work at first, but it catches entire categories of bugs before your program even runs.

2. Primitive Data Types

Java has 8 built-in primitive types. These are not objects — they hold raw values directly in memory.

💡 Most of the time you’ll use: int, double, boolean, char, and String.

Declaring Primitive Variables

int age = 25;
double price = 19.99;
boolean isLoggedIn = false;
char grade = 'A';       // Note: single quotes for char

3. Reference Types

Everything that is not a primitive is a reference type — it stores a reference (memory address) to an object, not the value itself.