Java Fundamentals – Getting Started

Java is a programming language.

A programming language is a way for humans to give instructions to a computer. Think of it like learning a new language — but instead of talking to people, you're talking to machines.

Computers do nothing by themselves. They only follow instructions exactly as written. Your job is to write those instructions clearly.

Java is designed so that:

• Your code runs on any computer — Windows, Mac, Linux, Android
• Errors are caught before your program runs (not during)
• Large teams can work on the same codebase without chaos

Java is used for:

• Android apps (Instagram, Spotify were built with Java)
• Banking and financial systems (nearly every bank uses Java)
• Web servers and APIs (Netflix, Amazon, LinkedIn)
• Big data processing (Hadoop, Apache Spark)
• Enterprise software and government systems
• Game development (Minecraft was written in Java!)

A program is simply a list of instructions that a computer follows, one by one, from top to bottom.

Example in human language:

1. Show a message on the screen
2. Add two numbers together
3. Display the result

In Java, this looks like:

System.out.println("Let me add some numbers...");
int result = 5 + 3;
System.out.println("5 + 3 = " + result);

Don't worry about understanding every word yet. The key idea is: each line is one instruction, and Java runs them in order, top to bottom.

What does System.out.println() do?

System.out.println() tells Java: "Show this on the screen."

It's the most basic way to make Java display something to you.

Example:

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

Let's break this down piece by piece:

• System → a built-in Java class that gives you access to system functions
• out → the output stream (where text goes — usually your screen)
• println → "print line" — displays text and moves to the next line
• ("Hello, World!") → the text you want to display (must be in quotes!)
• ; → the semicolon at the end — every statement in Java must end with one

System.out.print("Hello ");
System.out.print("World");
// Output: Hello World  (all on one line)

This is one of the most important things to understand early:

Java must know what is text and what is code.

Text MUST be wrapped in double quotes. Without quotes, Java gets confused.

This works:

System.out.println("Hello");   // ✅ Correct

This does NOT work:

System.out.println(Hello);     // ❌ Error!

Why does this fail?

Without quotes, Java thinks Hello is a variable name (a container for data). But no variable called Hello exists yet — so Java throws an error:

error: cannot find symbol
    symbol: variable Hello

In programming, text is called a "String":

"Hello"              // This is a String
"I love Java!"       // This is a String
"12345"              // This is also a String (not a number!)
""                   // This is an empty String

Important: "123" is a String (text), but 123 (no quotes) is a number. They are completely different things in Java.

In Java, every instruction (called a statement) must end with a semicolon ;

Think of it like a period at the end of a sentence. Without it, Java doesn't know where one instruction ends and the next begins.

System.out.println("Line 1");   // ✅ Good
System.out.println("Line 2");   // ✅ Good
System.out.println("Line 3")    // ❌ Missing semicolon!

What happens without a semicolon?

The compiler gives you an error like:

error: ';' expected

This is the #1 most common error for beginners. If you see this error, just add the missing ; at the end of the line.

public class MyApp {              // No semicolon here
    public static void main(String[] args) {  // No semicolon here
        System.out.println("Hi");  // Semicolon here ✅
    }
}

Curly braces { } group code together into blocks. Every opening brace { must have a matching closing brace }.

public class HelloWorld {          // Opens the class
    public static void main(String[] args) {  // Opens the method
        System.out.println("Hi");  // Inside both blocks
    }                              // Closes the method
}                                  // Closes the class

Comments are notes you leave in your code. Java completely ignores them — they exist only for you and other developers to read.

// This is a single-line comment
System.out.println("Hello");  // You can also put them after code

/* This is a multi-line comment.
   It can span as many lines
   as you need. */

/** This is a Javadoc comment.
 *  Used to generate documentation.
 *  You'll use these when you're more advanced.
 */

When should you use comments?

• To explain why you did something (not what the code does)
• To leave notes for yourself or teammates
• To temporarily disable code while testing

// print hello
System.out.println("Hello");

// Greet user before showing menu
System.out.println("Hello");

Java is different from languages like Python or JavaScript. Java needs two steps to run:

HelloWorld.java  →  javac  →  HelloWorld.class  →  JVM  →  Output
   (your code)     (compile)    (bytecode)        (run)   (screen)

Errors are completely normal. Even experienced developers get errors constantly. The key is learning to read them.

Error 1: Missing semicolon

System.out.println("Hello")   // forgot the ;

HelloWorld.java:3: error: ';' expected
        System.out.println("Hello")
                                   ^

Java tells you: the file, the line number (3), and exactly where the problem is (^). Just add the ;!

Error 2: Wrong class name

// File is called HelloWorld.java but code says:
public class Helloworld {   // lowercase 'w' — doesn't match!

error: class Helloworld is public, should be declared
in a file named Helloworld.java

Fix: make sure the class name matches the filename exactly (including uppercase/lowercase).

Error 3: Wrong capitalization

system.out.println("Hello");   // lowercase 's'!

error: package system does not exist

Fix: it's System with a capital S. Java is case-sensitive!