Read through this page to understand the rules. Then as you write your first programs, refer back when something doesn't compile. Within a few weeks, correct Java syntax will feel completely natural.

The Basic Structure of Every Java Program

Every Java program — from the simplest Hello World to a production server — follows this same skeleton:

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public class ClassName {

    public static void main(String[] args) {
        // statements go here
    }

}

Three nested layers, every time:

›Class — the outermost container. Everything lives inside a class.
›Method — a named block of code inside the class. main is the entry point.
›Statements — the actual instructions inside the method.

Rule 1 — Java is Case-Sensitive

Java treats uppercase and lowercase letters as completely different. This applies to everything — class names, method names, variable names, and keywords.

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// These are all different things to Java:
String name = "Arjun";   // ✅ correct
string name = "Arjun";   // ❌ error — 'string' doesn't exist
String Name = "Arjun";   // different variable from 'name'

// These method calls are all different:
System.out.println("Hello");   // ✅ correct
system.out.println("Hello");   // ❌ error — 'system' doesn't exist
System.out.Println("Hello");   // ❌ error — 'Println' doesn't exist
System.out.printLn("Hello");   // ❌ error — 'printLn' doesn't exist

⚠️ Case sensitivity is the #1 cause of beginner compile errors. When your code doesn't compile and you can't see why — check capitalization first.

Rule 2 — Every Statement Ends with a Semicolon

A statement is a single instruction. In Java, every statement ends with a semicolon ;.

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System.out.println("Hello");        // ✅ statement with semicolon
int age = 25;                        // ✅ statement with semicolon
String name = "Arjun";              // ✅ statement with semicolon

Forget the semicolon and you get a compile error:

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System.out.println("Hello") // ❌ error: ';' expected

💡 Think of the semicolon as a period at the end of a sentence. Just as English sentences end with a period, Java statements end with a semicolon. It tells the compiler: "this instruction is complete."

What does NOT need a semicolon:

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public class HelloWorld {           // ← no semicolon — class declaration
    public static void main(...) {  // ← no semicolon — method declaration
        System.out.println("Hi");   // ← semicolon — this is a statement
    }                               // ← no semicolon — closing brace
}                                   // ← no semicolon — closing brace

The rule is: statements get semicolons. Declarations with {} blocks do not.

Rule 3 — Curly Braces Define Blocks

Blocks are sections of code enclosed in curly braces { }. Every class and every method has a block.

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public class Example {          // opens class block
    public static void main(String[] args) {  // opens method block
        System.out.println("Inside the method block");
    }                           // closes method block
}                               // closes class block

Rules for braces:

›Every { must have a matching }
›Blocks can be nested — methods inside classes, if-statements inside methods
›The content inside a block is indented (not required by Java, but required for readability)

A missing or extra brace is one of the most common compile errors:

Java

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public class Example {
    public static void main(String[] args) {
        System.out.println("Hello");
    }
// ← missing closing } for class

Error: error: reached end of file while parsing

💡 Brace matching tip: Most code editors highlight matching braces when your cursor is on one. In VS Code and IntelliJ, clicking on a { highlights its matching }. Use this constantly when debugging.

Rule 4 — The Class Name Must Match the Filename

In Java, the public class name must exactly match the filename — including capitalization.

File: HelloWorld.java  →  class must be named HelloWorld
File: BankAccount.java  →  class must be named BankAccount
File: myclass.java  →  class must be named myclass

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// File: HelloWorld.java
public class HelloWorld {      // ✅ names match
    ...
}

// File: HelloWorld.java
public class helloworld {      // ❌ error — 'helloworld' ≠ 'HelloWorld'
    ...
}

Error: class helloworld is public, should be in a file named helloworld.java

Rule 5 — Whitespace is Ignored (Mostly)

Java ignores extra spaces, tabs, and blank lines between tokens. This means you control your code's visual layout freely — the compiler doesn't care.

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// These are identical to the compiler:

// Compact:
public class A{public static void main(String[] args){System.out.println("Hi");}}

// Readable:
public class A {
    public static void main(String[] args) {
        System.out.println("Hi");
    }
}

Both compile and run identically. But only one is readable.

Exception — whitespace inside Strings:

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System.out.println("Hello World");    // prints: Hello World
System.out.println("Hello  World");   // prints: Hello  World (two spaces)

Whitespace inside String literals is preserved exactly.

Naming Rules — What You Can and Cannot Name Things

Java has strict rules for identifiers — the names you give to classes, methods, and variables.

Legal identifier rules:

›Can contain letters, digits, underscores _, and dollar signs $
›Must not start with a digit
›Must not be a Java keyword (like class, int, if, return)
›No spaces allowed

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// ✅ Valid identifiers:
int age = 25;
String firstName = "Arjun";
double _salary = 50000.0;
int count1 = 0;

// ❌ Invalid identifiers:
int 1count = 0;        // starts with digit
String my name = "";   // contains space
int class = 5;         // 'class' is a keyword
double my-salary;      // hyphen not allowed

Naming Conventions — How Java Developers Actually Name Things

Beyond the legal rules, Java has strong conventions — community-agreed styles that make code readable. The compiler doesn't enforce these, but every Java developer follows them.

Classes — PascalCase

Every word starts with an uppercase letter. No underscores.

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public class HelloWorld { }
public class BankAccount { }
public class StudentGradeCalculator { }

Variables and Methods — camelCase

First word lowercase, every subsequent word starts uppercase. No underscores.

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int age = 25;
String firstName = "Arjun";
double accountBalance = 5000.0;

public void calculateTax() { }
public String getStudentName() { }

Constants — UPPER_SNAKE_CASE

All uppercase, words separated by underscores. Used with static final.

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static final int MAX_SIZE = 100;
static final double PI = 3.14159;
static final String APP_NAME = "DevStackFlow";

Packages — all lowercase

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package com.devstackflow.utils;
package org.example.models;

Comments — Ignoring Code

Comments are lines the compiler completely ignores. They exist for humans reading the code.

Single-line comment

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// This is a single-line comment
int age = 25;  // you can also put comments at the end of a line

Multi-line comment

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/*
   This is a multi-line comment.
   It can span as many lines as you need.
   The compiler ignores everything between the markers.
*/
int salary = 50000;

Javadoc comment

Used to generate documentation automatically from your code:

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/**
 * Calculates the area of a rectangle.
 * @param width  the width of the rectangle
 * @param height the height of the rectangle
 * @return the area as a double
 */
public double calculateArea(double width, double height) {
    return width * height;
}

💡 When to comment: Don't comment what the code obviously does (// adds 1 to count). Comment why you made a decision, or explain something non-obvious. Good code mostly explains itself through clear naming.

Keywords — Words Reserved by Java

Java has 67 reserved keywords — words with specific meaning that you cannot use as identifiers. Here are the most important ones you'll use regularly:

Category	Keywords
Data types	`int`, `double`, `boolean`, `char`, `long`, `float`, `byte`, `short`
Class & OOP	`class`, `interface`, `extends`, `implements`, `new`, `this`, `super`
Access	`public`, `private`, `protected`
Control flow	`if`, `else`, `switch`, `case`, `for`, `while`, `do`, `break`, `continue`, `return`
Other common	`static`, `final`, `void`, `null`, `true`, `false`, `import`, `package`

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// ❌ You cannot use keywords as variable names:
int class = 5;      // error — 'class' is a keyword
String return = ""; // error — 'return' is a keyword
boolean true = false; // error — 'true' is a keyword

A Complete Example — All Rules Applied

Here's a program that demonstrates every syntax rule covered on this page:

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// File: BankAccount.java

public class BankAccount {           // PascalCase class name matches filename

    // Constant — UPPER_SNAKE_CASE
    static final double INTEREST_RATE = 0.05;

    // Instance variables — camelCase
    String accountHolder;
    double balance;

    // Method — camelCase, returns void
    public void deposit(double amount) {
        balance = balance + amount;   // statement ends with ;
        System.out.println("Deposited: " + amount);
        System.out.println("New balance: " + balance);
    }

    // Entry point
    public static void main(String[] args) {
        BankAccount account = new BankAccount();
        account.accountHolder = "Arjun";
        account.balance = 1000.0;

        account.deposit(500.0);

        System.out.println("Account holder: " + account.accountHolder);
        System.out.println("Interest rate: " + INTEREST_RATE);
    }

}

Output:

Deposited: 500.0
New balance: 1500.0
Account holder: Arjun
Interest rate: 0.05

Every concept from this page is used here — PascalCase class, camelCase methods and variables, UPPER_SNAKE_CASE constant, semicolons on statements, braces on blocks, and comments throughout.

Common Syntax Mistakes — Quick Reference

Mistake	Example	Fix
Missing semicolon	`System.out.println("Hi")`	Add `;` at the end
Wrong capitalization	`system.out.println(...)`	Use `System.out.println(...)`
Mismatched braces	`public class A { void m() { }`	Add closing `}` for class
Filename ≠ class name	File: `hello.java`, class: `Hello`	Match exactly: `Hello.java`
Keyword as identifier	`int class = 5;`	Use a non-keyword name
Starting name with digit	`int 1count = 0;`	Start with a letter: `int count1`
Space in identifier	`String my name;`	Remove space: `String myName;`

The 3 Things to Remember From This Page

›Every statement ends with ; — declarations with {} do not. This single rule eliminates the most common category of compile errors.
›Java is fully case-sensitive — System, String, println must be capitalized exactly right. When code won't compile and you can't see why, check capitalization first.
›Follow naming conventions — PascalCase for classes, camelCase for variables and methods, UPPER_SNAKE_CASE for constants. The compiler doesn't enforce these, but every Java developer follows them.

Summary

›Every Java program has three nested layers: class → method → statements
›Java is completely case-sensitive — String and string are different things
›Every statement ends with a semicolon ; — class and method declarations with {} do not
›Curly braces { } define blocks — every opening { needs a matching closing }
›The public class name must exactly match the filename including capitalization
›Extra whitespace (spaces, blank lines) is ignored by the compiler — use it freely for readability
›Identifiers cannot start with a digit, cannot contain spaces, and cannot be Java keywords
›Naming conventions: PascalCase for classes, camelCase for variables and methods, UPPER_SNAKE_CASE for constants
›Comments (// and /* */) are ignored by the compiler — use them to explain your code