Java Syntax

Every Java program follows this skeleton:

java Run in compiler
// 1. Optional package declaration
package com.codekilla;

// 2. Optional imports
import java.util.Scanner;

// 3. Class declaration — file MUST be named ClassName.java
public class Demo {

    // 4. Main method — entry point
    public static void main(String[] args) {
        // 5. Statements go here
        System.out.println("Hi!");
    }
}

A statement is one instruction. Every Java statement ends with a semicolon ;.

java
int x = 10;                          // statement 1
System.out.println(x);               // statement 2

Forgetting ; is the #1 beginner error: ';' expected.

Code surrounded by { } is a block. Classes, methods, loops, ifs — all use blocks.

java
if (x > 0) {
    System.out.println("positive");
    System.out.println("number");
}

Three flavours:

java
// Single-line comment

/* Multi-line
   comment */

/** Javadoc — used by IDEs and the `javadoc` tool to generate HTML docs.
 *  @param args command-line arguments
 */

Java is case-sensitive:

java
int Age = 21;
System.out.println(age);  // ERROR — 'age' is not declared (it's 'Age')

• Start with a letter, $, or _: score, _total, $value, userName.
• Can contain letters, digits, _, $: score1, total_amount.
• Cannot start with a digit: ❌ 1score.
• Cannot be a keyword: ❌ int, ❌ class, ❌ return.

abstract, boolean, break, byte, case, catch, char, class, const, continue, default, do, double, else, enum, extends, final, finally, float, for, if, implements, import, instanceof, int, interface, long, new, null, package, private, protected, public, return, short, static, super, switch, this, throw, throws, try, void, volatile, while.

java
System.out.print("Hello ");          // no newline
System.out.println("Codekilla!");    // adds newline
System.out.printf("%s is %d%n", "Java", 30);  // C-style printf

java
import java.util.Scanner;

Scanner sc = new Scanner(System.in);
System.out.print("Age? ");
int age = sc.nextInt();
System.out.println("Hello, " + age + "-year-old!");

java Run in compiler
import java.util.Scanner;

public class Greet {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Your name: ");
        String name = sc.nextLine();
        System.out.print("Your age: ");
        int age = sc.nextInt();
        System.out.printf("Hello %s, you are %d!%n", name, age);
    }
}

Output (input Aarav, 21):

Your name: Aarav
Your age: 21
Hello Aarav, you are 21!

• Missing semicolon — int x = 10 without ; triggers ';' expected.
• File name mismatch — public class Greet must live in Greet.java.
• Calling System.out.println outside a method — must be inside main or another method.
• Forgetting import — using Scanner without import java.util.Scanner; → cannot find symbol.
• Mismatched braces — every { needs a }. The compiler reports far from the actual mistake.

1. Sum two numbers — Read two ints with Scanner and print their sum. Hint: sc.nextInt().
2. Fix the bugs — System.out.println("Hello") (missing ;) and Scanner sc = Scanner(System.in); (missing new). Hint: read the compiler messages.
3. Echo your name — Read a String and print Hello, NAME!. Hint: nextLine() reads a full line including spaces.
4. Comment cleanup — Take any 10-line Java snippet and add // comments explaining each line. Hint: comments make code readable for the next developer.

💡 Think Like a Programmer: Syntax is the grammar of code. Get it wrong and the compiler refuses to talk to you. Get it right and the compiler becomes your friendliest teacher — its error messages will guide every fix.