Java Syntax

The structure of a Java program

Every Java program is built from classes, and every statement lives inside a method that lives inside a class. When you run a program, Java looks for a special method called main and starts there. Understanding this skeleton makes every later lesson easier.

public class Main {
  public static void main(String[] args) {
    System.out.println("Hello, World!");
  }
}

Breaking down the pieces

• public class Main declares a class named Main; the file must be saved as Main.java
• public static void main(String[] args) is the entry point where execution begins
• System.out.println(...) prints a line of text to the console
• Curly braces { } group the code that belongs together into a block
• The semicolon ; marks the end of each statement

Statements and blocks

A statement is a single instruction, and in Java it almost always ends with a semicolon. A block is a group of statements wrapped in curly braces. Blocks define where a class or method begins and ends, and they can be nested inside one another.

Whitespace and indentation

Java ignores extra spaces and blank lines, so indentation does not change how a program behaves. However, consistent indentation makes code far easier to read, and by convention each nested block is indented one level deeper than its parent.

public class Main {
  public static void main(String[] args) {
    System.out.println("Line one");
    System.out.println("Line two");
    System.out.println("Line three");
  }
}