Java Program Structure

Java programs have a specific structure. Let's take a closer look at the Hello World program — line by line!

Java programs have a specific structure in how the code is written. There are key elements that all Java programs share.

The Program

We have the text of a program inside the file called HelloWorld.java.

// This program outputs the message "Hello World!" to the monitor

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

This program writes Hello World! to your terminal when run.

Case-Sensitivity

Java is a case-sensitive language. Case sensitivity means that syntax, the words our computer understands, must match the case. For example, the Java command for outputting text to the screen is System.out.println(). If you were to type system.out.println() or System.Out.println(), the compiler would not know that your intention was to use System or out.

Let’s go over this HelloWorld.java program line by line:

Comments

// This program outputs the message "Hello World!" to the monitor

This is a single-line comment that documents the code. The compiler will ignore everything after // to the end of the line. Comments provide information outside the syntax of the language.

Classes

public class HelloWorld { 
  // class code
}

This is the class of the file. All Java programs are made of at least one class. The class name must match the file: our file is HelloWorld.java and our class is HelloWorld. We capitalize every word, a style known as pascal case. Java variables and methods are named in a similar style called camel case where every word after the first is capitalized.

The curly braces, { and }, mark the scope of the class. Everything inside the curly braces is part of the class.

Methods

  public static void main(String[] args) {
   // Statements
  }

Every Java program must have a method called main(). A method is a sequence of tasks for the computer to execute. This main() method holds all of the instructions for our program.

Statements

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

This code uses the method println() to send the text “Hello World!” to the terminal as output. println() comes from an object called out, which is responsible for various types of output. Objects are packages of state and behavior, and they’re often modeled on real-world things.

out is located within System, which is another object responsible for representing our computer within the program! We can access parts of an object with a ., which is known as dot notation.

This line of code is a statement, because it performs a single task. Statements always conclude with a semicolon.

Whitespace

Java programs allow judicious use of whitespace (tabs, spaces, newlines) to create code that is easier to read. The compiler ignores whitespace, but humans need it! Use whitespace to indent and separate lines of code. Whitespace increases the readability of your code.

Practice

The structure of a Java program will feel familiar the more you work with this language. Continue learning at Codecademy and you’ll be a Java pro in no time!