Codecademy will be down from 00:00 to 02:00 ET on November 15, 2019 for scheduled database maintenance. This will affect the website, learning environment, forums, and mobile app. We will provide updates as necessary.
Cheat Sheets are in BETA. Give us your feedback.

Java Classes

In Java, a class represents a single concept.

A Java program must have one class whose name is the same as the program filename.

In the example, the Person class must be declared in a program file named

public class Person { public static void main(String[] args) { System.out.println("I am a person, not a computer."); } }

main() Method in Java

In Java, every application must contain a main() method, which is the entry point for the application. All other methods are invoked from the main() method.

The signature of the method is public static void main(String[] args) { }. It accepts a single argument: an array of elements of type String.

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

Print Line in Java

In Java, System.out.println() can print to the console:

  • System is a class from the core library provided by Java
  • out is an object that controls output
  • println() is a method associated with that object that receives a single argument
System.out.println("Hello, world!"); // Output: Hello, world!

Comments in Java

In Java, comments are bits of text that are ignored by the compiler. They are used to increase the readability of a program.

Single line comments are made by using // and multi-line comments are made by starting with /* and ending with */.

// I am a single line comment! /* I am a multi-line comment! */

Whitespace in Java

Whitespace, including spaces and newlines, between statements is ignored.

System.out.println("Example of a statement"); System.out.println("Another statement"); // Output: // Example of a statement // Another statement

Compiling in Java

In Java, when we compile a program, each individual class is converted into a .class file, which is known as byte code.

The JVM (Java virtual machine) is used to run the byte code.

# Compile the class file: javac # Execute the compiled file: java first


In Java, a statement is a line of code that executes a task and is terminated with a ;.

System.out.println("Hello World"); System.out.println("Java Programming Language");