Learn Java

What is Java?

Java is a high-level, class-based programming language first released in 1995 and now maintained by Oracle. Its guiding idea is "write once, run anywhere": you compile your source code a single time, and the resulting program can run on any device that has a Java runtime installed, regardless of the underlying hardware or operating system.

Because it is strongly typed and object-oriented, Java encourages clean, well-organized code. It powers a huge share of the software running today, which makes it a dependable first language to learn and a valuable skill for a long-term career.

Where Java is used

• Android mobile applications
• Large-scale business and banking systems
• Web servers and backend APIs
• Desktop applications with graphical interfaces
• Big data tools and scientific computing
• Embedded devices and smart cards

Why learn Java?

Java runs on billions of devices and is consistently one of the most in-demand languages in the job market. It is portable across platforms, has a massive collection of ready-made libraries, and is backed by a large, active community, so help and documentation are never far away.

How Java runs your code

You write source code in a file ending with .java. The Java compiler turns that source into bytecode, an intermediate format stored in a .class file. The Java Virtual Machine (JVM) then reads the bytecode and executes it on the current machine. This two-step process is what lets the same compiled program run unchanged on Windows, macOS, and Linux.

A first look

Here is a complete Java program that prints a message to the screen. Do not worry about every detail yet; the following lessons explain each part step by step.

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