Lesson 0 - How does Java work?

Java intro

What do you need to know?

• In Java, your code must reside inside a class (meaning inside the of the code block containing the keyword “class”).
• Every command must end with a semicolon (;)
• The public class in your java file will be the thing your file is named after e.g.

// The class:
public class HelloWorld{
}

would have to exist in a file called HelloWorld.java

• Java is a compiled language (more on that in Java internals)

Setup

Java doesn’t come preinstalled on most systems. In order to use it, you’ll have to install the necessary tools.

I will list the primary methods for installation on Linux and Windows:

• Linux:
  • Debian-based: sudo apt install default-jdk
  • Fedora-based: Link to tutorial
  • Arch-based: you can figure this out on your own btw ;)
• Windows: Link to tutorial

Java Internals

Java code doesn’t just run on your computer. It needs tools to run.

Java code first gets translated to Bytecode through the Java compiler(javac). Next, the bytecode gets interpreted and executed by the Java Virtual Machine (JVM). Java code > Bytecode > Interpretation + Execution

What you need to do

To run your code you just need to type java <your-code-name>.java since Java 11.

Before you would have to type javac <your-code-name>.java and java <your-code-name> to run your code.

The first command just batches these two commands together (1st java code to bytecode, 2nd bytecode interpretation+execution).

Technical Jargon

• Java Runtime Environment (JRE): provides the environment and libraries for the JVM to execute the bytecode.
• Java Development Kit (JDK): a packet that includes all tools necessary to execute Java code (compiler, JVM, JRE)