Java Fundamentals

Java is both a programming language and a platform. You write source files in the Java language. The compiler turns them into class files. A Java Virtual Machine, or JVM, runs those class files.

That separation is the central mental model. Source code expresses your program. Bytecode is the compiled form. The JVM supplies the runtime that loads classes, executes code, and manages memory.

The Java Development Kit, or JDK, is the development toolkit. It includes the compiler, the launcher, documentation, and development tools. Java SE is the standard platform for general-purpose computing. Its APIs are organized into modules, including the foundational java.base module.

Start with one small program

A Java program is organized into types. A class is a type that can hold state and behavior. A method holds executable behavior. The Java launcher starts a class that has a suitable main method.

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

Compile source with javac. Run the compiled class with java.

javac Hello.java
java Hello