Don't Panic - 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 (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 (JDK) includes the compiler, launcher, and development tools. Java SE is the standard platform for general-purpose computing, with APIs organized into modules such as java.base.

A program is organized into types. A class holds state and behavior. A method holds executable behavior. The launcher starts a class with a suitable main method. Compile with javac and run with java, even when an IDE performs both for you. Keep those steps distinct when diagnosing classpath or version problems.

Every variable and expression has a type. Primitive types represent simple values such as integers, floating-point values, booleans, and characters. Reference types refer to objects, arrays, interfaces, enums, records, and other class instances. An object combines state and behavior. Constructors initialize new objects. Use a class when data and the operations on that data form one useful unit.

Null applies to references, not primitives. Equality needs care: == compares reference identity, while equals compares value when defined. Class files carry a version the runtime must support; upgrading the compiler without the matching JVM produces UnsupportedClassVersionError.

Read the Intro for the platform model and first program. Use the Cheatsheet when you need the type and tool maps. Updates tracks Oracle's Java SE Support Roadmap for which JDK lines remain supported.