1.1 Java Fundamentals

Learning Objectives

•Describe the edit → compile → run cycle for a Java program.
•Explain the role of the JDK, JRE, and JVM.
•Identify the structure of a minimal Java class with a main method.
•Compile and run a Java program from the command line.

Introduction

Java is a general-purpose, object-oriented, statically-typed language designed to run on any device via the Java Virtual Machine (JVM). Every Java application starts life as human-readable source code (.java files), gets compiled to platform-independent bytecode (.class files), and is then executed by the JVM — the "write once, run anywhere" promise that made Java a dominant enterprise and Android language.

Key Concepts

JDK (Java Development Kit) — compiler + tools + JRE, used to build Java software.

JRE (Java Runtime Environment) — the libraries + JVM needed to run compiled Java programs.

JVM (Java Virtual Machine) — executes platform-independent bytecode.

Every standalone Java program needs exactly one `public static void main(String[] args)` entry point per run.

Java is case-sensitive and statically typed — every variable's type is fixed at compile time.

Java Syntax

Code Examples

`public class HelloWorld` declares a class named HelloWorld — the file must be saved as HelloWorld.java. `main` is where the JVM starts execution. `System.out.println` writes a line of text to the console.

Common Mistakes

⚠Naming the file differently from the public class it contains (Java requires them to match, e.g. `Student.java` for `public class Student`).
⚠Forgetting `static` on `main`, which prevents the JVM from finding an entry point without first creating an instance.
⚠Confusing the JDK (needed to develop) with the JRE (only needed to run already-compiled programs).

Practice

Practice 1

Multiple Choiceeasy

Which component actually executes compiled Java bytecode?

The JDK

The JVM

The IDE

The compiler (javac)

Practice 2

Short Answereasy

If a file contains `public class Account { ... }`, what must the file be named?

Summary

Java source is compiled to bytecode and executed by the JVM, giving it cross-platform portability. Every program's execution begins in a `public static void main(String[] args)` method inside a public class whose name matches the file name.

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