Introduction to Java

Beginner ~20 min read

Java is one of the most popular programming languages in the world, powering millions of applications from enterprise software to Android apps. In this lesson, you'll learn what Java is, why it's so widely used, and understand the key components that make Java work: JDK, JRE, and JVM.

What is Java?

Java is a high-level, object-oriented, class-based programming language designed to have as few implementation dependencies as possible. Developed by Sun Microsystems (now owned by Oracle) and released in 1995, Java was designed with the principle of "Write Once, Run Anywhere" (WORA).

Key Features of Java:

Platform Independent: Java code compiles to bytecode that runs on any device with a Java Virtual Machine (JVM)
Object-Oriented: Everything in Java is an object, making code modular and reusable
Secure: Built-in security features and sandbox execution
Robust: Strong memory management and exception handling
Multithreaded: Built-in support for concurrent programming

JDK, JRE, and JVM

Understanding the difference between JDK, JRE, and JVM is crucial for Java development. These three components work together to enable Java's platform independence.

JVM (Java Virtual Machine)

The JVM is a virtual machine that executes Java bytecode. It's platform-specific (different JVM for Windows, Linux, macOS) but provides a consistent runtime environment. The JVM:

Loads bytecode (.class files)
Verifies bytecode for security
Executes bytecode by converting it to native machine code
Manages memory (garbage collection)

JRE (Java Runtime Environment)

The JRE is a software package that provides everything needed to run Java applications. It includes:

JVM (Java Virtual Machine)
Java class libraries (rt.jar, charsets.jar, etc.)
Supporting files and properties

Note: JRE is sufficient if you only want to run Java programs. You don't need JRE if you have JDK installed.

JDK (Java Development Kit)

The JDK is a complete development environment for Java. It includes:

Everything in JRE
Development tools (javac compiler, javadoc, jdb debugger, etc.)
Source code and documentation

For development: You need JDK installed. It contains all tools necessary to write, compile, and debug Java programs.

Component	Contains	Used For
JVM	Virtual machine that executes bytecode	Running Java programs
JRE	JVM + Java libraries	Running Java programs
JDK	JRE + Development tools	Developing Java programs

Memory Tip: Think of it like building a house:

JVM = The foundation (runs code)
JRE = Foundation + walls (can run programs)
JDK = Complete house with tools (can build and run programs)

Platform Independence

Java's most powerful feature is platform independence, enabled by the JVM. Here's how it works:

Write: You write Java source code (.java files) on any platform
Compile: The JDK compiler (javac) converts .java to .class bytecode files
Distribute: Bytecode is platform-independent - the same .class files work everywhere
Run: JVM (platform-specific) interprets bytecode for the specific operating system

Example: You can write Java code on a Windows machine, compile it, and the resulting .class files will run on Linux, macOS, or any other platform with JVM installed - without any modifications!

Java Versions

Java has evolved significantly since its release. Here are some major milestones:

Version	Year	Key Features
Java 1.0	1996	Initial release
Java 5	2004	Generics, autoboxing, annotations
Java 8	2014	Lambda expressions, Streams API, default methods
Java 11	2018	LTS version, HTTP client, local variable syntax
Java 17	2021	Current LTS, sealed classes, pattern matching

LTS Versions: Long Term Support (LTS) versions receive extended support and updates. Java 8, 11, and 17 are LTS versions and are widely used in production environments.

Installation

To start developing Java applications, you need to install the JDK. Here's how:

1. Download JDK

Visit Oracle's JDK download page or
Use Eclipse Temurin (Adoptium) for open-source JDK

2. Install JDK

Windows: Run the installer and follow the wizard
macOS: Use Homebrew: brew install openjdk@17
Linux: Use package manager: sudo apt install openjdk-17-jdk

3. Verify Installation

Open terminal/command prompt and run:

java -version
javac -version

Environment Variables: Make sure JAVA_HOME is set to your JDK installation directory, and that the bin directory is in your PATH environment variable.

Common Mistakes

1. Confusing JDK with JRE

Problem: Trying to compile Java code with only JRE installed

# Error: javac command not found
javac HelloWorld.java

Solution: Install JDK (which includes JRE) to get the compiler

2. Multiple Java Versions

Problem: Having multiple Java versions installed without proper configuration

Solution: Use JAVA_HOME environment variable to specify which version to use, or use tools like jenv (macOS/Linux) to manage versions

Summary

Java is a platform-independent, object-oriented programming language
JVM executes Java bytecode and is platform-specific
JRE = JVM + Java libraries (for running programs)
JDK = JRE + Development tools (for developing programs)
Java's "Write Once, Run Anywhere" is enabled by bytecode and JVM
Install JDK to develop Java applications

What's Next?

Now that you understand what Java is and how it works, you're ready to write your first Java program! In the next lesson, you'll learn how to create a Java class, write the main method, and see your code come to life.

Next First Program →