The Complete Java Learning Guide

Java is a general-purpose, statically typed programming language used for applications, services, Android development, and large software systems.

How Java works

Java source code is compiled into bytecode that runs on a Java Virtual Machine (JVM). Types are checked as code is compiled, and the runtime manages memory for objects.

What you will learn

You will start with a small class and main method, then learn types, control flow, object-oriented design, collections, exceptions, and application structure.

How to use this guide

1. Start with Chapter 1 and move through the chapters in order. New ideas build on the earlier ones.
2. Type each example yourself, make a small change, and predict what will happen before running it.
3. When a chapter is ready, its lesson will appear at the chapter link. These pages are currently blank lesson starters.

A step-by-step learning path

1. Chapters 1-7: Learn to write instructions. Compile a first program, use typed variables, work with input and output, make decisions, repeat tasks, and write methods.
2. Chapters 8-12: Work with collections and objects. Use arrays and lists, process strings, understand references, manage resources, and design classes and other types.
3. Chapters 13-18: Organize and verify applications. Learn scope, packages, the Java libraries, files, exceptions, debugging, and tests.
4. Chapters 19-24: Grow into larger programs. Study algorithms, advanced Java features, safe and efficient design, build tools, and guided projects.

Java ideas you will meet early

• Class: a named definition that describes a kind of object and the operations it can perform. A Java application starts running from a main method.
• Type: describes what kind of value a variable can hold. Java checks many type mistakes while compiling.
• Method: a named operation associated with a class. A method can receive parameters and return a result.
• Object reference: a value used to reach an object. A reference is not the object itself, and it can be null.

Your first Java program

Save this class in a file named Hello.java:

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

Hello is the class name and matches the file name. The main method is where this simple program starts. System.out.println displays a line of text. With a Java Development Kit installed, compile and run it from the file's folder:

javac Hello.java
java Hello

A simple way to study

• Keep the public class name and file name the same, including capitalization.
• Read compiler errors carefully; Java often reports the file and line where a syntax or type problem was noticed.
• Practice a small class and method before adding more classes, inheritance, or frameworks.
• Run the program after each small change so you can identify which change caused a problem.

First file: save your first program as Hello.java. Compile with javac Hello.java, then run with java Hello.