Java Tutorial

Java is a free, general purpose programming language. It was created by James Gosling at Sun Microsystems and was first released in 1995. Today Oracle looks after it, and a big community…

Java is a programming language and also a platform. When people say “Java” they can mean the language (the code you write), or the whole platform (the tools and the runtime which…

To write Java programs you must install a JDK (Java Development Kit). There are many free builds of the same OpenJDK, from Eclipse Temurin, Oracle, Amazon Corretto, Microsoft and others. They all…

Let us write the Hello World program, which prints a message on the screen. We will do it two times: first the classic way, which every Java book and every older project…

Syntax means the rules of writing code. Java has strict rules, and the compiler tells you when you break one. Here are the most important ones. A Java program The rules All…

Comments are notes inside the code. The compiler ignores them. They help other people (and you after six months) to understand the code. Single line comment Starts with // and goes to…

A variable is a named box in the memory where you keep a value. In Java every variable has a type, which says what kind of value it can keep. Declare and…

Java has two groups of data types: primitive types (simple values) and reference types (objects like String, arrays and classes). The 8 primitive types Type Size What it keeps Example byte 8…

Type casting means changing a value from one data type to another. Java has two kinds: widening (automatic) and narrowing (manual). Widening casting (automatic) A smaller type goes into a bigger type…

Operators do work on values and variables. Java has these groups. Arithmetic operators Operator Name Example Result + Addition 10 + 3 13 – Subtraction 10 – 3 7 * Multiplication 10…

A String is a sequence of characters, like a name or a sentence. It is an object (not a primitive) and it is used in almost every program. Create a string Join…

The String class has many methods. These are the ones you will use most. All the examples use this text. Basic methods Find and cut The positions start at 0. For substring(start,…

A normal String can not be changed, so every join creates a new object. When you build a long text in a loop, this is slow. StringBuilder is a changeable text, made…

The Scanner class reads input from the keyboard (or from a file or a text). You import it from java.util and give it System.in. Read text and numbers In the examples below…

The Math class (in java.lang, no import needed) has static methods for common maths. You call them like Math.sqrt(16). Common methods Rounding Method What it does round(x) Nearest whole number (as long…

Programs must take decisions. The if statement runs a block of code only when a condition is true. if if … else else if Use else if to check many conditions. Java…

switch chooses one path from many, based on the value of a variable. It is cleaner than many else if. Classic switch with break Without break Java goes on and runs the…

A loop repeats a block of code. Use the for loop when you know how many times to repeat. Syntax Initialization runs once at the start. The condition is checked before every…

Use a while loop when you do not know how many times you need to repeat, and you only know the condition. while loop The condition is checked before each round. If…

break and continue change the normal flow of a loop. break break stops the loop at once. continue continue skips the rest of the current round and goes to the next round….

An array keeps many values of the same type in one variable. The size is fixed when you create it, and the items are numbered from 0. Create an array Default values…

A multidimensional array is an array of arrays. The most common is the two dimensional (2D) array, like a table with rows and columns. Create a 2D array Loop with nested loops…

A method is a named block of code which you can run many times. Methods make programs short, clear and easy to test. In other languages they are called functions. Declare and…

Overloading means many methods with the same name but different parameters (different number or different types). Java chooses the right one from the arguments you give. Example Rules The parameter list must…

Recursion means a method calls itself. It is a good way to solve a big problem by solving smaller copies of the same problem. Two parts of every recursion Base case: the…

Java is an object oriented language. A class is a blueprint, and an object is a real thing made from that blueprint. A class Car is the idea of a car, and…

A constructor is a special method which runs when you create an object with new. It sets the starting values of the object. It has the same name as the class and…

static and this are two keywords which every Java developer must understand. static fields: one copy for the class A normal (instance) field has a copy in every object. A static field…

Encapsulation means keeping the data of a class private and giving controlled access through methods. So the outside code can not put the object in a wrong state. The problem without it…

Inheritance lets a class (the subclass or child) take the fields and methods of another class (the superclass or parent). It gives you code reuse and an “is a” relation: a Dog…

Polymorphism means “many forms”. One method call can behave in different ways depending on the real object. It is the most powerful idea of object oriented programming. Parent reference, child object The…

An abstract class is a class which you can not create objects of. It is made to be a base for other classes. It can have abstract methods (no body, only the…

An interface is a contract: it lists the methods that a class promises to have. Classes implement the interface. A class can implement many interfaces, so it solves the “only one parent”…

An enum is a special class for a fixed set of constants, like days of the week, order status or sizes. It is safer than plain numbers or strings, because the compiler…

A record (Java 16+) is a short way to write a class which only carries data. With one line you get the fields, the constructor, the getters, equals(), hashCode() and toString(). A…

A package is a folder of related classes. Packages keep big projects organized and avoid name clashes: two different classes can both be named Date if they are in different packages. Built…

An exception is an error which happens while the program is running, like dividing by zero or opening a file which does not exist. If you do not handle it, the program…

An ArrayList is a list which can grow and shrink, unlike an array which has a fixed size. It is part of the Collections framework in java.util. Create and use Loop through…

A HashMap keeps data as key and value pairs, like a dictionary. You find a value very fast by its key. Keys are unique. Basic use Loop over a map Count the…

The Collections framework has many types. We already saw ArrayList and HashMap. Here are three more which you will use often. HashSet: unique values A Set does not allow duplicates. HashSet is…

Generics let you write classes and methods which work with many types, and still keep the type safe. You already use them in List<String> and Map<String, Integer>. The type in the angle…

A lambda expression (Java 8) is a short way to write an anonymous function. It is mostly used to pass a small piece of behavior to a method, for example how to…

A stream (Java 8) is a pipeline to process a collection of data in a clear way: take data, filter it, change it and collect the result. It does not change the…

Optional (Java 8) is a box which may or may not have a value. A method returns an Optional to say clearly: “the result can be missing”. It helps you to avoid…

The modern date and time API is in the package java.time (Java 8+). Its objects are immutable and easy to use. Do not use the old java.util.Date and Calendar in new code….

Java can create, read, write and delete files. The modern way is the java.nio.file package (Files and Path), which is simple and short. Write and read a text file Append and write…

A thread is a path of execution. A Java program starts with one thread (main). With more threads, a program can do many jobs at the same time, like downloading a file…

Modern Java (17, 21 and 25) has features which make code shorter and safer. The three most important work together: sealed types, records and pattern matching. Sealed classes and interfaces A sealed…

A quick reference of the most used Java syntax. Use it to revise before an exam or an interview. Every example is tested on Java 25. Program Variables and types Type Example…