Learn Java – Beginner Guide | Algo Infinity Verse

01

Introduction to Java

Java is a high-level, object-oriented programming language developed by Sun Microsystems (now Oracle) and first released in 1995. It follows the principle of "Write Once, Run Anywhere" (WORA), meaning compiled Java code runs on any platform that supports the Java Virtual Machine (JVM).

Why Learn Java?

Platform Independent – Runs on any device with a JVM
Object-Oriented – Encourages reusable, modular code
Massive Ecosystem – Spring, Android, Hibernate, and more
Industry Standard – Used by 90% of Fortune 500 companies
Strong Typing – Catches errors at compile time

Your first Java program is the classic "Hello, World!":

Java

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

Output

Hello, World!

💡

Every Java application needs a main method — it's the entry point where the program starts executing. The class name must match the filename (HelloWorld.java).

02

Installation & Setup

To write and run Java code, you need the Java Development Kit (JDK) and a text editor or IDE. The JDK includes the compiler (javac), runtime (java), and libraries.

Installing the JDK

Download the latest JDK from oracle.com/java or use an open-source build like OpenJDK. Version 17 LTS or 21 LTS is recommended for beginners.

Bash

# Verify your Java installation
java -version
javac -version

Choosing an IDE

IntelliJ IDEA Community – Free, feature-rich, recommended for beginners
Eclipse – Free, widely used in industry
VS Code – Lightweight with Java extension pack

Bash

# Compile and run a Java program
javac HelloWorld.java    # compiles to HelloWorld.class
java HelloWorld          # runs the compiled bytecode

✅

Java is a compiled language: you write .java source files, compile them to .class bytecode with javac, and run the bytecode with java on the JVM.

03

Variables & Data Types

In Java, every variable must have a declared type. Java is statically typed — once you declare a variable's type, it cannot hold a different type of value.

Java

String name = "Alice";    // Text
int age = 25;                // Whole number
double height = 5.7;         // Decimal number
boolean isStudent = true;    // true / false
char grade = 'A';            // Single character

System.out.println(name);
System.out.println(age);

Output

Alice
25

Type	Size	Example	Description
`byte`	8-bit	`127`	Small integer
`int`	32-bit	`42`	Whole number
`long`	64-bit	`100L`	Large integer
`float`	32-bit	`3.14f`	Single precision decimal
`double`	64-bit	`3.14159`	Double precision decimal
`char`	16-bit	`'A'`	Single Unicode character
`boolean`	1-bit	`true/false`	Logical value

💡

Java also has reference types like String, arrays, and objects. Unlike primitives, reference types store a reference (memory address) to the actual data.

Practice: Variables & Data Types

Create variables of different primitive types and print them:

Create an int variable score with value 95
Create a double variable gpa with value 3.85
Create a String variable city with value "New York"
Create a boolean variable passed set to true
Print each variable using System.out.println()

Java

int score = 95;
double gpa = 3.85;
String city = "New York";
boolean passed = true;

System.out.println("Score: " + score);
System.out.println("GPA: " + gpa);
System.out.println("City: " + city);
System.out.println("Passed: " + passed);

04

Input & Output

Java provides System.out.println() for output and the Scanner class for reading user input.

System.out — Displaying Output

System.out.println() prints text followed by a newline. Use System.out.print() to print without a newline.

Java

String name = "Alice";
int age = 25;

System.out.println("My name is " + name + " and I am " + age + " years old.");
System.out.printf("Name: %s | Age: %d%n", name, age);

Output

My name is Alice and I am 25 years old.
Name: Alice | Age: 25

Scanner — Reading User Input

The Scanner class reads input from the console. Import it from java.util.

Java

import java.util.Scanner;

public class Greeting {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("What is your name? ");
        String name = sc.nextLine();

        System.out.print("How old are you? ");
        int age = sc.nextInt();

        System.out.println("Hello, " + name + "! In 10 years you will be " + (age + 10) + ".");
        sc.close();
    }
}

⚠️

Always call sc.close() to release the scanner resource. nextLine() reads a whole line; nextInt() reads an integer. Be careful about mixing them — nextInt() leaves a newline that nextLine() will consume!

Practice: Input & Output

Write a program that asks the user for their name and favorite color, then prints a greeting.

Import java.util.Scanner
Ask for name and favorite color using nextLine()
Print: "Hello {name}, your favorite color is {color}!"

Java

import java.util.Scanner;

public class Greeting {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter your name: ");
        String name = sc.nextLine();

        System.out.print("Enter your favorite color: ");
        String color = sc.nextLine();

        System.out.println("Hello " + name + ", your favorite color is " + color + "!");
        sc.close();
    }
}

05

Operators

Operators perform operations on variables and values. Java supports arithmetic, assignment, comparison, logical, and more.

Category	Operators	Example
Arithmetic	`+ - * / %`	`10 + 5 → 15`
Assignment	`= += -= *= /=`	`x += 5`
Comparison	`== != < > <= >=`	`10 > 5 → true`
Logical	`&& \|\| !`	`true && false → false`
Unary	`++ -- !`	`i++`

Java

int a = 10;
int b = 5;

// Arithmetic
int sum = a + b;
int product = a * b;
int remainder = a % b;

// Comparison & Logical
boolean isGreater = a > b;
boolean bothPositive = (a > 0) && (b > 0);

System.out.println("Sum: " + sum);
System.out.println("Product: " + product);
System.out.println("Remainder: " + remainder);
System.out.println("Is a > b? " + isGreater);
System.out.println("Both positive? " + bothPositive);

Output

Sum: 15
Product: 50
Remainder: 0
Is a > b? true
Both positive? true

Practice: Operators

Calculate the area and perimeter of a rectangle with width 8 and length 12.

Create int variables width and length
Calculate area = width * length
Calculate perimeter = 2 * (width + length)
Print both results

Java

int width = 8;
int length = 12;
int area = width * length;
int perimeter = 2 * (width + length);

System.out.println("Area: " + area);
System.out.println("Perimeter: " + perimeter);

06

Conditional Statements

Conditionals let your program make decisions. Java supports if, else if, else, and switch statements.

Java

int score = 85;
char grade;

if (score >= 90) {
    grade = 'A';
} else if (score >= 80) {
    grade = 'B';
} else if (score >= 70) {
    grade = 'C';
} else {
    grade = 'F';
}

System.out.println("Score: " + score + " → Grade: " + grade);

Output

Score: 85 → Grade: B

switch Statement

The switch statement is useful when checking a variable against many specific values.

Java

int day = 3;
String dayName;

switch (day) {
    case 1:  dayName = "Monday";    break;
    case 2:  dayName = "Tuesday";   break;
    case 3:  dayName = "Wednesday"; break;
    case 4:  dayName = "Thursday";  break;
    case 5:  dayName = "Friday";    break;
    default: dayName = "Weekend";
}

System.out.println(dayName);

Output

Wednesday

Practice: Conditionals

Write a program that reads a number and prints whether it is positive, negative, or zero.

Use Scanner to read an integer
Use if/else if/else to check the number
Print the result

Java

import java.util.Scanner;

public class CheckNumber {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = sc.nextInt();

        if (num > 0) {
            System.out.println("Positive");
        } else if (num < 0) {
            System.out.println("Negative");
        } else {
            System.out.println("Zero");
        }
        sc.close();
    }
}

07

Loops (for, while, do-while)

Loops let you repeat a block of code. Java has three loop types: for, while, and do-while.

for Loop

Use for when you know how many times to iterate.

Java

// Count from 0 to 4
for (int i = 0; i < 5; i++) {
    System.out.println("Count: " + i);
}

// Enhanced for-loop (for-each)
String[] fruits = {"apple", "banana", "cherry"};
for (String fruit : fruits) {
    System.out.println(fruit);
}

Output

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4
apple
banana
cherry

while & do-while Loops

while checks the condition first; do-while runs the body at least once.

Java

// while loop
int count = 0;
while (count < 5) {
    System.out.println("While: " + count);
    count++;
}

// do-while loop
int num = 0;
do {
    System.out.println("Do-while: " + num);
    num++;
} while (num < 5);

⚠️

Always ensure your loop condition eventually becomes false to avoid infinite loops. Use break to exit early and continue to skip the current iteration.

Practice: Loops

Write two programs:

Use a for loop to print all even numbers from 2 to 20
Use a while loop to calculate the sum of numbers from 1 to 100

Java

// Even numbers 2-20
for (int i = 2; i <= 20; i += 2) {
    System.out.println(i);
}

// Sum of 1 to 100
int sum = 0;
int n = 1;
while (n <= 100) {
    sum += n;
    n++;
}
System.out.println("Sum: " + sum);

08

Methods

Methods (also called functions) are reusable blocks of code. They help you avoid repetition and organize your program into logical pieces.

Java

public class MethodExamples {

    // Method with parameters and return value
    public static int add(int a, int b) {
        return a + b;
    }

    // Void method (no return value)
    public static void greet(String name) {
        System.out.println("Hello, " + name + "!");
    }

    public static void main(String[] args) {
        int sum = add(10, 5);
        System.out.println("Sum: " + sum);

        greet("Alice");
    }
}

Output

Sum: 15
Hello, Alice!

Method Overloading

Java allows multiple methods with the same name as long as parameters differ (number, type, or order).

Java

public static int multiply(int a, int b) {
    return a * b;
}

public static double multiply(double a, double b) {
    return a * b;
}

// Usage
System.out.println(multiply(5, 3));         // 15
System.out.println(multiply(2.5, 4.0));     // 10.0

Practice: Methods

Write a method isEven(int n) that returns true if n is even
Write a method factorial(int n) that returns the factorial of n
Write a method reverseString(String s) that returns the reversed string

Java

public static boolean isEven(int n) {
    return n % 2 == 0;
}

public static int factorial(int n) {
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

public static String reverseString(String s) {
    return new StringBuilder(s).reverse().toString();
}

System.out.println(isEven(4));               // true
System.out.println(factorial(5));            // 120
System.out.println(reverseString("hello")); // olleh

09

Arrays & Strings

Arrays store multiple values of the same type. Strings represent text and come with many built-in methods.

Arrays — Fixed-Size Collections

Arrays in Java have a fixed length. You can create them with new or with curly braces.

Java

// Creating arrays
int[] numbers = new int[5];         // [0, 0, 0, 0, 0]
String[] fruits = {"apple", "banana", "cherry"};

// Access and modify
numbers[0] = 10;
System.out.println(numbers[0]);     // 10
System.out.println(fruits[1]);      // banana
System.out.println("Length: " + fruits.length);

String Methods

Strings in Java are objects with many useful methods.

Java

String text = "Hello, Java!";

System.out.println(text.length());             // 12
System.out.println(text.toUpperCase());        // HELLO, JAVA!
System.out.println(text.toLowerCase());        // hello, java!
System.out.println(text.substring(7));         // Java!
System.out.println(text.replace("Java", "World")); // Hello, World!
System.out.println(text.contains("Java"));     // true

Practice: Arrays & Strings

Create an array of 5 integers and calculate their sum using a for loop
Write code to count how many vowels are in a given string

Java

// Sum of array
int[] nums = {10, 20, 30, 40, 50};
int sum = 0;
for (int n : nums) {
    sum += n;
}
System.out.println("Sum: " + sum);

// Count vowels
String str = "hello world";
int vowels = 0;
for (char c : str.toCharArray()) {
    if ("aeiou".indexOf(c) != -1) {
        vowels++;
    }
}
System.out.println("Vowels: " + vowels);

10

Classes & Objects

A class is a blueprint for creating objects. An object is an instance of a class with its own state (fields) and behavior (methods).

Java

// Define a class
class Student {
    String name;
    int age;

    // Constructor
    Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    void introduce() {
        System.out.println("Hi, I'm " + name + " and I'm " + age + " years old.");
    }
}

// Create objects
Student alice = new Student("Alice", 20);
Student bob = new Student("Bob", 22);

alice.introduce();
bob.introduce();

Output

Hi, I'm Alice and I'm 20 years old.
Hi, I'm Bob and I'm 22 years old.

💡

this refers to the current object's fields. Constructors have the same name as the class and no return type. The new keyword creates an object in memory.

Practice: Classes & Objects

Create a Book class with fields title, author, and year. Add a constructor and a method displayInfo() that prints the book details. Then create two book objects.

Java

class Book {
    String title;
    String author;
    int year;

    Book(String title, String author, int year) {
        this.title = title;
        this.author = author;
        this.year = year;
    }

    void displayInfo() {
        System.out.println("\"" + title + "\" by " + author + " (" + year + ")");
    }
}

// Usage
Book book1 = new Book("1984", "George Orwell", 1949);
Book book2 = new Book("To Kill a Mockingbird", "Harper Lee", 1960);
book1.displayInfo();
book2.displayInfo();

11

Basic Object-Oriented Programming

Java is built around Object-Oriented Programming (OOP). The four main pillars are: Encapsulation, Inheritance, Polymorphism, and Abstraction.

1. Encapsulation

Bundle data and methods together, and restrict direct access to fields using private with public getters/setters.

Java

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() { return name; }
    public int getAge() { return age; }
    public void setAge(int age) {
        if (age > 0) this.age = age;
    }
}

2. Inheritance

Create a new class based on an existing one using the extends keyword. The child class inherits fields and methods from the parent.

Java

// Parent class
class Animal {
    String name;
    Animal(String name) { this.name = name; }
    void speak() { System.out.println(name + " makes a sound"); }
}

// Child class
class Dog extends Animal {
    Dog(String name) { super(name); }

    @Override
    void speak() { System.out.println(name + " barks!"); }
}

// Polymorphism
Animal a = new Dog("Buddy");
a.speak();  // Buddy barks! (runtime polymorphism)

Output

Buddy barks!

3. Polymorphism & Abstraction

Polymorphism – Same method name, different behavior (method overriding & overloading)
Abstraction – Hide implementation details using abstract classes or interfaces

Java

// Abstract class
abstract class Shape {
    abstract double area();
}

class Circle extends Shape {
    double radius;
    Circle(double r) { radius = r; }

    @Override
    double area() { return Math.PI * radius * radius; }
}

class Rectangle extends Shape {
    double w, h;
    Rectangle(double w, double h) { this.w = w; this.h = h; }

    @Override
    double area() { return w * h; }
}

// Polymorphism in action
Shape[] shapes = {new Circle(5), new Rectangle(4, 6)};
for (Shape s : shapes) {
    System.out.println(s.area());
}

Output

78.53981633974483
24.0

Practice: OOP Concepts

Create a Vehicle base class and a Car subclass:

Vehicle has a brand field and a method honk()
Car extends Vehicle with an additional model field
Override honk() in Car to print a car-specific sound
Create a Car object and call its methods

Java

class Vehicle {
    String brand;
    Vehicle(String brand) { this.brand = brand; }
    void honk() { System.out.println(brand + " goes beep beep!"); }
}

class Car extends Vehicle {
    String model;
    Car(String brand, String model) {
        super(brand);
        this.model = model;
    }
    @Override
    void honk() { System.out.println(brand + " " + model + " goes HONK HONK!"); }
}

// Usage
Car myCar = new Car("Toyota", "Camry");
myCar.honk();  // Toyota Camry goes HONK HONK!

Algo Infinity Verse

Start Your Journey With Java Programming Beginner-Friendly Concepts, Code & Practice

☕ Learn Java

Introduction to Java

Why Learn Java?

Installation & Setup

Installing the JDK

Choosing an IDE

Variables & Data Types

Input & Output

System.out — Displaying Output

Scanner — Reading User Input

Operators

Conditional Statements

switch Statement

Loops (for, while, do-while)

for Loop

while & do-while Loops

Methods

Method Overloading

Arrays & Strings

Arrays — Fixed-Size Collections

String Methods

Classes & Objects

Basic Object-Oriented Programming

1. Encapsulation

2. Inheritance

3. Polymorphism & Abstraction