✨ Introduction to Java

smallbean

☕ What is Java?

Java is a high-level, object-oriented, and platform-independent programming language. It follows the principle “write once, run anywhere”, meaning Java programs can run on any device with the Java Virtual Machine (JVM).

It’s widely used in:

  • Backend systems
  • Mobile apps (Android)
  • Desktop applications
  • Large enterprise platforms
  • Cloud and distributed systems

🔤 Basic Syntax and Rules

1. Variables

String name = "Doby";      // Text
int age = 20;              // Whole number
double height = 1.75;      // Decimal number
boolean isHappy = true;    // True or false
final int MAX = 100;       // Constant (cannot change)

2. Data Types

String text = "hello";       // String
int number = 10;             // Integer
double pi = 3.14;            // Floating point
boolean isCool = true;       // Boolean
char letter = 'A';           // Single character

// Objects
Person p = new Person("Doby", 20);

🔁 Logic and Control Flow

1. Conditionals

if (name.equalsIgnoreCase("Doby")) {
    System.out.println("Welcome back, Doby!");
} else if (name.equals("Guest")) {
    System.out.println("Hello, Guest!");
} else {
    System.out.println("Access denied.");
}

2. Loops

For Loop

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

While Loop

int count = 0;
while (count < 3) {
    System.out.println("Counting: " + count);
    count++;
}

🔧 Functions (Methods)

public static String greet(String name) {
    return "Hello, " + name + "!";
}

public static void main(String[] args) {
    System.out.println(greet("Doby"));
}

🧱 Core Data Structures

Arrays

String[] colors = { "red", "green", "blue" };
System.out.println(colors[0]);

ArrayList (Flexible List)

import java.util.ArrayList;

ArrayList<String> items = new ArrayList<>();
items.add("coffee");
items.add("energy drink");

System.out.println(items.get(1));

Objects (Classes)

class Person {
    String name;
    int age;

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

Person p = new Person("Doby", 25);
System.out.println(p.name);

📦 JSON in Java

Java uses libraries for JSON handling. Example using Jackson:

ObjectMapper mapper = new ObjectMapper();

Person user = new Person("Doby", 25);

// Convert to JSON
String json = mapper.writeValueAsString(user);

// Convert back to Java object
Person parsed = mapper.readValue(json, Person.class);

System.out.println(parsed.name);

🖥️ Input and Output

import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
System.out.print("Enter your name: ");
String nameInput = scanner.nextLine();

System.out.println("Hello, " + nameInput + "!");

🧩 Packages and Classes

Java organizes code into packages for structure and reuse.

// file: greetings/Greet.java
package greetings;

public class Greet {
    public static String hello(String name) {
        return "Hello, " + name;
    }
}

// file: App.java
import greetings.Greet;

public class App {
    public static void main(String[] args) {
        System.out.println(Greet.hello("Doby"));
    }
}

📘 Learning Resources

  • Java Tutorial – Oracle Docs
  • w3schools Java
  • GeeksForGeeks Java
  • freeCodeCamp Java
  • Codecademy Java Course

💡 Final Thoughts

Java is a powerful language that’s everywhere — phones, servers, banking systems, satellites, even giant enterprise infrastructures. Once you understand the basics, you can build anything from small utilities to massive distributed applications.

Take your time, compile often, and let your curiosity lead you.

← Back