DEV Community: Abinesh.R

Basic of SQL

Abinesh.R — Sun, 23 Aug 2026 06:04:27 +0000

OOP (Object-Oriented Programming).

Abinesh.R — Sun, 23 Aug 2026 04:55:34 +0000

JAVA OOPS CONCEPTS
│
├── 1. Class
├── 2. Object
├── 3. Encapsulation
├── 4. Inheritance
├── 5. Polymorphism
└── 6. Abstraction

1. Class

Blueprint for creating objects. Defines what data (fields) and behavior (methods) objects will have.
Example
class Student {
String name;
int age;
void study() { System.out.println(name + " is studying"); }
}

2. Object
An actual instance created from the class, using new.
Example
Student s1 = new Student(); // object
s1.name = "Abinesh";
**

Encapsulation** Hiding data using private, exposing it safely via getters/setters.

Example
class Account {
private double balance;
public double getBalance() { return balance; }
public void deposit(double amt) { balance += amt; }
}

4. Inheritance
Child class reuses parent class's code via extends.
Example
class Animal {
void eat() { System.out.println("eating"); }
}
class Dog extends Animal {
void bark() { System.out.println("barking"); }
}

5. Polymorphism
Same method name, different behavior.
Overloading (compile-time) — same method, different parameters, same class.
Overriding (runtime) — child class redefines parent's method.
Example
// Overloading
void add(int a, int b) {}
void add(int a, int b, int c) {}

// Overriding
class Animal { void sound() { System.out.println("sound"); } }
class Dog extends Animal { void sound() { System.out.println("bark"); } }

6. Abstraction

Hiding implementation details, showing only essential features. Done via abstract class or interface.

example

abstract class Shape {
abstract void draw(); // no implementation
}
class Circle extends Shape {
void draw() { System.out.println("drawing circle"); }
}

important oops concept **
**1.Constructor

Special method that runs automatically when an object is created. Same name as class, no return type. Used to initialize values.
Example
class Student {
String name;
Student(String n) { // constructor
name = n;
}
}
Student s1 = new Student("Abinesh"); // auto-called

2. this keyword

Refers to the current object. Mainly used to fix naming conflicts between instance variables and parameters.

example
class Student {
String name;
Student(String name) {
this.name = name; // this.name = instance var, name = parameter
}
}

String name;
Student(String name) {
    this.name = name;  // this.name = instance var, name = parameter
}

}
5. Instance vs Static
**
Instance members belong to each object separately.
Static members belong to the class itself — shared by all objects.
**Example
class Counter {
int id; // instance — different for each object
static int count = 0; // static — shared, one copy only
Counter() {
count++;
id = count;
}
}

basci of java

Abinesh.R — Sun, 23 Aug 2026 04:22:17 +0000

what is java?
Java is a highly popular, object-oriented programming language used to build mobile apps, web applications, and large enterprise systems.

It follows the rule "Write Once, Run Anywhere" (WORA), meaning Java code can run on any computer that supports Java without being rewritten.

📦 The Development Tools (JDK vs JRE vs JVM)
Think of these three structural pieces as boxes packed inside each other.
JVM:
The inner engine that executes the program bytes line-by-line.
JRE (Java Runtime Environment):
The middle box containing the JVM engine plus standard libraries to help run the code.
JDK (Java Development Kit):
The complete outer kit containing the JRE, compilers, and debuggers needed to build programs from scratch.

🧱 Core Concepts of Java
To understand Java, you need to know how it runs and how it handles data.1.
How Java RunsJVM (Java Virtual Machine): The engine that actually runs the Java code.
JRE (Java Runtime Environment): The toolbox that contains the JVM and packages your code.
**JDK (Java Development Kit): **The full developer kit containing the JRE and tools to write code.

2. Variables and Data Types
Variables hold data in memory.
Java requires you to state the type of data first.

int: Holds whole numbers (e.g., int age = 25;).
double: Holds decimal numbers (e.g., double price = 19.99;).
char: Holds a single letter (e.g., char grade = 'A';).
boolean: Holds true or false values (e.g., boolean isJavaFun = true;).
String: Holds text (e.g., String name = "Developer";).

3. Control Flow (Decision Making)
Control flow structures let your program make decisions or repeat tasks.

if-else statements: Run code only if a condition is true.
for and while loops:
Repeat code blocks multiple times.
💻 Your First Java Program Here is the traditional "Hello World" program in Java.
class Main {
public static void main(String[] args) {

    System.out.println("Hello, World!");
}

}
Object-Oriented Programming (OOP)
Java organizes code into "Objects" based on real-world things.
OOP relies on four main pillars:
Class: A blueprint or template for creating objects (e.g., a "Car" blueprint).
Object: An instance of a class (e.g., a specific "Red Tesla" built from the blueprint).
Inheritance: Letting a new class adopt properties of an existing class.
Polymorphism: Allowing different objects to respond to the same action in their own way.