Object-Oriented Programming in Java

Object-Oriented Programming Basics

Java is built around OOP principles. The key ideas are:

• Encapsulation – grouping data and methods inside a class
• Abstraction – exposing only essential features to users
• Inheritance – reusing and extending existing classes
• Polymorphism – performing actions in different ways

A class defines structure and behavior. An object is a runtime instance of the class.

Classes and Object Creation

A class contains fields (data) and methods (operations). Objects are created using the new keyword.

class Student {
  int roll;
  String name;

  void show() {
    System.out.println(roll + " - " + name);
  }
}

Student s = new Student();
s.roll = 101;
s.name = "Pushkar";
s.show();

Diagram: class-object-diagram.png

Constructors and Finalizer

A constructor initializes an object at creation. It has no return type, not even void. Java supports constructor overloading.

class Point {
  int x, y;

  Point() {        // default constructor
    x = 0; y = 0;
  }

  Point(int x, int y) {   // parameterized constructor
    this.x = x;
    this.y = y;
  }
}

Point p1 = new Point();
Point p2 = new Point(5, 8);

Java no longer recommends using finalizers. Modern Java uses try-with-resources and garbage collection.

Visibility and Access Modifiers

Java provides four levels of access control:

• public – accessible everywhere
• private – accessible only inside class
• protected – class + subclasses + same package
• default – class + same package

class Employee {
  private int id;
  public String name;

  public void setId(int id) {  // encapsulation
    this.id = id;
  }

  public int getId() {
    return id;
  }
}

Methods and the this Reference

Methods operate on object data. The this keyword refers to the current object and resolves naming conflicts.

class Box {
  private int side;

  Box(int side) {
    this.side = side;    // resolves shadowing
  }

  int area() {
    return side * side;
  }
}

Useful Built-in Classes (String, Character, StringBuffer)

Java provides several commonly used classes for text processing.

• String – immutable text
• Character – wrapper for char type
• StringBuffer – mutable and thread-safe

String name = "Pushkar";
String upper = name.toUpperCase();

Character ch = 'A';
boolean isDigit = Character.isDigit('5');

StringBuffer sb = new StringBuffer("open");
sb.append("CSE");  // openCSE

Diagram: string-immutability.png