OOP Principles

#What is OOP?

OOP stands for Object Oriented Programming. Everything revolves around:

• Objects
• Classes
• Relationships

Instead of writing functions separately, we bundle data and methods inside one object.

For example, instead of keeping deposit(), withdraw() and balance as separate pieces, we create an Account that contains everything:

This is called an Object.

#What is a Class?

A class is a blueprint. A house blueprint is not a real house, but using the blueprint we build House1, House2, House3.

Similarly, class Employee is only a blueprint. The objects are Employee e1, Employee e2, Employee e3.

class Employee {

    int id;
    String name;

}

Creating objects:

Employee e1 = new Employee();
Employee e2 = new Employee();

Memory — each object has its own state:

#What is an Object?

Object = Instance of a Class.

Car c = new Car();

Here Car is the class and c is the object.

#Four Pillars of OOP

There are only four:

1. Encapsulation
2. Inheritance
3. Polymorphism
4. Abstraction

#1. Encapsulation

Definition

Binding data + methods into one unit.

class Employee {

    private int salary;

    public void setSalary(int salary) {
        this.salary = salary;
    }

    public int getSalary() {
        return salary;
    }

}

salary is hidden. It can be accessed only through methods.

Why private?

Imagine anyone could write salary = -50000. Anyone can modify it — that is wrong.

Instead, setSalary() can validate:

public void setSalary(int salary) {

    if (salary > 0)
        this.salary = salary;

}

Now the object is protected.

Real-life example: ATM

You cannot access balance directly. You can only Withdraw, Deposit and Check Balance. That is exactly encapsulation.

Advantages

• Data Hiding
• Security
• Easy Maintenance
• Loose Coupling

#2. Inheritance

Definition

Acquiring properties from another class.

class Animal {

    void eat() {}

}

class Dog extends Animal {

}

Dog automatically gets eat().

Memory:

Advantages

• Code Reuse
• Easy Maintenance
• Less Duplicate Code

Types of Inheritance

Java supports Single, Multilevel and Hierarchical inheritance.

Java does not support Multiple Inheritance (with classes) because of the Diamond Problem.

class A {

    void show() {}

}

class B extends A {

    void show() {}

}

class C extends A {

    void show() {}

}

class D extends B, C {   // not allowed in Java

}

The compiler cannot decide which show() should execute. Hence Java avoids this.

#3. Polymorphism

Poly = Many, Morph = Forms. One object, many behaviours.

There are two types:

• Compile Time
• Run Time

Compile Time — Method Overloading

add(int)
add(double)
add(String)

Same method, different parameters. The compiler decides.

Run Time — Method Overriding

Animal a = new Dog();

a.sound();

Dog Barking

The decision happens during runtime.

Example

class Animal {

    void sound() {
        System.out.println("Animal");
    }

}

class Dog extends Animal {

    void sound() {
        System.out.println("Bark");
    }

}

Animal a = new Dog();

a.sound();

Bark

#4. Abstraction

Showing only essential details. Hiding implementation.

Example — ATM: you click Withdraw. You don't know about the SQL, the server or the cash counting. All of that is hidden.

Using Abstract Class

abstract class Animal {

    abstract void sound();

}

class Dog extends Animal {

    void sound() {
        System.out.println("Bark");
    }

}

Using Interface

interface Vehicle {

    void start();

}

class Car implements Vehicle {

    public void start() {

    }

}

Abstract Class vs Interface