basics of OOP in java

OOP-Main.java

package com.oop;

import com.oop.encapsulation.EncapsClass;

import java.util.Calendar;

public class Main {

    public static void main(String[] args) {

        // class properties and methods
        Employee em = new Employee();
        em.name = "Martin Garrix";
        em.birth_year = 1995;
        System.out.println(em.name + " is almost "+ em.getAge(true)+ " days old");

        // constructor
        Employer ey1 = new Employer("Google");
        Employer ey2 = new Employer("Facebook");
        System.out.println(ey1.name+" Total companies "+Employer.count);

        // inheritance
        Person p1 = new Person("bob",2000);
        Person p2 = new Person("sana",2004);
        Person p3 = new Person();

        Manager m1 = new Manager("Raj",1996,"Cloud");
        Person pm = new Manager("Dinesh",2002,"IT");
        System.out.println(m1.name +" is "+m1.getAge()+" years old.");
        System.out.println(m1.walk());
        System.out.println(p1.walk());
        System.out.println(p1.salary());

        // Encapsulation and access modifiers

        /* public      everywhere
           protected   class and child class
           default     within package
           private     within class
        */

        EncapsClass encap = new EncapsClass();
        encap.doWork();

        // Abstraction
        Tesla t1 = new Tesla();
        t1.startCar();

        // Interfaces
        BMW b1 = new BMW();
        b1.start();

    }
}

class Employee {
    String name;
    int birth_year;

    // Compile time polymorphism

    int getAge(){
        return Math.abs(birth_year - Calendar.getInstance().get(Calendar.YEAR));
    }

    int getAge(boolean in_days){
        return 365*Math.abs(birth_year - Calendar.getInstance().get(Calendar.YEAR));
    }
}

class Employer{
    String name;

    // static use for Class property which shared by all the instances of class
    static int count;

    // Constructor
    public Employer(String name){
        this.name = name;
        count++;
    }
}

// Inheritance

class Person {
    protected String name;
    int birth_year;
    static int population;

    public Person(){
        population++;
    }
    public Person(String name,int year){
        this();
        this.name = name;
        this.birth_year =year;
    }

    int getAge() {
        return Math.abs(birth_year - Calendar.getInstance().get(Calendar.YEAR));
    }

    String walk(){
        return this.name +" is walking";
    }

    String salary(){
        return this.name +" is making "+ this.getSalary()+" a year.";
    }

    // only access within class
    private int getSalary(){
        return 100000;
    }
}

class Manager extends Person{
    String department;

    public Manager(String name,int year,String department){
        super(name,year);
        this.department = department;
    }

    String walk(){
        return super.name +" is walking in "+this.department+" department.";
    }
}

// abstraction

abstract class Car {
    String brand;
    protected int price;
    abstract String getEngine();

    void startCar(){
        System.out.println("Car is starting ");
    }
}

class Tesla extends Car{
    void startCar(){
        super.startCar();
        System.out.println("Tesla is started");
    }
    public String getEngine(){
        return "Electric Engine";
    }
}

// Interfaces

interface Bike {
    void start();
}

class BMW implements Bike{

    @Override
    public void start() {
        System.out.println("BMW is started ");
    }
}