Learn simple Java abstract class points and examples

A few points to note when using abstract classes:

Classes containing one or more abstract methods must be declared as abstract classes.
Declare a class as an abstract class, not necessarily with abstract methods.
It is generally believed that concrete methods should not be included in abstract classes, and it is recommended that common fields and methods be placed in superclasses whenever possible.
Abstract classes cannot be instantiated. Objects of this class cannot be created
Example code:

import java.util.*;

public class PersonTest
{
   public static void main(String[] args)
   {
      Person[] people = new Person[2];
      // fill the people array with Student and Employee objects
      people[0] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
      people[1] = new Student("Maria Morris", "computer science");
      // print out names and descriptions of all Person objects
      for (Person p : people)
         System.out.println(p.getName() + ", " + p.getDescription());
   }
}
abstract class Person
{
   public Person(String n)
   {
      name = n;
   }
   public abstract String getDescription();
   public String getName()
   {
      return name;
   }
   private String name;
}
class Employee extends Person
{
   public Employee(String n, double s, int year, int month, int day)
   {
      super(n);
      salary = s;
      GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
      hireDay = calendar.getTime();
   }
   public double getSalary()
   {
      return salary;
   }
   public Date getHireDay()
   {
      return hireDay;
   }
   public String getDescription()
   {
      return String.format("an employee with a salary of $%.2f", salary);
   }
   public void raiseSalary(double byPercent)
   {
      double raise = salary * byPercent / 100;
      salary += raise;
   }
   private double salary;
   private Date hireDay;
}
class Student extends Person
{
   
   public Student(String n, String m)
   {
      // pass n to superclass constructor
      super(n);
      major = m;
   }
   public String getDescription()
   {
      return "a student majoring in " + major;
   }
   private String major;
}

In the code block:

 for (Person p : people)
         System.out.println(p.getName() + ", " + p.getDescription());

P.getdescription (), which references methods of subclass objects of concrete subclasses.

You cannot omit getDescription() from the Person class because the compiler only allows you to call methods declared in the class.