why do we create abstract class in java code example

Example 1: abstract class in java

Sometimes we may come across a situation where we cannot provide 
implementation to all the methods in a class. We want to leave the 
implementation to a class that extends it. In such case we declare a class
as abstract.To make a class abstract we use key word abstract. 
Any class that contains one or more abstract methods is declared as abstract. 
If we don’t declare class as abstract which contains abstract methods we get 
compile time error.
  
  1)Abstract classes cannot be instantiated
  2)An abstarct classes contains abstract method, concrete methods or both.
  3)Any class which extends abstarct class must override all methods of abstract
    class
  4)An abstarct class can contain either 0 or more abstract method.

Example 2: abstract class java constructor

/**
 * Simple Java program to prove that abstract class can have constructor in Java.
 * @author http://java67.blogspot.com
 */
public class AbstractConstructorTest {

    public static void main(String args[]) {
       Server server = new Tomcat("Apache Tomcat");
       server.start();
    }
}

abstract class Server{
    protected final String name;
   
    public Server(String name){
        this.name = name;
    }
   
    public abstract boolean start();
}

class Tomcat extends Server{
   
    public Tomcat(String name){
        super(name);
    }

    @Override
    public boolean start() {
       System.out.println( this.name + " started successfully");
       return true;
    }
   
}

Output:
Apache Tomcat started successfully

Example 3: java abstract

abstract class Pesan {
   public void success() {
     System.out.println("Mobil Berhasil Dibeli");
   }
   
   public void error() {
     System.out.println("Uang Anda Tidak Cukup");
   }
}
 
class Car extends Pesan {
    protected String nama = "toyota supra";
    protected String warna = "merah";
    protected int harga = 2000000000;
    protected String brand = "toyota";
}
 
class ShowRoom extends Car {
  protected String namaShowroom = "Catur Sentosa Raya";
  protected String alamatShowroom = "Jl.siliwangin kec pancoranmas kota depok 16436";
}
 
class Pembeli extends ShowRoom {
    protected String namaPembeli = "anto jayabaya";
    protected String alamatPembeli = "jl.swadaya rt.01/rw.04 no.112 kec pancoranmas kota depok";
    protected int saldoPembeli = 50000000;
}
 
class BeliMobil extends Pembeli {
 
 public BeliMobil(String nama, String warna, int harga, String brand, String nsr, String asr, String np, String ap, int sdp) {
    super();
    
    super.nama = nama;
    super.warna = warna;
    super.harga = harga;
    super.brand = brand;
    super.namaShowroom = nsr;
    super.alamatShowroom = asr;
    super.namaPembeli = np;
    super.alamatPembeli = ap;
    super.saldoPembeli = sdp;
 }
    
void getResult(String nama, String warna, int harga, String brand, String np, String ap) {
    
     if(super.harga > super.saldoPembeli) {
       System.out.println("=======================");
       super.error();
       System.out.println("=======================");
     } else {
       System.out.println("=======================");
       super.success();
       System.out.println("=======================");
       System.out.println("");
       System.out.println("=======================");
       System.out.println("Jenis Mobil");
       System.out.println("=======================");
       System.out.println("");
       System.out.println("Nama Mobil:" + nama);
       System.out.println("Warna Mobil:" + warna);
       System.out.println("Harga Mobil:" + harga);
       System.out.println("Brand Mobil:" + brand);
       System.out.println("");
       System.out.println("=======================");
       System.out.println("Nama Pembeli Mobil");
       System.out.println("=======================");
       System.out.println("Nama Pembeli:" + np);
       System.out.println("Nama Pembeli:" + ap);
     }
  }
    
 public static void main(String[] args) {
   BeliMobil beli = new BeliMobil("avanza", "hitam", 128000000, "toyota", "Jaya Mobil", "Jakarta", "Anton", "Depok", 228000000);
   beli.getResult(beli.nama, beli.warna, beli.harga, beli.brand, beli.namaPembeli, beli.alamatPembeli);
  }
}