create object using abstract classes

Question

I am trying to understand better oop ,but I don't understand how to use abstract classes. I have a car class, and a car can be mercedes , audi and volvo. Mercedes cars can have 5 passengers ,can open door and block windows; audi car can have 6 passengers, and just can open door. Volvo car can only have 3 passengers. So, in order to do this, I created an interface:

public interface Car{
void openDoor();
void blockWindows();
int passengers=0;
}

then for each car I created an abstract class:

public abstract class  Mercedes implements Car{
public void openDoor(){
    System.out.println("Mercedes opendoor");
}
  public void blockWindow(){
    System.out.println("Mercedes blockwindow");
}
public Mercedes()
{
int passengers=5;
}



 public abstract class  Audi implements Car{
public void openDoor(){
    System.out.println("Audi opendoor");
}
public Audi()
{
int passengers=6;
}
  }

public abstract class  Volvo implements Car{

public Volvo()
{
int passengers=6;
}

Now, I need to create an object that can transport maximum 15 cars. So I wrote:

public class TransportCars{
Car[] transport=new Car[15];}

//now I need to put in transport array differents types of cars. But I can not instantiate abstract classes. Should I use anything else? I used abstract classes because I can implement an interface and use just o part of it

Answer 1

Basically your design is completely wrong, as you are yet new to java, you first need to understand basic.

Design should be like this :

Car is a Vehical, so is-a relationship.

So you can create a Class Vehicle.

class Vehicle {
   // properties of Vehicle like type of Vehicle, numberOfWheels etc.
   String vType;
   int numberOfWheels;
   int passengers;
}

// Car is a Vehicle so it should extend Vehicle
class Car extends Vehicle {
    String type; // sedan or hatchback
    String manufacturer; // Mercedes, BMW, Audi, Volvo etc.
}

If you want to restrict Vehicle not to be instantiated, you can declare it as an abstract class

Answer 2

Interfaces and abstract classes have some similarities, but are not the same. For your example, you should probably make Car an abstract and have Mercedes, Audi, and Volvo extend that abstract class. Then make sure to implement any abstract methods in Car in each of your classes which extend it. Doing so will make them concrete classes, which can be instantiated.

One thing you can do with abstract classes which you cannot with interfaces, is include data. I see you're already doing that with your interface for number of passengers, but in an interface, the value will be static and final.