How to compare two data types in Java

Question

I have a class Vehicule and 3 subclasses Voiture, Camion and Moto. I also have a method getVehicules for the all vehicules (i.e. camion, voiture, moto) but I want to print a specific type like Voiture.

I don't know how to solve it .

 List<Vehicule> vehicules = new ArrayList<>();

    public void getVehicules() {
            vehicules.forEach(vehicule->{
                System.out.println(vehicule);
            });     

     }
    
    public void getVoitures() {
        Iterator iterator = vehicules.iterator();
        while(iterator.hasNext()) {

      //I want to do a condtion if vehicule is voiture it will disaplay

            if(vehicules.equals(voiture)) {
                System.out.println(iterator);
            }
        }
    }

Answer 1

Pattern matching with switch

A new feature being previewed in Java 20 enables using a switch statement to test for subclasses. See: JEP 433: Pattern Matching for switch (Fourth Preview).

switch ( véhicule )
{
    case Voiture v -> System.out.println( "Voici une voiture." );
    case Camion c -> System.out.println( "Voici un camion." );
    case Moto m -> System.out.println( "Voici une moto." );
    default -> System.out.println( "Ooops! Il n'y a pas de véhicule." );
}

Wrap that in a for-each loop. Here is a complete example .java file.

package work.basil.example.subs;

import java.util.List;

public class App
{
    public static void main ( String[] args )
    {
        List < Véhicule > véhicules = List.of( new Voiture() , new Camion() , new Moto() );

        for ( Véhicule véhicule : véhicules )
        {
            switch ( véhicule )
            {
                case Voiture v -> System.out.println( "Voici une voiture." );
                case Camion c -> System.out.println( "Voici un camion." );
                case Moto m -> System.out.println( "Voici une moto." );
                default -> System.out.println( "Ooops! Il n'y a pas de véhicule." );
            }
        }
    }
}

class Véhicule { }

class Voiture extends Véhicule { }

class Camion extends Véhicule { }

class Moto extends Véhicule { }

When run:

Voici une voiture.
Voici un camion.
Voici une moto.

default case

Notice the default at the bottom of the switch to catch any other subclasses that may defined at the end.

Another new feature delivered in Java 17 can eliminate the need for that default, if we choose to disallow further direct subclasses. The new feature is sealed classes. See JEP 409: Sealed Classes. This feature defines a way to declare a list of known subclasses, and to declare that no more will be allowed. Therefore the compiler knows at compile-time all possible subclasses. The compiler can then check that your switch statement has covered all the possibilities.

sealed class Véhicule permits Voiture, Camion, Moto { }

final class Voiture extends Véhicule { }

final class Camion extends Véhicule { }

final class Moto extends Véhicule { }

Answer 2

How to compare two data type in Java You can get the type (a java.lang.Class) of an object using object.getClass().

You can compare two classes using equals(...); e.g.

if (obj1.getClass().equals(obj2.getClass())

You can test if an object has a specific class in a couple of ways

if (obj.getClass().equals(Car.class)) 

or

if (obj instanceof Car)

(They do slightly different things. The latter tests if obj is a Car or a subclass of Car and will return false if obj is null.)

There are methods on the Class class that will give the classes simple name or its full name ... or you can just use the Class.toString() method.

---

Just for the record, another way to solve the problem that doesn't involve using the class names would be to define a second (abstract) method in the superclass that returns a String that denotes the kind of the vehicle; e.g.

   public abstract class Vehicle {
      ...
      public abstract String getType();
   }

   public class Car extends Vehicle {
      
      @Override
      public String getType() {
          return "Car";
      }
   }

Answer 3

Check this out, instanceof would help here :-

class Vehicule {

}

class Voiture extends Vehicule {

    @Override
    public String toString() {
        return "Voiture";
    }
}

class Camion extends Vehicule {

    @Override
    public String toString() {
        return "Camion";
    }
}

class Moto extends Vehicule {
    @Override
    public String toString() {
        return "Moto";
    }
}

public class Operation {
    
    static List<Vehicule> vehicules = new ArrayList<>();
    
    public static void main(String[] args) {
        vehicules.add(new Camion());
        vehicules.add(new Moto());
        vehicules.add(new Voiture());
        //Prints all objects
        getVehicules();
        //Prints only voiture
        getVoitures();

    }

    public static void getVehicules() {
        vehicules.forEach(vehicule -> {
            System.out.println(vehicule);
        });

    }

    public static void getVoitures() {
        Iterator iterator = vehicules.iterator();
        while (iterator.hasNext()) {
            Object vehicule = iterator.next();
            if (vehicule instanceof Voiture) {
                System.out.println(vehicule);
            }
        }
    }


}