# Java types: "primitives" vs "reference type" variables

The Java, we are working with 2 kinds of types:

primitives : in the variable we keep the value (Ex.: boolean, double, float, long, int, short, byte, char)
references : in the variable we keep the reference to the memory address where we keep the object/value.

Info

Because the primitives cannot be referenced, the values are bound to the stack.
The referenced variables keeps the pointer in the stack, but the object stays in the HEAP.

Info

When a referenced type variables is passed as parameter, the reference is passed as parameter not the object !

We consider the following example (created with Spring Boot):

package com.example.demo;

public class Car {
  String plateNumber;
  String color;

  void changePlateNumber(String plateNumber){
    this.plateNumber = plateNumber;
    System.out.println("Car :> Plate number changed.");
  }

  void changeColor(String color){
    this.color = color;
    System.out.println("Car :> Color changed.");
  }

  public Car(String plateNumber, String color) {
    this.plateNumber = plateNumber;
    this.color = color;
  }
}

package com.example.demo;

public class ProfessionalCarPeinting {

    Car peintInBlue(Car car) {
        System.out.println("Car identity (3) = "+car+" >> ProfessionalCarPeinting");
        car.color = "Blue";
        return car;
    }

}

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Demo1Application {

	public static void main(String[] args) throws InterruptedException {
		SpringApplication.run(Demo1Application.class, args);

		Car car1 = new Car("B 3333", "Red");
		System.out.println("Car identity (1) = "+car1+" >> main");

		var pcp = new ProfessionalCarPeinting();

		Car car2 = pcp.peintInBlue(car1);
		System.out.println("Car identity (2) = "+car2+" >> main");

		System.out.println("Car1 color = "+car1.color);

		car2.changePlateNumber("AD 22");
		System.out.println("Car1 plate number = "+car1.plateNumber);
	}
}

When we run we can see the following result:

Car identity (1) = com.example.demo.Car@70925b45 >> main
Car identity (3) = com.example.demo.Car@70925b45 >> ProfessionalCarPeinting
Car identity (2) = com.example.demo.Car@70925b45 >> main
Car1 color = Blue
Car :> Plate number changed.
Car1 plate number = AD 22