# Casting in Java

# Casting for primitives

In Java, for primitives we have 2 kinds of casting: widening/upcasting & narrowing/down casting.

Widening casting (Upcasting) (could be done automatically)

double <- float <- long <- int <- short <- byte

Widening casting
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);

		int myInt = 7;

		// Automatic casting: int -> double (no loose of data)
		double myDouble = myInt;

		System.out.println(myInt);
		System.out.println(myDouble);
	}
}

And when we run we receive:

7
7.0

Narrowing casting (Downcasting) (must be done manually)

double -> float -> long -> int -> short -> byte

Info

-32 768 <= short <= 32 767

-128 <= byte <= 127

Narrowing casting
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);

		double myDouble = 7.8;
		int myInt;

		// Manual casting: double -> int (loose of data could exist)
		myInt  = (int)myDouble;

		System.out.println(myInt);
		System.out.println(myDouble);
	}
}

And when we run we receive:

7
7.8

# Casting for reference type variables

The main idea of the casting is the same. Please take a look at the following examples.

Casting examples
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);

		Vehicle vehicle = new Vehicle("VE 100");
		Car car = new Car("CA 100");

		System.out.println("Widening casting (Upcasting)");
		System.out.println("----------------------------");

		Vehicle vehicle2 = car;
		vehicle2.changePlateNumber("AAA");

		System.out.println(" ");
		System.out.println("Narrowing casting (Downcasting)");
		System.out.println("----------------------------");

		Car car2 = (Car) vehicle2;
		car2.changePlateNumber("AAA");
		car2.changeColor("Blue");
	}
}

When we run the code above we will see in the console:

Widening casting (Upcasting)
----------------------------
Car :> Plate number changed.
 
Narrowing casting (Downcasting)
----------------------------
Car :> Plate number changed.
Car :> Change color.

Remarks

The casting is done automatically when we have no data loss.
The casting is done between the same variable type.
A primitive type value can be cast to a reference type by boxing conversion.
A reference type value can be cast to a primitive type by unboxing conversion.

Here we have a Boxing/ unboxing example:

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);

    int x = 10;

    //Boxing
    Integer x1 = x;
    Integer x2 = x1;
    x1 = x1 * 10;

    //Unboxing
    int int1 = x1;

    x = 8;

    System.out.println("x="+x);
    System.out.println("x1="+x1);
    System.out.println("x2="+x2);
    System.out.println("int1="+int1);
  }
}

... and the result is :

x=8
x1=100
x2=10
int1=100

Remarks

Boxing conversion creates a Wrapper Class (are the immutable classes) for a primitive. Wrapper Classes are using "pass by value" pattern, and not "pass by reference".
The usage of the Wrapper Classes costs memory and speed, but the main reason to create wrapper classes is their usage in Collection Framework as collections can only accept objects.
The Wrapper Classes are stored on HEAP memory as other objects from the HEAP memory is supposed to use these Wrapper Classes.