What is the difference using a += b and a += (int)b ?

Question

public class Program{
public static void main(String[] args) {

int a = 2;
double b = 2.7;

b += a;
a += (int)b;

System.out.println(a);
}
}

I have this code, the output is 6, but I don't understand the difference between a += b and a += (int)b, the output is the same, so, is not necessary in this case to use a += (int)b ?

Answer 1

So after re-reading the question I may have re-editted my post a bunch 😜.

a += b; is an 'automatic' convertion. The compiler will be able to interpret the result as an int due to the +=. However if you did a = a + b, that will for sure throw an error.
So a += b is equivalent to: a = (int)(a + (b));

a = a + b; will throw an error because an int cannot be given the value of a double.
So, you need to cast the double to an int, which will give you:
a = a + (int) b;

Just note a += (int) b Might actually be the same as: a = (int) (a +((int) b));
Though, I'm not 100% sure about this, since the compiler could be aware and just ignore the extra conversion.

Answer 2

In both cases the code is downcasting the value in b from double to int.

But doing it this way: a += (int)b - is preferable because it makes your intentions explicit.