Why don't Java's +=, -=, *=, /= compound assignment operators require casting?

Question

Until today, I thought that for example:

i += j;

Was just a shortcut for:

i = i + j;

But if we try this:<

Answer 1

The main difference is that with a = a + b, there is no typecasting going on, and so the compiler gets angry at you for not typecasting. But with a += b, what it's really doing is typecasting b to a type compatible with a. So if you do

int a=5;
long b=10;
a+=b;
System.out.println(a);

What you're really doing is:

int a=5;
long b=10;
a=a+(int)b;
System.out.println(a);