java - "+=" operator and int long usage

int a = 1L;

This doesn't compile (of course).
incompatible types: possible lossy conversion from long to int

int b = 0;
b += Long.MAX_VALUE;

This does compile!

But why is it allowed?

Answer

When you do += that's a compound statement and Compiler internally casts it. Where as in first case the compiler straight way shouted at you since it is a direct statement :)

The line

b += Long.MAX_VALUE;

Compiler version of it equivalent of

b += (int)Long.MAX_VALUE;

of course there will be lossy conversion from conversion of long to int.

http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.26.2

A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T) ((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.