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.
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