Primitive type casting

Question

I have been working with Java's byte primitive lately, and I came across a silly problem:

byte a = 10;
byte b = 9;
byte c = 8;
b += b*c;    // how come this statement is correct without any explicit type casting
b =  b*c;    // while this statement is incorrect; it requires explicit cast, of course
b = b+(b*c); // this is wrong too.

So my question is, does += specify any assignment other than just add and assign, or is this a bug in Java (which I am almost sure is not)?

Answer 1

Because b += b*c is equivalent to b += (byte) ((b) + (b*c)) .

From the Java language specification on compound assignment operators :

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.

Answer 2

All compound assignment operators not only performs the operation, but also cast automatically their result to the type of the left-hand side variable.

So your += not only adds variables and assign the result - it also cast the result to the right type.