from：java language specification

15.26.2 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.

For example, the following code is correct:


    short x = 3;
    x += 4.6;

and results in x having the value 7 because it is equivalent to:


    short x = 3;
    x = (short)(x + 4.6);