Java Integer underflow / overflow not happen during math operation?

It’s happening because the Java language says it must. See section 5.1.3, Narrowing Primitive Conversion.

Otherwise, if the floating-point number is not an infinity, the
floating-point value is rounded to an integer value V using the round
toward zero rounding policy (§4.2.4). Then there are two cases:
a. If
T is long, and this integer value can be represented as a long, then
the result of the first step is the long value V.
b. Otherwise, if
this integer value can be represented as an int, then the result of
the first step is the int value V.
• Otherwise, one of the following
two cases must be true:
a. The value must be too small (a negative
value of large magnitude or negative infinity), and the result of the
first step is the smallest representable value of type int or long.
b.
The value must be too large (a positive value of large magnitude or
positive infinity), and the result of the first step is the largest
representable value of type int or long

This last clause is what you are witnessing in action.