What is Arithmetic promotion assignment and casting?

In an assignment statement, i.e. if there's an equals sign, Java compares the type of the left hand side to the at last type of the right hand side. It won't change the type of the left hand side, but it will check to make sure in which the value it has (double, float, int or long) on the right hand side can fit in the type on the left hand side. Anything can fit in a double. Anything except a double could fit in a float. Any integral type can fit in a long, but a float or a double can't, and ints, shorts, and bytes can fit inside ints. If the right hand side can fit inside the left hand side, the assignment takes place along with no additional ado.

Assigning long values to int variables or double values to float variables can be equally troublesome. In fact it's so troublesome the compiler won't let you do it unless you tell it you actually mean it with a cast. When it's essential to force a value within a particular type, use a cast. To cast a variable or a literal or an expression to a different data type just precede it with the kinds in parentheses. For example:

int i = (int) (9.0/4.0);

A cast lets the compiler know in which you're serious about the conversion you plan to make.

While a value is cast down before assignment, series of operations takes place to chop the right hand side down to size. For a conversion among a floating point number and an int or a long, the fractional part of the floating point number is truncated (rounded toward zero). This generates an integer. If the integer is small sufficient to fit in the left hand side, the assignment is completed. Instead if the number is too large, then the integer is set to the largest probable value of its type. If the floating point number is too little the integer is set to the smallest probable value of its type.

This can be a nasty bug in your code. It can also be hard to search since everything might work perfectly 99 times out of a hundred and only on rare occasions will the rounding become a problem. Therefore while it does there will be no warning or error message. You required being extremely careful when assigning floating point values to integer types.