Type promotion Java

Question

I`m learning Java with the Herbert Schildt book's: Java a Beginner's Guide. In that book appears this code:

// A promotion surprise!
class PromDemo{
    public static void main(String args[]){
        byte b;
        int i;
        b = 10;
        i = b * b;      // OK, no cast needed

        b = 10;
        b = (byte) (b * b);     // cast needed!!

        System.out.println("i and b: " + i + " " + b);
    }
}

I don't understand why I must use (byte) in the line:

b = (byte) (b * b);     // cast needed!!

b was defined as a byte and the result of b * b is 100 which is right value for a byte (-128...127).

Thank you.

Answer 1

The JLS (5.6.2. Binary Numeric Promotion) gives rules about combining numeric types with a binary operator, such as the multiplication operator (*):

• If either of the operands is of type double, the other one will be converted to a double.
• Otherwise, if either of the operands is of type float, the other one will be converted to a float.
• Otherwise, if either of the operands is of type long, the other one will be converted to a long.
• Otherwise, both operands will be converted to an int.

The last point applies to your situation, the bytes are converted to ints and then multiplied.