java - 类型提升 Java

Question

我正在通过 Herbert Schildt 的书学习 Java:Java 初学者指南。在那本书中出现了这段代码:

// 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);
    }
}

我不明白为什么我必须在行中使用 (byte):

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

b 被定义为一个字节，b * b 的结果是 100，这是一个字节的正确值 (-128...127)。

谢谢。

Answer 1

JLS ( 5.6.2. Binary Numeric Promotion ) 给出了有关将数字类型与二元运算符组合的规则，例如乘法运算符 (*):

• 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.

最后一点适用于你的情况，字节被转换为整数然后相乘。