java - 两个字节相乘

Question

有人能解释一下为什么我不能用这种方式将两个字节相乘吗？

byte a = 1;
byte b = 1;
byte c = a*b;

或

byte a = 1;
byte b = 1;
short c = a*b;

为什么我必须这样做？

byte a = 1;
byte b = 1;
byte c = (byte)(a*b);

或

byte a = 1;
byte b = 1;
int/double/float/long c = a*b;

Answer 1

当使用byte 执行数学运算时，二进制数字提升 会发生，如JLS, Section 5.6.2 所指定的那样.

When an operator applies binary numeric promotion to a pair of operands, each of which must denote a value that is convertible to a numeric type, the following rules apply, in order:

1. If any operand is of a reference type, it is subjected to unboxing conversion (§5.1.8).

2. Widening primitive conversion (§5.1.2) is applied to convert either or both operands as specified by the following rules:

   • If either operand is of type double, the other is converted to double.

   • Otherwise, if either operand is of type float, the other is converted to float.

   • Otherwise, if either operand is of type long, the other is converted to long.

   • Otherwise, both operands are converted to type int.

(强调我的)

这会强制您分配给至少与 int 一样宽的类型，或者转换回 byte。