java - 为什么 Long (Wrapper) 在转换为字节时没有拆箱为原始 long？

Question

为什么Long(包装器)不拆箱为原始长整型，然后转换为原始字节？

Long lo = 4L;

byte b = (byte) lo;     // c.ERR !!!

Byte bW = (byte) lo;    // c.ERR !!!

Answer 1

因为 Java 语言规范是这么说的!

JLS § 5.5 Casting Contexts

Casting contexts allow the use of one of:

• an identity conversion (§5.1.1)

• a widening primitive conversion (§5.1.2)

• a narrowing primitive conversion (§5.1.3)

• a widening and narrowing primitive conversion (§5.1.4)

• a widening reference conversion (§5.1.5) optionally followed by either an unboxing conversion (§5.1.8) or an unchecked conversion (§5.1.9)

• a narrowing reference conversion (§5.1.6) optionally followed by either an unboxing conversion (§5.1.8) or an unchecked conversion (§5.1.9)

• a boxing conversion (§5.1.7) optionally followed by a widening reference conversion (§5.1.5)

• an unboxing conversion (§5.1.8) optionally followed by a widening primitive conversion (§5.1.2).

• Value set conversion (§5.1.13) is applied after the type conversion.

上面的列表不包含您在此处尝试执行的转换，即拆箱转换(Long 到 long)，然后是缩小转换( >长到字节)。有趣的是，如果您缩小范围然后拆箱，这是允许的!

本节后面的表格也显示了这一点:

最后一行是 -，不允许进行强制转换。