java - 为什么分配 'int constant -> byte variable' 有效，但 'long constant -> int variable' 无效？

Question

我有这段代码:

int i = 5l; // not valid (compile error)
byte b = 5; // valid

你怎么看？

为什么？

Answer 1

这是在 the JLS #5.2 (Assignment conversion) 中定义的:

If the expression is a constant expression (§15.28) of type byte, short, char, or int, a narrowing primitive conversion may be used if the type of the variable is byte, short, or char, and the value of the constant expression is representable in the type of the variable.

所以:

byte b = 5; //ok: b is a byte and 5 is an int between -128 and 127
byte b = 1000; //not ok: 1000 is an int but is not representable as a byte (> 127)
byte b = 5L; //not ok: 5L is a long (and not a byte, short, char or int)
int i = 5L; //not ok: i is not a byte, short or char
int i = 5; byte b = i; //not ok: i is not a constant
final int i = 5; byte b = i; //ok: i is a constant and b is a byte