问题描述
请考虑以下代码段:
class TypeCast{
public static void main(String[] args){
byte by = 4; //compiler casts int literal to byte
doCasting(4); //Compilation Error: external type casting is required. WHY ?
}
public static void doCasting(byte by){
}
}
我认为上面的代码片段是不言自明的.当 int 文字分配为 byte 类型时，编译器自动执行所需的强制转换.当我们调用带有int常量的 byte 参数的方法时，不会发生相同的事情.为什么?
I think above code snippet is quite self-explanatory. While int literal assignment to byte type, compiler does the required cast automatically. Same thing does not happen when we call a method taking byte parameter with int literal. Why?
推荐答案
这是分配上下文( JLS 5.2 )和调用上下文( JLS 5.3 )进行转换.
This is the difference between an assignment context (JLS 5.2) and an invocation context (JLS 5.3) for conversions.
分配上下文转换包括:
- 如果变量的类型为byte，short或char，并且常量表达式的值可表示为变量的类型，则可以使用缩窄基元转换.
在调用上下文转换中不存在 .
That isn't present in the invocation context conversions.
除了可能简化重载解析(如果您有)之外，我不清楚为什么要用这种方式设计语言.
It's not clear to me why the language was designed that way, other than to possibly simplify overload resolution - if you had:
doCasting(5);
...
doCasting(int x) {}
doCasting(byte b) {}
然后，您可以为它们中的任何一个辩称是最佳匹配".- byte 是比 int 更具体的类型，但是如果您认为文字是 int 类型，则 byte重载使得需要进行转换，而 int 重载则不需要转换.
then you could argue for either of them being the "best match" - byte is a more specific type than int, but if you think of the literal as being of type int, then the byte overload makes requires a conversion whereas the int overload doesn't.
通过使 byte 重载根本不适用，问题得以解决.
By making the byte overload simply not applicable, the question is removed.
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