Java integer到短轉換規則

Question

在代碼中，只有第一種情況（整數變量到 short）有編譯錯誤（從 int 到 short 的有損轉換）。 為什么其他情況（整數文字短而最終 integer 變量短）沒有同樣的編譯錯誤？

public class Test
{   
    public static void main(String[] args)
    {
        short shortNum = 0;
        
        //integer variable to short
        int intNum = 12;
        shortNum = intNum;
        
        //integer literal to short
        shortNum = 12;

        //final integer variable to short
        final int finalNum = 12;
        shortNum = finalNum;
    }
}

Answer 1

將答案視為評論：

short shortNum = 0;

//integer variable to short
int intNum = 12;
shortNum = intNum; //doesn't work, because casting primitives is required any time you are going from a larger numerical data type to a smaller numerical data type.
        
//integer literal to short
shortNum = 12; //This is not integer literal per se, you're initializing your short variable with 12, which fits in short data type, so it's short.

//final integer variable to short
final int finalNum = 12; //your variable is final, and according to Java Language Specification, if final variable's value fits into the type you're casting it, then no explicit cast is needed.
shortNum = finalNum;

Answer 2

答案在這篇文章中： Type cast issue from int to byte using final keyword in java 。

如果 Java 知道該數字是常數並且適合另一種類型，那么您可以使用它。

在非常量值的情況下，您無法知道變量將如何演變......在這種情況下，為了保護，它不會允許類型更改。