Java 整数到短的转换规则

Posted 2023-02-16

【中文标题】Java 整数到短的转换规则

【英文标题】：Java integer to short conversion rules

【发布时间】：2020-07-30 16:20:37

【问题描述】：

在代码中，只有第一种情况（整数变量到 short）有编译错误（从 int 到 short 的有损转换）。为什么其他情况（整数文字转短，最终整数变量转短）没有同样的编译错误？

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;
    

【问题讨论】：

编译器可以看到常量值12 适合一个short。

【参考方案1】：

将答案视为 cmets：

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;

【讨论】：

还要注意byte、short 和char 数据类型在与算术运算符一起使用时有一个棘手的部分。例如short x = 2; short y = 3; short z = x + y; 将无法编译，因为结果被提升为int

这可以这样表达：From OCA Java SE 8 Book: short values are automatically promoted to int when applying any arithmetic operator, with the resulting value being of type int.

【参考方案2】：

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

如果 Java 知道该数字是一个常量并且适合另一种类型，那么您可以使用它。

在非常量值的情况下，你无法知道变量将如何演变......在这种情况下，为了保护，它不会允许类型更改。