java - Java JLS 是否指定提升原始包装器类型？

Question

我对这个程序的输出有点迷惑:

public class xx {
    public static void main(String[] args) throws Exception {
        Number x = false ? new Long(123) : new Integer(456);
        System.out.println(x + " isa " + x.getClass().getName());
    }
}

这是它的输出:

456 isa java.lang.Long

编译器似乎正在将类型为 Integer 的对象“提升”为 Long，就像它通常会提升原始值一样。我从未听说过对象提升，这种行为似乎非常令人惊讶。

我的问题:根据 JLS，这真的是正确的行为吗？如果是这样的话，我希望能看到一份引用资料。

或者这是某种自动装箱疯狂的编译器错误？

我正在使用:

java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)

Answer 1

这实际上是二进制数字提升 ( JLS, Section 5.6.2 )。

When an operator applies binary numeric promotion to a pair of operands, each of which must denote a value that is convertible to a numeric type, the following rules apply, in order:

1. If any operand is of a reference type, it is subjected to unboxing conversion (§5.1.8).

2. Widening primitive conversion (§5.1.2) is applied to convert either or both operands as specified by the following rules:

   • If either operand is of type double, the other is converted to double.

   • Otherwise, if either operand is of type float, the other is converted to float.

   • Otherwise, if either operand is of type long, the other is converted to long.

   • Otherwise, both operands are converted to type int.

Binary numeric promotion is performed on the operands of certain operators:

...

• In certain cases, the conditional operator ? : (§15.25)

因此，操作数被取消装箱，456 被扩展为 456L。

此外，Long 和 Integer 的特定情况在条件运算符 JLS Section 15.25, Table 15.25-C 的 JLS 部分中明确涵盖。

bnp(Long,Integer)

其中“bnp”表示二进制数字提升。因此，条件运算符表达式的类型为 long，它被装箱到 Long 以分配给 Number。