“错误"在 Java 中使用 if 与三元运算符时的返回类型

问题描述

在下面的类中，两个方法的返回类型与三元运算符的想法不一致:

In the following class, the return type of the two methods is inconsistent with the idea that the ternary operator:

return condition?a:b;

相当于

if(condition) {
    return a;
} else{
    return b;
}

第一个返回一个 Double，第二个返回一个 Long:

The first returns a Double and the second a Long:

public class IfTest {
    public static Long longValue = 1l;
    public static Double doubleValue = null;

    public static void main(String[] args) {
        System.out.println(getWithIf().getClass());// outpus Long
        System.out.println(getWithQuestionMark().getClass());// outputs Double
    }

    public static Object getWithQuestionMark() {
        return doubleValue == null ? longValue : doubleValue;
    }

    public static Object getWithIf() {
        if (doubleValue == null) {
            return longValue;
         } else {
            return doubleValue;
        }
    }
}

我可以想象这与编译器窄范围转换 getWithQuestionMark() 的返回类型有关，但这种语言是否明智?这当然不是我所期望的.

I can imagine this has to do with the compiler narrow casting the return type of getWithQuestionMark() but is that language wise ok? It's certainly not what I would have expected.

欢迎提供任何见解！

下面有很好的答案.此外，@sakthisundar 引用的以下问题探讨了三元运算符中发生的类型提升的另一个副作用:Java 中棘手的三元运算符 - 自动装箱

there's very good answers below. Additionally, the following question referenced by @sakthisundar explores another side effect of the type promotion occurring in the ternary operator: Tricky ternary operator in Java - autoboxing

推荐答案

基本上遵循了JLS 第 15.25 节，特别是:

Basically it's following the rules of section 15.25 of the JLS, specifically:

否则，如果第二个和第三个操作数具有可转换(第 5.1.8 节)为数字类型的类型，则有几种情况:

• [...]

• [...]

否则，对操作数类型应用二进制数字提升(第 5.6.2 节)，条件表达式的类型是第二个和第三个操作数的提升类型.

Otherwise, binary numeric promotion (§5.6.2) is applied to the operand types, and the type of the conditional expression is the promoted type of the second and third operands.

所以 第 5.6 节.2 之后，这将基本上涉及拆箱 - 所以这使您的表达式工作，好像 longValue 和 doubleValue 是 long 类型和double，对long进行加宽提升得到double的整体结果类型.

So section 5.6.2 is followed, which will basically involves unboxing - so this makes your expression work as if longValue and doubleValue were of types long and double respectively, and the widening promotion is applied to the long to get an overall result type of double.

那个 double 然后被装箱以便从方法中返回一个 Object.

That double is then boxed in order to return an Object from the method.

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