为什么三元运算给出空指针而它的 ifelse 对应物没有? [英] Why ternary operation gives nullpointer while its ifelse counterpart doesn't?

问题描述

我在下面的一个实例中收到 NullPointerException，而其对应的运行流畅.

I am getting NullPointerException in one instance below while its counterpart runs smooth.

public static void main(String[] args){
    System.out.println(withTernary(null, null)); //Null Pointer
    System.out.println(withIfElse(null, null));  //No Exception
}

private static Boolean withTernary(String val, Boolean defVal){
    return val == null ? defVal : "true".equalsIgnoreCase(val);
}

private static Boolean withIfElse(String val, Boolean defVal){
    if (val == null) return defVal;
    else return "true".equalsIgnoreCase(val);
}

网络版

在线版本，main 中的行颠倒了，输出 null 从withIfElse 然后在 withTernary 中失败.

Online version with the lines in main reversed, which outputs null from withIfElse and then fails in withTernary.

我正在使用以下 java 版本

I am using following java version

java version "1.6.0_65"
Java(TM) SE Runtime Environment (build 1.6.0_65-b14-462-11M4609)
Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-462, mixed mode)

推荐答案

这是来自 规范(第 15.25.2 节):

布尔条件表达式是独立的表达式(§15.2).

Boolean conditional expressions are standalone expressions (§15.2).

布尔条件表达式的类型确定如下:

The type of a boolean conditional expression is determined as follows:

• 如果第二个和第三个操作数都是Boolean类型，则条件表达式的类型为Boolean.

• If the second and third operands are both of type Boolean, the conditional expression has type Boolean.

否则，条件表达式的类型为boolean.

Otherwise, the conditional expression has type boolean.

因此，整个表达式的类型被认为是boolean，并且Boolean 值被自动拆箱，导致NullPointerException.

Therefore, the overall expression's type is considered to be boolean, and the Boolean value is autounboxed, causing a NullPointerException.

正如评论中提到的，为什么以下不会引发异常?

As mentioned in the comments, why doesn't the following raise an exception?

return val == null ? null : "true".equalsIgnoreCase(val);

好吧，规范的上述摘录仅适用于布尔条件表达式，这些表达式被指定为此处(第 15.25 节):

Well, the above excerpt from the spec specifically only applies to boolean conditional expressions, which are specified here (§15.25):

如果第二个和第三个操作数表达式都是布尔表达式，则条件表达式为布尔条件表达式.

If both the second and the third operand expressions are boolean expressions, the conditional expression is a boolean conditional expression.

为了对条件进行分类，以下表达式是布尔表达式:

For the purpose of classifying a conditional, the following expressions are boolean expressions:

• 独立表单的表达式(§15.2) 类型为 boolean 或 Boolean.

带括号的 boolean 表达式 (§15.8.5).

A parenthesized boolean expression (§15.8.5).

一个类实例创建表达式(§15.9) 用于 Boolean 类.

A class instance creation expression (§15.9) for class Boolean.

一个方法调用表达式(§15.12) 选择最具体的方法 (§15.12.2.5) 具有返回类型 boolean 或 Boolean.
(请注意，对于泛型方法，这是实例化方法的类型参数之前的类型.)

A method invocation expression (§15.12) for which the chosen most specific method (§15.12.2.5) has return type boolean or Boolean.
(Note that, for a generic method, this is the type before instantiating the method's type arguments.)

一个 boolean 条件表达式.

由于null 不是布尔表达式，所以整个条件表达式也不是布尔条件表达式.参考表 15.2(在同一节后面)，我们可以看到这样的表达式被认为具有 Boolean 类型，因此不会发生拆箱，也不会引发异常.

Since null is not a boolean expression, the overall conditional expression is not a boolean conditional expression. Referring to Table 15.2 (later in the same section), we can see that such an expression is considered to have a Boolean type, so no unboxing occurs, and no exception is raised.

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