为什么三元运算给出了nullpointer，而ifelse对应的呢？

问题描述

我在下面的一个实例中得到NullPointerException，而它的对应物运行顺畅。

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);
}

，从 withIfElse 输出 null ，然后在<$中失败c $ c> 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

以下是：

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:

• 如果第二个第三个操作数都是 Boolean 类型，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 ，以及 Boolean 值是自动装箱的，导致 NullPointerException 。

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

如评论中所述，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);

嗯，上面的规范摘录特别适用于布尔条件表达式，指定：

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

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（）类型为 boolean 或 Boolean 。

带括号的布尔表达式（）。

类实例创建表达式（）类布尔。

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

方法调用表达式（）选择最具体的方法（）返回类型 boolean 或 Boolean 。

（注意，对于泛型方法，这是在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.)

A 布尔条件表达式。

由于 null 不是布尔表达式，整体条件表达式不是布尔值条件表达式。参考表15.2（稍后在同一节中），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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