java - 为什么空数组引用的数组访问表达式不抛出 NullPointerException？

Question

考虑以下代码:

int[] r = null;
r[0] = 1 % 0;

我预计这会抛出一个NullPointerException:根据JLS Sec 15.7.1 :

The left-hand operand of a binary operator appears to be fully evaluated before any part of the right-hand operand is evaluated.

= 是一个二元运算符(在 JLS Sec 15.2 中显示 - JLS Sec 15.26 描述了赋值运算符)，完全计算左侧操作数将导致 NullPointerException。但是，将抛出 ArithmeticException，表示在完全计算左侧操作数之前计算右侧操作数。

为什么？

Answer 1

the simple assignment operator的规范描述了这种行为:

...

If the left-hand operand is an array access expression (§15.10.3), possibly enclosed in one or more pairs of parentheses, then:

• First, the array reference subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the index subexpression (of the left-hand operand array access expression) and the right-hand operand are not evaluated and no assignment occurs.

正常完成。

• Otherwise, the index subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and the right-hand operand is not evaluated and no assignment occurs.

正常完成。

• Otherwise, the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.

这突然完成，并出现 ArithmeticException。

• Otherwise, if the value of the array reference subexpression is null, then no assignment occurs and a NullPointerException is thrown.

这永远不会执行。

因此，在第 15.7.1 节的引述中似乎存在不一致 - 或者过度简化，至少。

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有趣的是，复合赋值运算符没有观察到相同的行为，例如

int[] arr = null;
arr[0] += 1 % 0;

确实产生NullPointerException。

JLS Sec 15.26.2描述这个。不过，这也许并不令人惊讶，因为:

A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T) ((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.

换句话说，这段代码(大致)等同于:

arr[0] = arr[0] + 1 % 0;

因此 NullPointerException 发生在评估简单赋值的右手操作数时。