Java中方法调用和参数之间的求值顺序 [英] Evaluation order between a method call and arguments in Java

问题描述

处理另一个SO问题，我想知道下面的代码是否具有未定义的行为:

Dealing with another SO question, I was wondering if the code below has an undefined behavior:

if (str.equals(str = getAnotherString())) {
  // [...]
}

我倾向于认为equals()调用所依据的str引用是在之前作为参数传递的进一步的str赋值进行评估的.是否有消息来源?

I tend to think the str reference from which the equals() call is made is evaluated before the further str assignment passed as argument. Is there a source about it?

推荐答案

在

在运行时，方法调用需要五个步骤.首先，可以计算目标参考.其次，对参数表达式进行求值. [...]

At run time, method invocation requires five steps. First, a target reference may be computed. Second, the argument expressions are evaluated. [...]

什么是目标参考"?你问?这将在下一部分中指定:

What's a "target reference" you ask? This is specified in the next subsection:

15.12.4.1.计算目标参考(如有必要)

...

• 如果form是ExpressionName. [TypeArguments]标识符，然后:
  • 如果调用模式是静态的，则没有目标引用.对ExpressionName进行评估，但结果将被丢弃.
  • 否则，目标引用是ExpressionName表示的值.
  • If form is ExpressionName . [TypeArguments] Identifier, then:
    • If the invocation mode is static, then there is no target reference. The ExpressionName is evaluated, but the result is then discarded.
    • Otherwise, the target reference is the value denoted by ExpressionName.

    因此，目标参考"是str.equals中的str位-您要在其上调用该方法的表达式.

    So the "target reference" is the str bit in str.equals - the expression on which you are calling the method.

    如第一个引号所述，首先对目标引用进行求值，然后对参数进行然后.因此，str.equals(str = getAnotherString())仅在getAnotherString返回与赋值表达式之前的字符与str相同的字符串的情况下计算为true.

    As the first quote says, the target reference is evaluated first, then the arguments. Therefore, str.equals(str = getAnotherString()) only evaluates to true if getAnotherString returns a string that has the same characters as str before the assignment expression.

    是的，您倾向于认为的事情是正确的.但这不是"未定义的行为".

    So yeah, the thing that you tend to think is correct. But this is not "undefined behaviour".

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