java – 当一个语句被认为是单入口/单出口时,当它不是时？

public int stringLength(String s) {
  return s.length();
}

两个出口点方法：

public int stringLength(String s) {
  if(s == null) {
    return 0;
  }
  return s.length();
}

以下是Martin Fowler的书Refactoring的引用：

I often find I use Replace Nested Conditional with Guard Clauses when I’m working with a programmer who has been taught to have only one entry point and one exit point from a method. One entry point is enforced by modern languages,and one exit point is really not a useful rule. Clarity is the key principle: if the method is clearer with one exit point,use one exit point; otherwise don’t.

并举例说明上述陈述,比较这两种方法的代码：

double getPayAmount() { 
    double result; 
    if (_isDead) result = deadAmount(); 
    else {
        if (_isSeparated) result = separatedAmount(); 
        else {
            if (_isRetired) result = retiredAmount(); 
            else result = normalPayAmount();
        };
    } 
    return result; 
};

并有一些退出点：

double getPayAmount() { 
    if (_isDead) return deadAmount(); 
    if (_isSeparated) return separatedAmount(); 
    if (_isRetired) return retiredAmount();    
    return normalPayAmount();
};

Nested conditional code often is written by programmers who are taught to have one exit point from a method. I’ve found that is a too simplistic rule. When I have no further interest in a method,I signal my lack of interest by getting out. Directing the reader to look at an empty else block only gets in the way of comprehension.