java - 什么时候说语句是单次进入/单次退出，什么时候不是？

Question

我不确定我是否理解得很好，例如Java中的if语句被称为单入口/单导出语句。如果它的条件为真，这是否被认为是它的单一入口点，如果条件为假，它是否被认为是它的单一导出点？

if(someCondition)
   doSomething();

非-(单次进入/单次退出)语句的示例是什么？

Answer 1

一个导出点法(single-exit):

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

两种退出点方法:

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

下面引用自 Martin Fowler 的书Refactoring:

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.

和上面语句的一个例子，比较这两种方法的代码做同样的事情:

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

还有一些导出点:

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.