java : Function objects as strategies

Question

我正在阅读 Effective Java。在讨论使用函数对象作为策略的部分中，存在以下段落。

Because the strategy interface serves as a type for all of its concrete strategy instances, a concrete strategy class needn’t be made public to export a concrete strategy. Instead, a “host class” can export a public static field (or static factory method) whose type is the strategy interface, and the concrete strategy class can be a private nested class of the host

   // Exporting a concrete strategy
  class Host {
        private static class StrLenCmp
               implements Comparator<String>, Serializable {

        public int compare(String s1, String s2) {
            return s1.length() - s2.length();
        }
    }

    // Returned comparator is serializable
    public static final Comparator<String>
    STRING_LENGTH_COMPARATOR = new StrLenCmp();
    ... // Bulk of class omitted
  }        

我的问题是，使用上述方式有什么特别的优势吗？通过公开具体策略来导出策略有什么问题？

Answer 1

是的，有。这样你返回的是接口(interface)而不是具体类，所以如果你更改 Comparator 接口(interface)的具体实现，你不必也修改客户端类(我认为这是使用接口(interface)的最重要原因)。

例如:

//inside aClass

Comparator c = Host.STRING_LENGTH_COMPARATOR; //Programming against interfaces is different from:
StrLenCmp  c = Host.STRING_LENGTH_COMPARATOR; //programming against concrete class

假设将来您将使用另一个实现(我们称之为 NewStrLenCmp)更改 StrLenCmp，而不是如果您针对接口(interface) Comparator 进行编程则不必修改 aClass。

Comparator c = Host.STRING_LENGTH_COMPARATOR; //still work because interface doesn't changed
NewStrLenCmp  c = Host.STRING_LENGTH_COMPARATOR; // problem: you need to modify the client class in order to use the new concrete type: bad idea