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                ## Chapter 8. Methods(方法) ### Item 51: Design method signatures carefully(仔細設計方法簽名) This item is a grab bag of API design hints that don’t quite deserve items of their own. Taken together, they’ll help make your API easier to learn and use and less prone to errors. 本條目是一個 API 設計提示的大雜燴,它們還不完全值得擁有獨立的條目。總之,它們將幫助你使 API 更容易學習和使用,并且更不容易出錯。 **Choose method names carefully.** Names should always obey the standard naming conventions (Item 68). Your primary goal should be to choose names that are understandable and consistent with other names in the same package. Your secondary goal should be to choose names consistent with the broader consensus, where it exists. Avoid long method names. When in doubt, look to the Java library APIs for guidance. While there are plenty of inconsistencies— inevitable, given the size and scope of these libraries—there is also a fair amount of consensus. **仔細選擇方法名稱。** 名稱應始終遵守標準的命名約定([Item-68](/Chapter-9/Chapter-9-Item-68-Adhere-to-generally-accepted-naming-conventions.md))。你的主要目標應該是選擇可理解的、與同一包中的其他名稱風格一致的名稱。你的第二個目標應該是選擇被廣泛認可的名字。避免長方法名。如果有疑問,可以參考 Java 庫 API,盡管其中也存在大量的矛盾(考慮到這些庫的規模和范圍,這是不可避免的)但也達成了相當多的共識。 **Don’t go overboard in providing convenience methods.** Every method should “pull its weight.” Too many methods make a class difficult to learn, use, document, test, and maintain. This is doubly true for interfaces, where too many methods complicate life for implementors as well as users. For each action supported by your class or interface, provide a fully functional method. Consider providing a “shorthand” only if it will be used often. **When in doubt, leave it out.** **不要提供過于便利的方法。** 每種方法都應該各司其職。太多的方法使得類難以學習、使用、記錄、測試和維護。對于接口來說更是如此,在接口中,太多的方法使實現者和用戶的工作變得復雜。對于類或接口支持的每個操作,請提供一個功能齊全的方法。只有在經常使用時才考慮提供便捷方式。**但如果有疑問,就不要提供。** **Avoid long parameter lists.** Aim for four parameters or fewer. Most programmers can’t remember longer parameter lists. If many of your methods exceed this limit, your API won’t be usable without constant reference to its documentation. Modern IDEs help, but you are still much better off with short parameter lists. **Long sequences of identically typed parameters are especially harmful**. Not only won’t users be able to remember the order of the parameters, but when they transpose parameters accidentally, their programs will still compile and run. They just won’t do what their authors intended. **避免長參數列表。** 設定四個或更少的參數。大多數程序員記不住更長的參數列表。如果你的許多方法超過了這個限制,而用戶沒有對文檔的不斷查看,你的 API 將無法使用。現代 IDE 會有所幫助,但是使用簡短的參數列表仍然會讓情況好得多。**長序列的同類型參數尤其有害**。用戶不僅不能記住參數的順序,而且當他們不小心轉置參數時,他們的程序仍然會編譯和運行。它們只是不會按照作者的意圖去做。 There are three techniques for shortening overly long parameter lists. One is to break the method up into multiple methods, each of which requires only a subset of the parameters. If done carelessly, this can lead to too many methods, but it can also help reduce the method count by increasing orthogonality. For example, consider the java.util.List interface. It does not provide methods to find the first or last index of an element in a sublist, both of which would require three parameters. Instead it provides the subList method, which takes two parameters and returns a view of a sublist. This method can be combined with the indexOf or lastIndexOf method, each of which has a single parameter, to yield the desired functionality. Moreover, the subList method can be combined with any method that operates on a List instance to perform arbitrary computations on sublists. The resulting API has a very high power-to-weight ratio. 有三種方法可以縮短過長的參數列表。一種方法是將方法分解為多個方法,每個方法只需要參數的一個子集。如果操作不當,這可能導致產生太多的方法,但它也可以通過增加正交性來幫助減少方法數量。例如,考慮 `java.util.List` 接口。它不提供查找子列表中元素的第一個或最后一個索引的方法,這兩個方法都需要三個參數。相反,它提供了 subList 方法,該方法接受兩個參數并返回子列表的視圖。此方法可以與 indexOf 或 lastIndexOf 方法組合使用以達到所需的功能,其中每個方法都有一個參數。此外,subList 方法可以與操作 List 實例的任何方法組合使用,以執行子列表上的任意操作。這樣的 API 就具有非常高的 power-to-weight 比。 A second technique for shortening long parameter lists is to create helper classes to hold groups of parameters. Typically these helper classes are static member classes (Item 24). This technique is recommended if a frequently occurring sequence of parameters is seen to represent some distinct entity. For example, suppose you are writing a class representing a card game, and you find yourself constantly passing a sequence of two parameters representing a card’s rank and its suit. Your API, as well as the internals of your class, would probably benefit if you added a helper class to represent a card and replaced every occurrence of the parameter sequence with a single parameter of the helper class. 縮短長參數列表的第二種技術是創建 helper 類來保存參數組。通常,這些 helper 類是靜態成員類([Item-42](/Chapter-7/Chapter-7-Item-42-Prefer-lambdas-to-anonymous-classes.md))。如果經常出現的參數序列表示一些不同的實體,則推薦使用這種技術。例如,假設你正在編寫一個表示紙牌游戲的類,你發現會不斷地傳遞一個序列,其中包含兩個參數,分別表示紙牌的等級和花色。如果你添加一個 helper 類來表示一張卡片,并用 helper 類的一個參數替換參數序列中的每個出現的參數,那么你的 API 以及類的內部結構都可能受益。 A third technique that combines aspects of the first two is to adapt the Builder pattern (Item 2) from object construction to method invocation. If you have a method with many parameters, especially if some of them are optional, it can be beneficial to define an object that represents all of the parameters and to allow the client to make multiple “setter” calls on this object, each of which sets a single parameter or a small, related group. Once the desired parameters have been set, the client invokes the object’s “execute” method, which does any final validity checks on the parameters and performs the actual computation. 結合前兩個方面討論的第三種技術是,從對象構建到方法調用都采用建造者模式([Item-2](/Chapter-2/Chapter-2-Item-2-Consider-a-builder-when-faced-with-many-constructor-parameters.md))。如果你有一個方法帶有許多參數,特別是其中一些參數是可選的,最好定義一個對象來表示所有參數,并允許客戶端多次調用「setter」來使用這個對象,每一次都設置一個參數或較小相關的組。一旦設置了所需的參數,客戶機就調用對象的「execute」方法,該方法對參數進行最終有效性檢查并執行實際操作。 **For parameter types, favor interfaces over classes** (Item 64). If there is an appropriate interface to define a parameter, use it in favor of a class that implements the interface. For example, there is no reason to ever write a method that takes HashMap on input—use Map instead. This lets you pass in a HashMap, a TreeMap, a ConcurrentHashMap, a submap of a TreeMap, or any Map implementation yet to be written. By using a class instead of an interface, you restrict your client to a particular implementation and force an unnecessary and potentially expensive copy operation if the input data happens to exist in some other form. **對于參數類型,優先選擇接口而不是類**([Item-64](/Chapter-9/Chapter-9-Item-64-Refer-to-objects-by-their-interfaces.md))。如果有合適的接口來定義參數,那么使用它來支持實現該接口的類。例如,沒有理由編寫一個輸入使用 HashMap 的方法,而應該使用 Map。這允許你傳入 HashMap、TreeMap、ConcurrentHashMap、TreeMap 的子映射或任何尚未編寫的 Map 實現。通過使用類而不是接口,你可以將客戶端限制在特定的實現中,如果輸入數據碰巧以某種其他形式存在,則會強制執行不必要的、可能代價很高的復制操作。 **Prefer two-element enum types to boolean parameters,** unless the meaning of the boolean is clear from the method name. Enums make your code easier to read and to write. Also, they make it easy to add more options later. For example, you might have a Thermometer type with a static factory that takes this enum: **雙元素枚舉類型優于 boolean 參數,** 除非布爾值的含義在方法名中明確。枚舉使代碼更容易讀和寫。此外,它們使以后添加更多選項變得更加容易。例如,你可能有一個 Thermometer 類型與靜態工廠,采用枚舉: ``` public enum TemperatureScale { FAHRENHEIT, CELSIUS } ``` Not only does Thermometer.newInstance(TemperatureScale.CELSIUS) make a lot more sense than Thermometer.newInstance(true), but you can add KELVIN to TemperatureScale in a future release without having to add a new static factory to Thermometer. Also, you can refactor temperaturescale dependencies into methods on the enum constants (Item 34). For example, each scale constant could have a method that took a double value and converted it to Celsius. `Thermometer.newInstance(TemperatureScale.CELSIUS)` 不僅比 `Thermometer.newInstance(true)` 更有意義,而且你可以在將來的版本中向 TemperatureScale 添加 KELVIN,而不必向 Thermometer 添加新的靜態工廠。此外,你還可以將 TemperatureScale 依賴項重構為 enum 常量([Item-34](/Chapter-6/Chapter-6-Item-34-Use-enums-instead-of-int-constants.md))上的方法。例如,每個刻度單位都有一個方法,該方法帶有 double 值并將其轉換為攝氏度。 --- **[Back to contents of the chapter(返回章節目錄)](/Chapter-8/Chapter-8-Introduction.md)** - **Previous Item(上一條目):[Item 50: Make defensive copies when needed(在需要時制作防御性副本)](/Chapter-8/Chapter-8-Item-50-Make-defensive-copies-when-needed.md)** - **Next Item(下一條目):[Item 52: Use overloading judiciously(明智地使用重載)](/Chapter-8/Chapter-8-Item-52-Use-overloading-judiciously.md)**
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