## 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)**
- Chapter 2. Creating and Destroying Objects(創建和銷毀對象)
- Item 1: Consider static factory methods instead of constructors(考慮以靜態工廠方法代替構造函數)
- Item 2: Consider a builder when faced with many constructor parameters(在面對多個構造函數參數時,請考慮構建器)
- Item 3: Enforce the singleton property with a private constructor or an enum type(使用私有構造函數或枚舉類型實施單例屬性)
- Item 4: Enforce noninstantiability with a private constructor(用私有構造函數實施不可實例化)
- Item 5: Prefer dependency injection to hardwiring resources(依賴注入優于硬連接資源)
- Item 6: Avoid creating unnecessary objects(避免創建不必要的對象)
- Item 7: Eliminate obsolete object references(排除過時的對象引用)
- Item 8: Avoid finalizers and cleaners(避免使用終結器和清除器)
- Item 9: Prefer try with resources to try finally(使用 try-with-resources 優于 try-finally)
- Chapter 3. Methods Common to All Objects(對象的通用方法)
- Item 10: Obey the general contract when overriding equals(覆蓋 equals 方法時應遵守的約定)
- Item 11: Always override hashCode when you override equals(當覆蓋 equals 方法時,總要覆蓋 hashCode 方法)
- Item 12: Always override toString(始終覆蓋 toString 方法)
- Item 13: Override clone judiciously(明智地覆蓋 clone 方法)
- Item 14: Consider implementing Comparable(考慮實現 Comparable 接口)
- Chapter 4. Classes and Interfaces(類和接口)
- Item 15: Minimize the accessibility of classes and members(盡量減少類和成員的可訪問性)
- Item 16: In public classes use accessor methods not public fields(在公共類中,使用訪問器方法,而不是公共字段)
- Item 17: Minimize mutability(減少可變性)
- Item 18: Favor composition over inheritance(優先選擇復合而不是繼承)
- Item 19: Design and document for inheritance or else prohibit it(繼承要設計良好并且具有文檔,否則禁止使用)
- Item 20: Prefer interfaces to abstract classes(接口優于抽象類)
- Item 21: Design interfaces for posterity(為后代設計接口)
- Item 22: Use interfaces only to define types(接口只用于定義類型)
- Item 23: Prefer class hierarchies to tagged classes(類層次結構優于帶標簽的類)
- Item 24: Favor static member classes over nonstatic(靜態成員類優于非靜態成員類)
- Item 25: Limit source files to a single top level class(源文件僅限有單個頂層類)
- Chapter 5. Generics(泛型)
- Item 26: Do not use raw types(不要使用原始類型)
- Item 27: Eliminate unchecked warnings(消除 unchecked 警告)
- Item 28: Prefer lists to arrays(list 優于數組)
- Item 29: Favor generic types(優先使用泛型)
- Item 30: Favor generic methods(優先使用泛型方法)
- Item 31: Use bounded wildcards to increase API flexibility(使用有界通配符增加 API 的靈活性)
- Item 32: Combine generics and varargs judiciously(明智地合用泛型和可變參數)
- Item 33: Consider typesafe heterogeneous containers(考慮類型安全的異構容器)
- Chapter 6. Enums and Annotations(枚舉和注解)
- Item 34: Use enums instead of int constants(用枚舉類型代替 int 常量)
- Item 35: Use instance fields instead of ordinals(使用實例字段替代序數)
- Item 36: Use EnumSet instead of bit fields(用 EnumSet 替代位字段)
- Item 37: Use EnumMap instead of ordinal indexing(使用 EnumMap 替換序數索引)
- Item 38: Emulate extensible enums with interfaces(使用接口模擬可擴展枚舉)
- Item 39: Prefer annotations to naming patterns(注解優于命名模式)
- Item 40: Consistently use the Override annotation(堅持使用 @Override 注解)
- Item 41: Use marker interfaces to define types(使用標記接口定義類型)
- Chapter 7. Lambdas and Streams(λ 表達式和流)
- Item 42: Prefer lambdas to anonymous classes(λ 表達式優于匿名類)
- Item 43: Prefer method references to lambdas(方法引用優于 λ 表達式)
- Item 44: Favor the use of standard functional interfaces(優先使用標準函數式接口)
- Item 45: Use streams judiciously(明智地使用流)
- Item 46: Prefer side effect free functions in streams(在流中使用無副作用的函數)
- Item 47: Prefer Collection to Stream as a return type(優先選擇 Collection 而不是流作為返回類型)
- Item 48: Use caution when making streams parallel(謹慎使用并行流)
- Chapter 8. Methods(方法)
- Item 49: Check parameters for validity(檢查參數的有效性)
- Item 50: Make defensive copies when needed(在需要時制作防御性副本)
- Item 51: Design method signatures carefully(仔細設計方法簽名)
- Item 52: Use overloading judiciously(明智地使用重載)
- Item 53: Use varargs judiciously(明智地使用可變參數)
- Item 54: Return empty collections or arrays, not nulls(返回空集合或數組,而不是 null)
- Item 55: Return optionals judiciously(明智地的返回 Optional)
- Item 56: Write doc comments for all exposed API elements(為所有公開的 API 元素編寫文檔注釋)
- Chapter 9. General Programming(通用程序設計)
- Item 57: Minimize the scope of local variables(將局部變量的作用域最小化)
- Item 58: Prefer for-each loops to traditional for loops(for-each 循環優于傳統的 for 循環)
- Item 59: Know and use the libraries(了解并使用庫)
- Item 60: Avoid float and double if exact answers are required(若需要精確答案就應避免使用 float 和 double 類型)
- Item 61: Prefer primitive types to boxed primitives(基本數據類型優于包裝類)
- Item 62: Avoid strings where other types are more appropriate(其他類型更合適時應避免使用字符串)
- Item 63: Beware the performance of string concatenation(當心字符串連接引起的性能問題)
- Item 64: Refer to objects by their interfaces(通過接口引用對象)
- Item 65: Prefer interfaces to reflection(接口優于反射)
- Item 66: Use native methods judiciously(明智地使用本地方法)
- Item 67: Optimize judiciously(明智地進行優化)
- Item 68: Adhere to generally accepted naming conventions(遵守被廣泛認可的命名約定)
- Chapter 10. Exceptions(異常)
- Item 69: Use exceptions only for exceptional conditions(僅在確有異常條件下使用異常)
- Item 70: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors(對可恢復情況使用 checked 異常,對編程錯誤使用運行時異常)
- Item 71: Avoid unnecessary use of checked exceptions(避免不必要地使用 checked 異常)
- Item 72: Favor the use of standard exceptions(鼓勵復用標準異常)
- Item 73: Throw exceptions appropriate to the abstraction(拋出能用抽象解釋的異常)
- Item 74: Document all exceptions thrown by each method(為每個方法記錄會拋出的所有異常)
- Item 75: Include failure capture information in detail messages(異常詳細消息中應包含捕獲失敗的信息)
- Item 76: Strive for failure atomicity(盡力保證故障原子性)
- Item 77: Don’t ignore exceptions(不要忽略異常)
- Chapter 11. Concurrency(并發)
- Item 78: Synchronize access to shared mutable data(對共享可變數據的同步訪問)
- Item 79: Avoid excessive synchronization(避免過度同步)
- Item 80: Prefer executors, tasks, and streams to threads(Executor、task、流優于直接使用線程)
- Item 81: Prefer concurrency utilities to wait and notify(并發實用工具優于 wait 和 notify)
- Item 82: Document thread safety(文檔應包含線程安全屬性)
- Item 83: Use lazy initialization judiciously(明智地使用延遲初始化)
- Item 84: Don’t depend on the thread scheduler(不要依賴線程調度器)
- Chapter 12. Serialization(序列化)
- Item 85: Prefer alternatives to Java serialization(優先選擇 Java 序列化的替代方案)
- Item 86: Implement Serializable with great caution(非常謹慎地實現 Serializable)
- Item 87: Consider using a custom serialized form(考慮使用自定義序列化形式)
- Item 88: Write readObject methods defensively(防御性地編寫 readObject 方法)
- Item 89: For instance control, prefer enum types to readResolve(對于實例控制,枚舉類型優于 readResolve)
- Item 90: Consider serialization proxies instead of serialized instances(考慮以序列化代理代替序列化實例)