## Chapter 6. Enums and Annotations(枚舉和注解)
### Item 36: Use EnumSet instead of bit fields(用 EnumSet 替代位字段)
If the elements of an enumerated type are used primarily in sets, it is traditional to use the int enum pattern (Item 34), assigning a different power of 2 to each constant:
如果枚舉類型的元素主要在 Set 中使用,傳統上使用 int 枚舉模式([Item-34](/Chapter-6/Chapter-6-Item-34-Use-enums-instead-of-int-constants.md)),通過不同的 2 平方數為每個常量賦值:
```
// Bit field enumeration constants - OBSOLETE!
public class Text {
public static final int STYLE_BOLD = 1 << 0; // 1
public static final int STYLE_ITALIC = 1 << 1; // 2
public static final int STYLE_UNDERLINE = 1 << 2; // 4
public static final int STYLE_STRIKETHROUGH = 1 << 3; // 8
// Parameter is bitwise OR of zero or more STYLE_ constants
public void applyStyles(int styles) { ... }
}
```
This representation lets you use the bitwise OR operation to combine several constants into a set, known as a bit field:
這種表示方式稱為位字段,允許你使用位運算的 OR 操作將幾個常量組合成一個 Set:
```
text.applyStyles(STYLE_BOLD | STYLE_ITALIC);
```
The bit field representation also lets you perform set operations such as union and intersection efficiently using bitwise arithmetic. But bit fields have all the disadvantages of int enum constants and more. It is even harder to interpret a bit field than a simple int enum constant when it is printed as a number. There is no easy way to iterate over all of the elements represented by a bit field. Finally, you have to predict the maximum number of bits you’ll ever need at the time you’re writing the API and choose a type for the bit field (typically int or long) accordingly. Once you’ve picked a type, you can’t exceed its width (32 or 64 bits) without changing the API.
位字段表示方式允許使用位運算高效地執行 Set 操作,如并集和交集。但是位字段具有 int 枚舉常量所有缺點,甚至更多。當位字段被打印為數字時,它比簡單的 int 枚舉常量更難理解。沒有一種簡單的方法可以遍歷由位字段表示的所有元素。最后,你必須預測在編寫 API 時需要的最大位數,并相應地為位字段(通常是 int 或 long)選擇一種類型。一旦選擇了一種類型,在不更改 API 的情況下,不能超過它的寬度(32 或 64 位)。
Some programmers who use enums in preference to int constants still cling to the use of bit fields when they need to pass around sets of constants. There is no reason to do this, because a better alternative exists. The java.util package provides the EnumSet class to efficiently represent sets of values drawn from a single enum type. This class implements the Set interface, providing all of the richness, type safety, and interoperability you get with any other Set implementation. But internally, each EnumSet is represented as a bit vector. If the underlying enum type has sixty-four or fewer elements—and most do—the entire EnumSet is represented with a single long, so its performance is comparable to that of a bit field. Bulk operations, such as removeAll and retainAll, are implemented using bitwise arithmetic, just as you’d do manually for bit fields. But you are insulated from the ugliness and errorproneness of manual bit twiddling: the EnumSet does the hard work for you.
一些使用枚舉而不是 int 常量的程序員在需要傳遞常量集時仍然堅持使用位字段。沒有理由這樣做,因為存在更好的選擇。`java.util` 包提供 EnumSet 類來有效地表示從單個枚舉類型中提取的值集。這個類實現了 Set 接口,提供了所有其他 Set 實現所具有的豐富性、類型安全性和互操作性。但在內部,每個 EnumSet 都表示為一個位向量。如果底層枚舉類型有 64 個或更少的元素(大多數都是),則整個 EnumSet 用一個 long 表示,因此其性能與位字段的性能相當。批量操作(如 removeAll 和 retainAll)是使用逐位算法實現的,就像手動處理位字段一樣。但是,你可以避免因手工修改導致產生不良代碼和潛在錯誤:EnumSet 為你完成了這些繁重的工作。
Here is how the previous example looks when modified to use enums and enum sets instead of bit fields. It is shorter, clearer, and safer:
當之前的示例修改為使用枚舉和 EnumSet 而不是位字段時。它更短,更清晰,更安全:
```
// EnumSet - a modern replacement for bit fields
public class Text {
public enum Style { BOLD, ITALIC, UNDERLINE, STRIKETHROUGH }
// Any Set could be passed in, but EnumSet is clearly best
public void applyStyles(Set<Style> styles) { ... }
}
```
Here is client code that passes an EnumSet instance to the applyStyles method. The EnumSet class provides a rich set of static factories for easy set creation, one of which is illustrated in this code:
下面是將 EnumSet 實例傳遞給 applyStyles 方法的客戶端代碼。EnumSet 類提供了一組豐富的靜態工廠,可以方便地創建 Set,下面的代碼演示了其中的一個:
```
text.applyStyles(EnumSet.of(Style.BOLD, Style.ITALIC));
```
Note that the applyStyles method takes a `Set<Style>` rather than an `EnumSet<Style>`. While it seems likely that all clients would pass an EnumSet to the method, it is generally good practice to accept the interface type rather than the implementation type (Item 64). This allows for the possibility of an unusual client to pass in some other Set implementation.
請注意,applyStyles 方法采用 `Set<Style>` 而不是 `EnumSet<Style>`。雖然似乎所有客戶端都可能將 EnumSet 傳遞給該方法,但通常較好的做法是接受接口類型而不是實現類型([Item-64](/Chapter-9/Chapter-9-Item-64-Refer-to-objects-by-their-interfaces.md))。這允許特殊的客戶端傳入其他 Set 實現的可能性。
In summary, **just because an enumerated type will be used in sets, there is no reason to represent it with bit fields.** The EnumSet class combines the conciseness and performance of bit fields with all the many advantages of enum types described in Item 34. The one real disadvantage of EnumSet is that it is not, as of Java 9, possible to create an immutable EnumSet, but this will likely be remedied in an upcoming release. In the meantime, you can wrap an EnumSet with Collections.unmodifiableSet, but conciseness and performance will suffer.
總之,**因為枚舉類型將在 Set 中使用,沒有理由用位字段表示它。** EnumSet 類結合了位字段的簡潔性和性能,以及 [Item-34](/Chapter-6/Chapter-6-Item-34-Use-enums-instead-of-int-constants.md) 中描述的枚舉類型的許多優點。EnumSet 的一個真正的缺點是,從 Java 9 開始,它不能創建不可變的 EnumSet,在未來發布的版本中可能會糾正這一點。同時,可以用 `Collections.unmodifiableSet` 包裝 EnumSet,但簡潔性和性能將受到影響。
---
**[Back to contents of the chapter(返回章節目錄)](/Chapter-6/Chapter-6-Introduction.md)**
- **Previous Item(上一條目):[Item 35: Use instance fields instead of ordinals(使用實例字段替代序數)](/Chapter-6/Chapter-6-Item-35-Use-instance-fields-instead-of-ordinals.md)**
- **Next Item(下一條目):[Item 37: Use EnumMap instead of ordinal indexing(使用 EnumMap 替換序數索引)](/Chapter-6/Chapter-6-Item-37-Use-EnumMap-instead-of-ordinal-indexing.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)