## Chapter 6. Enums and Annotations(枚舉和注解)
### Item 40: Consistently use the Override annotation(堅持使用 @Override 注解)
The Java libraries contain several annotation types. For the typical programmer, the most important of these is @Override. This annotation can be used only on method declarations, and it indicates that the annotated method declaration overrides a declaration in a supertype. If you consistently use this annotation, it will protect you from a large class of nefarious bugs. Consider this program, in which the class Bigram represents a bigram, or ordered pair of letters:
Java 庫包含幾種注解類型。對于大多數的程序員來說,其中最重要的是 `@Override`。此注解只能在方法聲明上使用,帶有該注解的方法聲明將覆蓋超類型中的聲明。如果你堅持使用這個注解,它將幫助你減少受到有害錯誤的影響。考慮這個程序,其中類 Bigram 表示一個二元語法,或有序的字母對:
```
// Can you spot the bug?
public class Bigram {
private final char first;
private final char second;
public Bigram(char first, char second) {
this.first = first;
this.second = second;
}
public boolean equals(Bigram b) {
return b.first == first && b.second == second;
}
public int hashCode() {
return 31 * first + second;
}
public static void main(String[] args) {
Set<Bigram> s = new HashSet<>();
for (int i = 0; i < 10; i++)
for (char ch = 'a'; ch <= 'z'; ch++)
s.add(new Bigram(ch, ch));
System.out.println(s.size());
}
}
```
The main program repeatedly adds twenty-six bigrams, each consisting of two identical lowercase letters, to a set. Then it prints the size of the set. You might expect the program to print 26, as sets cannot contain duplicates. If you try running the program, you’ll find that it prints not 26 but 260. What is wrong with it?
主程序重復地向一個集合中添加 26 個 bigram,每個 bigram 由兩個相同的小寫字母組成。然后它打印該集合的大小。如果你嘗試運行該程序,你會發現它打印的不是 26 而是 260。有什么問題嗎?
Clearly, the author of the Bigram class intended to override the equals method (Item 10) and even remembered to override hashCode in tandem (Item 11). Unfortunately, our hapless programmer failed to override equals, overloading it instead (Item 52). To override Object.equals, you must define an equals method whose parameter is of type Object, but the parameter of Bigram’s equals method is not of type Object, so Bigram inherits the equals method from Object. This equals method tests for object identity, just like the == operator. Each of the ten copies of each bigram is distinct from the other nine, so they are deemed unequal by Object.equals, which explains why the program prints 260.
顯然,Bigram 類的作者打算覆蓋 equals 方法([Item-10](/Chapter-3/Chapter-3-Item-10-Obey-the-general-contract-when-overriding-equals.md)),甚至還記得要一并覆蓋 hashCode([Item-11](/Chapter-3/Chapter-3-Item-11-Always-override-hashCode-when-you-override-equals.md))。不幸的是,我們的程序員沒有覆蓋 equals,而是重載了它([Item-52](/Chapter-8/Chapter-8-Item-52-Use-overloading-judiciously.md))。要覆蓋 `Object.equals`,你必須定義一個 equals 方法,它的參數是 Object 類型的,但是 Bigram 的 equals 方法的參數不是 Object 類型的,所以 Bigram 從 Object 繼承 equals 方法。這個繼承來的 equals 方法只能檢測對象同一性,就像 == 操作符一樣。每 10 個 bigram 副本為一組,每組中的每個 bigram 副本都不同于其他 9 個,因此 `Object.equals` 認為它們不相等,這就解釋了為什么程序最終打印 260。
Luckily, the compiler can help you find this error, but only if you help it by telling it that you intend to override Object.equals. To do this, annotate Bigram.equals with @Override, as shown here:
幸運的是,編譯器可以幫助你找到這個錯誤,但前提是你告訴它你打算覆蓋 `Object.equals`。為此,請使用 `@Override` 注解標記 `Bigram.equals`,如下所示:
```
@Override
public boolean equals(Bigram b) {
return b.first == first && b.second == second;
}
```
If you insert this annotation and try to recompile the program, the compiler will generate an error message like this:
如果你插入此注解并嘗試重新編譯程序,編譯器將生成如下錯誤消息:
```
Bigram.java:10: method does not override or implement a method from a supertype
@Override public boolean equals(Bigram b) {
^
```
You will immediately realize what you did wrong, slap yourself on the forehead, and replace the broken equals implementation with a correct one (Item 10):
你會立刻意識到自己做錯了什么,拍拍自己的額頭,用正確的方式替換不正確的 equals 實現([Item-10](/Chapter-3/Chapter-3-Item-10-Obey-the-general-contract-when-overriding-equals.md)):
```
@Override
public boolean equals(Object o) {
if (!(o instanceof Bigram))
return false;
Bigram b = (Bigram) o;
return b.first == first && b.second == second;
}
```
Therefore, you should **use the Override annotation on every method declaration that you believe to override a superclass declaration.** There is one minor exception to this rule. If you are writing a class that is not labeled abstract and you believe that it overrides an abstract method in its superclass, you needn’t bother putting the Override annotation on that method. In a class that is not declared abstract, the compiler will emit an error message if you fail to override an abstract superclass method. However, you might wish to draw attention to all of the methods in your class that override superclass methods, in which case you should feel free to annotate these methods too. Most IDEs can be set to insert Override annotations automatically when you elect to override a method.
因此,你應該在 **要覆蓋超類聲明的每個方法聲明上使用 @Override 注解。** 這條規則有一個小小的例外。如果你正在編寫一個沒有標記為 abstract 的類,并且你認為它覆蓋了其超類中的抽象方法,那么你不必費心在這些方法上添加 `@Override` 注解。在未聲明為抽象的類中,如果未能覆蓋抽象超類方法,編譯器將發出錯誤消息。但是,你可能希望讓類中覆蓋超類方法的所有方法更加引人注目,在這種情況下,你也可以自由選擇是否注解這些方法。大多數 IDE 都可以設置為在選擇覆蓋方法時自動插入覆蓋注解。
Most IDEs provide another reason to use the Override annotation consistently. If you enable the appropriate check, the IDE will generate a warning if you have a method that doesn’t have an Override annotation but does override a superclass method. If you use the Override annotation consistently, these warnings will alert you to unintentional overriding. They complement the compiler’s error messages, which alert you to unintentional failure to override. Between the IDE and the compiler, you can be sure that you’re overriding methods everywhere you want to and nowhere else.
大多數 IDE 都提供了一致使用 `@Override` 注解的另一個原因。如果啟用適當的檢查,如果你的方法沒有 `@Override` 注解,但確實覆蓋了超類方法,IDE 將生成警告。如果你一致地使用 `@Override` 注解,這些警告將提醒你防止意外覆蓋。它們補充編譯器的錯誤消息,這些錯誤消息會警告你無意的覆蓋錯誤。在 IDE 和編譯器的幫助下,你可以確保在任何你想要實施覆蓋的地方都覆蓋了,而沒有遺漏。
The Override annotation may be used on method declarations that override declarations from interfaces as well as classes. With the advent of default methods, it is good practice to use Override on concrete implementations of interface methods to ensure that the signature is correct. If you know that an interface does not have default methods, you may choose to omit Override annotations on concrete implementations of interface methods to reduce clutter.
`@Override` 注解可用于覆蓋接口和類聲明的方法聲明。隨著默認方法的出現,最好對接口方法的具體實現使用 `@Override` 來確保簽名是正確的。如果你知道接口沒有默認方法,你可以選擇忽略接口方法的具體實現上的 `@Override` 注解,以減少混亂。
In an abstract class or an interface, however, it is worth annotating all methods that you believe to override superclass or superinterface methods, whether concrete or abstract. For example, the Set interface adds no new methods to the Collection interface, so it should include Override annotations on all of its method declarations to ensure that it does not accidentally add any new methods to the Collection interface.
然而,在抽象類或接口中,標記覆蓋超類或超接口方法的所有方法是值得的,無論是具體的還是抽象的。例如,Set 接口不會向 Collection 接口添加任何新方法,因此它的所有方法聲明的應該包含 `@Override` 注解,以確保它不會意外地向 Collection 接口添加任何新方法。
In summary, the compiler can protect you from a great many errors if you use the Override annotation on every method declaration that you believe to override a supertype declaration, with one exception. In concrete classes, you need not annotate methods that you believe to override abstract method declarations (though it is not harmful to do so).
總之,如果你在每個方法聲明上都使用 `@Override` 注解來覆蓋超類型聲明(只有一個例外),那么編譯器可以幫助你減少受到有害錯誤的影響。在具體類中,可以不對覆蓋抽象方法聲明的方法使用該注解(即使這么做也并不會有害)。
---
**[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 39: Prefer annotations to naming patterns(注解優于命名模式)](/Chapter-6/Chapter-6-Item-39-Prefer-annotations-to-naming-patterns.md)**
- **Next Item(下一條目):[Item 41: Use marker interfaces to define types(使用標記接口定義類型)](/Chapter-6/Chapter-6-Item-41-Use-marker-interfaces-to-define-types.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)