## Chapter 5. Generics(泛型)
### Item 27: Eliminate unchecked warnings(消除 unchecked 警告)
When you program with generics, you will see many compiler warnings: unchecked cast warnings, unchecked method invocation warnings, unchecked parameterized vararg type warnings, and unchecked conversion warnings. The more experience you acquire with generics, the fewer warnings you’ll get, but don’t expect newly written code to compile cleanly.
當你使用泛型編程時,你將看到許多編譯器警告:unchecked 強制轉換警告、unchecked 方法調用警告、unchecked 可變參數類型警告和 unchecked 自動轉換警告。使用泛型的經驗越豐富,遇到的警告就越少,但是不要期望新編寫的代碼能夠完全正確地編譯。
Many unchecked warnings are easy to eliminate. For example, suppose you accidentally write this declaration:
許多 unchecked 警告很容易消除。例如,假設你不小心寫了這個聲明:
```
Set<Lark> exaltation = new HashSet();
```
The compiler will gently remind you what you did wrong:
編譯器會精確地提醒你做錯了什么:
```
Venery.java:4: warning: [unchecked] unchecked conversion
Set<Lark> exaltation = new HashSet();
^ required: Set<Lark>
found: HashSet
```
You can then make the indicated correction, causing the warning to disappear. Note that you don’t actually have to specify the type parameter, merely to indicate that it’s present with the diamond operator (<>), introduced in Java 7. The compiler will then infer the correct actual type parameter (in this case, Lark):
你可以在指定位置進行更正,使警告消失。注意,你實際上不必指定類型參數,只需給出由 Java 7 中引入的 diamond 操作符(<>)。然后編譯器將推斷出正確的實際類型參數(在本例中為 Lark):
```
Set<Lark> exaltation = new HashSet<>();
```
Some warnings will be much more difficult to eliminate. This chapter is filled with examples of such warnings. When you get warnings that require some thought, persevere! **Eliminate every unchecked warning that you can.** If you eliminate all warnings, you are assured that your code is typesafe, which is a very good thing. It means that you won’t get a ClassCastException at runtime, and it increases your confidence that your program will behave as you intended.
一些警告會更難消除。這一章充滿這類警告的例子。當你收到需要認真思考的警告時,堅持下去!**力求消除所有 unchecked 警告。** 如果你消除了所有警告,你就可以確信你的代碼是類型安全的,這是一件非常好的事情。這意味著你在運行時不會得到 ClassCastException,它增加了你的信心,你的程序將按照預期的方式運行。
**If you can’t eliminate a warning, but you can prove that the code that provoked the warning is typesafe, then (and only then) suppress the warning with an @SuppressWarnings("unchecked") annotation.** If you suppress warnings without first proving that the code is typesafe, you are giving yourself a false sense of security. The code may compile without emitting any warnings, but it can still throw a ClassCastException at runtime. If, however, you ignore unchecked warnings that you know to be safe (instead of suppressing them), you won’t notice when a new warning crops up that represents a real problem. The new warning will get lost amidst all the false alarms that you didn’t silence.
**如果不能消除警告,但是可以證明引發警告的代碼是類型安全的,那么(并且只有在那時)使用 SuppressWarnings("unchecked") 注解來抑制警告。** 如果你在沒有首先證明代碼是類型安全的情況下禁止警告,那么你是在給自己一種錯誤的安全感。代碼可以在不發出任何警告的情況下編譯,但它仍然可以在運行時拋出 ClassCastException。但是,如果你忽略了你知道是安全的 unchecked 警告(而不是抑制它們),那么當出現一個代表真正問題的新警告時,你將不會注意到。新出現的警告就會淹設在所有的錯誤警告當中。
The SuppressWarnings annotation can be used on any declaration, from an individual local variable declaration to an entire class. **Always use the SuppressWarnings annotation on the smallest scope possible.** Typically this will be a variable declaration or a very short method or constructor. Never use SuppressWarnings on an entire class. Doing so could mask critical warnings.
SuppressWarnings 注解可以用于任何聲明中,從單個局部變量聲明到整個類。**總是在盡可能小的范圍上使用 SuppressWarnings 注解。** 通常用在一個變量聲明或一個非常短的方法或構造函數。不要在整個類中使用 SuppressWarnings。這樣做可能會掩蓋關鍵警告。
If you find yourself using the SuppressWarnings annotation on a method or constructor that’s more than one line long, you may be able to move it onto a local variable declaration. You may have to declare a new local variable, but it’s worth it. For example, consider this toArray method, which comes from ArrayList:
如果你發現自己在一個超過一行的方法或構造函數上使用 SuppressWarnings 注解,那么你可以將其移動到局部變量聲明中。你可能需要聲明一個新的局部變量,但這是值得的。例如,考慮這個 toArray 方法,它來自 ArrayList:
```
public <T> T[] toArray(T[] a) {
if (a.length < size)
return (T[]) Arrays.copyOf(elements, size, a.getClass());
System.arraycopy(elements, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
```
If you compile ArrayList, the method generates this warning:
如果你編譯 ArrayList,這個方法會產生這樣的警告:
```
ArrayList.java:305: warning: [unchecked] unchecked cast
return (T[]) Arrays.copyOf(elements, size, a.getClass());
^ required: T[]
found: Object[]
```
It is illegal to put a SuppressWarnings annotation on the return statement, because it isn’t a declaration [JLS, 9.7]. You might be tempted to put the annotation on the entire method, but don’t. Instead, declare a local variable to hold the return value and annotate its declaration, like so:
將 SuppressWarnings 注釋放在 return 語句上是非法的,因為它不是聲明 [JLS, 9.7]。你可能想把注釋放在整個方法上,但是不要這樣做。相反,應該聲明一個局部變量來保存返回值并添加注解,如下所示:
```
// Adding local variable to reduce scope of @SuppressWarnings
public <T> T[] toArray(T[] a) {
if (a.length < size) {
// This cast is correct because the array we're creating
// is of the same type as the one passed in, which is T[].
@SuppressWarnings("unchecked") T[] result = (T[]) Arrays.copyOf(elements, size, a.getClass());
return result;
}
System.arraycopy(elements, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
```
The resulting method compiles cleanly and minimizes the scope in which unchecked warnings are suppressed.
生成的方法編譯正確,并將抑制 unchecked 警告的范圍減到最小。
**Every time you use a @SuppressWarnings("unchecked") annotation, add a comment saying why it is safe to do so.** This will help others understand the code, and more importantly, it will decrease the odds that someone will modify the code so as to make the computation unsafe. If you find it hard to write such a comment, keep thinking. You may end up figuring out that the unchecked operation isn’t safe after all.
**每次使用 SuppressWarnings("unchecked") 注解時,要添加一條注釋,說明這樣做是安全的。** 這將幫助他人理解代碼,更重要的是,它將降低其他人修改代碼而產生不安全事件的幾率。如果你覺得寫這樣的注釋很難,那就繼續思考合適的方式。你最終可能會發現,unchecked 操作畢竟是不安全的。
In summary, unchecked warnings are important. Don’t ignore them. Every unchecked warning represents the potential for a ClassCastException at runtime. Do your best to eliminate these warnings. If you can’t eliminate an unchecked warning and you can prove that the code that provoked it is typesafe, suppress the warning with a @SuppressWarnings("unchecked") annotation in the narrowest possible scope. Record the rationale for your decision to suppress the warning in a comment.
總之,unchecked 警告很重要。不要忽視他們。每個 unchecked 警告都代表了在運行時發生 ClassCastException 的可能性。盡最大努力消除這些警告。如果不能消除 unchecked 警告,并且可以證明引發該警告的代碼是類型安全的,那么可以在盡可能狹窄的范圍內使用 @SuppressWarnings("unchecked") 注釋來禁止警告。在注釋中記錄你決定隱藏警告的理由。
---
**[Back to contents of the chapter(返回章節目錄)](/Chapter-5/Chapter-5-Introduction.md)**
- **Previous Item(上一條目):[Item 26: Do not use raw types(不要使用原始類型)](/Chapter-5/Chapter-5-Item-26-Do-not-use-raw-types.md)**
- **Next Item(下一條目):[Item 28: Prefer lists to arrays(list 優于數組)](/Chapter-5/Chapter-5-Item-28-Prefer-lists-to-arrays.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)