## Chapter 10. Exceptions(異常)
### Item 72: Favor the use of standard exceptions(鼓勵復用標準異常)
An attribute that distinguishes expert programmers from less experienced ones is that experts strive for and usually achieve a high degree of code reuse. Exceptions are no exception to the rule that code reuse is a good thing. The Java libraries provide a set of exceptions that covers most of the exception-throwing needs of most APIs.
專家程序員與經驗較少的程序員之間的一個區別是,專家力求實現高度的代碼復用。代碼復用是一件好事,異常也不例外。Java 庫提供了一組異常,涵蓋了大多數 API 的大多數異常拋出需求。
Reusing standard exceptions has several benefits. Chief among them is that it makes your API easier to learn and use because it matches the established conventions that programmers are already familiar with. A close second is that programs using your API are easier to read because they aren’t cluttered with unfamiliar exceptions. Last (and least), fewer exception classes means a smaller memory footprint and less time spent loading classes.
復用標準異常有幾個好處。其中最主要的是,它使你的 API 更容易學習和使用,因為它符合程序員已經熟悉的既定約定。其次,使用你的 API 的程序更容易閱讀,因為它們不會因為不熟悉的異常而混亂。最后(也是最不重要的),更少的異常類意味著更小的內存占用和更少的加載類的時間。
The most commonly reused exception type is IllegalArgumentException (Item 49). This is generally the exception to throw when the caller passes in an argument whose value is inappropriate. For example, this would be the exception to throw if the caller passed a negative number in a parameter representing the number of times some action was to be repeated.
最常見的復用異常類型是 IllegalArgumentException([Item-49](/Chapter-8/Chapter-8-Item-49-Check-parameters-for-validity.md))。這通常是調用者傳入不合適的參數時拋出的異常。例如,如果調用者在表示某個操作要重復多少次的參數中傳遞了一個負數,則拋出這個異常。
Another commonly reused exception is IllegalStateException. This is generally the exception to throw if the invocation is illegal because of the state of the receiving object. For example, this would be the exception to throw if the caller attempted to use some object before it had been properly initialized.
另一個常被復用異常是 IllegalStateException。如果接收對象的狀態導致調用非法,則通常會拋出此異常。例如,如果調用者試圖在對象被正確初始化之前使用它,那么這將是拋出的異常。
Arguably, every erroneous method invocation boils down to an illegal argument or state, but other exceptions are standardly used for certain kinds of illegal arguments and states. If a caller passes null in some parameter for which null values are prohibited, convention dictates that NullPointerException be thrown rather than IllegalArgumentException. Similarly, if a caller passes an out-of-range value in a parameter representing an index into a sequence, IndexOutOfBoundsException should be thrown rather than IllegalArgumentException.
可以說,每個錯誤的方法調用都歸結為參數非法或狀態非法,但是有一些異常通常用于某些特定的參數非法和狀態非法。如果調用者在禁止空值的參數中傳遞 null,那么按照慣例,拋出 NullPointerException 而不是 IllegalArgumentException。類似地,如果調用者將表示索引的參數中的超出范圍的值傳遞給序列,則應該拋出 IndexOutOfBoundsException,而不是 IllegalArgumentException。
Another reusable exception is ConcurrentModificationException. It should be thrown if an object that was designed for use by a single thread (or with external synchronization) detects that it is being modified concurrently. This exception is at best a hint because it is impossible to reliably detect concurrent modification.
另一個可復用異常是 ConcurrentModificationException。如果一個對象被設計為由單個線程使用(或與外部同步),并且檢測到它正在被并發地修改,則應該拋出該異常。因為不可能可靠地檢測并發修改,所以該異常充其量只是一個提示。
A last standard exception of note is UnsupportedOperationException. This is the exception to throw if an object does not support an attempted operation. Its use is rare because most objects support all of their methods. This exception is used by classes that fail to implement one or more optional operations defined by an interface they implement. For example, an append-only List implementation would throw this exception if someone tried to delete an element from the list.
最后一個需要注意的標準異常是 UnsupportedOperationException。如果對象不支持嘗試的操作,則拋出此異常。它很少使用,因為大多數對象都支持它們的所有方法。此異常用于一個類沒有實現由其實現的接口定義的一個或多個可選操作。例如,對于只支持追加操作的 List 實現,試圖從中刪除元素時就會拋出這個異常。
**Do not reuse Exception, RuntimeException, Throwable, or Error directly.** Treat these classes as if they were abstract. You can't reliably test for these exceptions because they are superclasses of other exceptions that a method may throw.
**不要直接復用 Exception、RuntimeException、Throwable 或 Error。** 應當將這些類視為抽象類。你不能對這些異常進行可靠的測試,因為它們是方法可能拋出的異常的超類。
This table summarizes the most commonly reused exceptions:
此表總結了最常見的可復用異常:
| Exception | Occasion for Use |
|:-------:|:-------:|
| IllegalArgumentException | Non-null parameter value is inappropriate(非空參數值不合適) |
| IllegalStateException | Object state is inappropriate for method invocation(對象狀態不適用于方法調用) |
| NullPointerException | Parameter value is null where prohibited(禁止參數為空時仍傳入 null) |
| IndexOutOfBoundsException | Index parameter value is out of range(索引參數值超出范圍) |
| ConcurrentModificationException | Concurrent modification of an object has been detected where it is prohibited(在禁止并發修改對象的地方檢測到該動作) |
| UnsupportedOperationException | Object does not support method(對象不支持該方法調用) |
While these are by far the most commonly reused exceptions, others may be reused where circumstances warrant. For example, it would be appropriate to reuse ArithmeticException and NumberFormatException if you were implementing arithmetic objects such as complex numbers or rational numbers. If an exception fits your needs, go ahead and use it, but only if the conditions under which you would throw it are consistent with the exception’s documentation: reuse must be based on documented semantics, not just on name. Also, feel free to subclass a standard exception if you want to add more detail (Item 75), but remember that exceptions are serializable (Chapter 12). That alone is reason not to write your own exception class without good reason.
雖然到目前為止,這些是最常見的復用異常,但是在環境允許的情況下也可以復用其他異常。例如,如果你正在實現諸如復數或有理數之類的算術對象,那么復用 ArithmeticException 和 NumberFormatException 是合適的。如果一個異常符合你的需要,那么繼續使用它,但前提是你拋出它的條件與異常的文檔描述一致:復用必須基于文檔化的語義,而不僅僅是基于名稱。另外,如果你想添加更多的細節,可以隨意子類化標準異常(第75項),但是請記住,異常是可序列化的(Chapter 12)。如果沒有充分的理由,不要編寫自己的異常類。
Choosing which exception to reuse can be tricky because the “occasions for use” in the table above do not appear to be mutually exclusive. Consider the case of an object representing a deck of cards, and suppose there were a method to deal a hand from the deck that took as an argument the size of the hand. If the caller passed a value larger than the number of cards remaining in the deck, it could be construed as an IllegalArgumentException (the handSize parameter value is too high) or an IllegalStateException (the deck contains too few cards). Under these circumstances, the rule is to throw IllegalStateException if no argument values would have worked, otherwise throw IllegalArgumentException.
選擇復用哪個異常可能比較棘手,因為上表中的「使用場合」似乎并不相互排斥。考慮一個對象,表示一副牌,假設有一個方法代表發牌操作,該方法將手牌多少作為參數。如果調用者傳遞的值大于牌堆中剩余的牌的數量,則可以將其解釋為 IllegalArgumentException (handSize 參數值太大)或 IllegalStateException(牌堆中包含的牌太少)。在這種情況下,規則是:如果沒有參數值,拋出 IllegalStateException,否則拋出 IllegalArgumentException。
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- 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)