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                ## 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。 --- **[Back to contents of the chapter(返回章節目錄)](/Chapter-10/Chapter-10-Introduction.md)** - **Previous Item(上一條目):[Item 71: Avoid unnecessary use of checked exceptions(避免不必要地使用 checked 異常)](/Chapter-10/Chapter-10-Item-71-Avoid-unnecessary-use-of-checked-exceptions.md)** - **Next Item(下一條目):[Item 73: Throw exceptions appropriate to the abstraction(拋出能用抽象解釋的異常)](/Chapter-10/Chapter-10-Item-73-Throw-exceptions-appropriate-to-the-abstraction.md)**
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