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                ## 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) This item is similar in nature to Item 15, “Minimize the accessibility of classes and members.” By minimizing the scope of local variables, you increase the readability and maintainability of your code and reduce the likelihood of error. 本條目在性質上類似于 [Item-15](/Chapter-4/Chapter-4-Item-15-Minimize-the-accessibility-of-classes-and-members.md),即「最小化類和成員的可訪問性」。通過最小化局部變量的范圍,可以提高代碼的可讀性和可維護性,并降低出錯的可能性。 Older programming languages, such as C, mandated that local variables must be declared at the head of a block, and some programmers continue to do this out of habit. It’s a habit worth breaking. As a gentle reminder, Java lets you declare variables anywhere a statement is legal (as does C, since C99). 較老的編程語言,如 C 語言,強制要求必須在代碼塊的頭部聲明局部變量,一些程序員出于習慣目前繼續這樣做。這是一個應改變的習慣。溫馨提醒,Java 允許你在任何能夠合法使用語句的地方聲明變量(這與 C99 標準后 C 語言一樣)。 **The most powerful technique for minimizing the scope of a local variable is to declare it where it is first used.** If a variable is declared before it is used, it’s just clutter—one more thing to distract the reader who is trying to figure out what the program does. By the time the variable is used, the reader might not remember the variable’s type or initial value. **將局部變量的作用域最小化,最具說服力的方式就是在第一次使用它的地方聲明。** 如果一個變量在使用之前聲明了,代碼會變得很混亂,這是另一件分散讀者注意力的事情,因為讀者正在試圖弄清楚程序的功能。在使用到該變量時,讀者可能不記得變量的類型或初始值。 Declaring a local variable prematurely can cause its scope not only to begin too early but also to end too late. The scope of a local variable extends from the point where it is declared to the end of the enclosing block. If a variable is declared outside of the block in which it is used, it remains visible after the program exits that block. If a variable is used accidentally before or after its region of intended use, the consequences can be disastrous. 過早地聲明局部變量會導致其作用域開始得太早,而且結束得過晚。局部變量的范圍應該從聲明它的地方直到封閉塊的末尾。如果變量在使用它的代碼塊外部聲明,則在程序退出該塊之后它仍然可見。如果一個變量在其預期使用區域之前或之后意外使用,其后果可能是災難性的。 **Nearly every local variable declaration should contain an initializer.** If you don’t yet have enough information to initialize a variable sensibly, you should postpone the declaration until you do. One exception to this rule concerns try-catch statements. If a variable is initialized to an expression whose evaluation can throw a checked exception, the variable must be initialized inside a try block (unless the enclosing method can propagate the exception). If the value must be used outside of the try block, then it must be declared before the try block, where it cannot yet be “sensibly initialized.” For an example, see page 283. **每個局部變量聲明都應該包含一個初始化表達式。** 如果你還沒有足夠的信息來合理地初始化一個變量,你應該推遲聲明,直到條件滿足。這個規則的一個例外是 try-catch 語句。如果一個變量被初始化為一個表達式,該表達式的計算結果可以拋出一個 checked 異常,那么該變量必須在 try 塊中初始化(除非所包含的方法可以傳播異常)。如果該值必須在 try 塊之外使用,那么它必須在 try 塊之前聲明,此時它還不能「合理地初始化」。例子可參見 283 頁。 Loops present a special opportunity to minimize the scope of variables. The for loop, in both its traditional and for-each forms, allows you to declare loop variables, limiting their scope to the exact region where they’re needed. (This region consists of the body of the loop and the code in parentheses between the for keyword and the body.) Therefore, prefer for loops to while loops, assuming the contents of the loop variable aren’t needed after the loop terminates. 循環提供了一個特殊的機會來最小化變量的范圍。for 循環的傳統形式和 for-each 形式都允許聲明循環變量,將它們的作用域精確限制在需要它們的區域。(這個區域由循環的主體以及 for 關鍵字和主體之間括號中的代碼組成。)因此,假設循環結束后不再需要循環變量,for 循環就優于 while 循環。 For example, here is the preferred idiom for iterating over a collection (Item 58): 例如,下面是遍歷集合的首選習慣用法([Item-58](/Chapter-9/Chapter-9-Item-58-Prefer-for-each-loops-to-traditional-for-loops.md)): ``` // Preferred idiom for iterating over a collection or array for (Element e : c) { ... // Do Something with e } ``` If you need access to the iterator, perhaps to call its remove method, the preferred idiom uses a traditional for loop in place of the for-each loop: 如果你需要訪問 iterator,或者調用它的 remove 方法,首選的習慣用法是使用傳統的 for 循環來代替 for-each 循環: ``` // Idiom for iterating when you need the iterator for (Iterator<Element> i = c.iterator(); i.hasNext(); ) { Element e = i.next(); ... // Do something with e and i } ``` To see why these for loops are preferable to a while loop, consider the following code fragment, which contains two while loops and one bug: 要弄清楚為什么 for 循環比 while 循環更好,請考慮下面的代碼片段,其中包含兩個 while 循環和一個 bug: ``` Iterator<Element> i = c.iterator(); while (i.hasNext()) { doSomething(i.next()); } ... Iterator<Element> i2 = c2.iterator(); while (i.hasNext()) { // BUG! doSomethingElse(i2.next()); } ``` The second loop contains a copy-and-paste error: it initializes a new loop variable, i2, but uses the old one, i, which is, unfortunately, still in scope. The resulting code compiles without error and runs without throwing an exception, but it does the wrong thing. Instead of iterating over c2, the second loop terminates immediately, giving the false impression that c2 is empty. Because the program errs silently, the error can remain undetected for a long time. 第二個循環包含一個復制粘貼錯誤:它計劃初始化一個新的循環變量 i2,卻誤用了舊的變量 i,不幸的是,i 仍然在作用域中。生成的代碼編譯時沒有錯誤,運行時沒有拋出異常,但是它做了錯誤的事情。第二個循環并沒有遍歷 c2,而是立即終止,從而產生 c2 為空的假象。因為程序會靜默地出錯,所以很長一段時間內都無法檢測到錯誤。 If a similar copy-and-paste error were made in conjunction with either of the for loops (for-each or traditional), the resulting code wouldn’t even compile. The element (or iterator) variable from the first loop would not be in scope in the second loop. Here’s how it looks with the traditional for loop: 如果將類似的復制粘貼錯誤發生在 for 循環(for-each 循環或傳統循環),則生成的代碼甚至無法編譯。對于第二個循環,第一個循環中的(或 iterator)變量已經不在作用域中。下面是它與傳統 for 循環的樣子: ``` for (Iterator<Element> i = c.iterator(); i.hasNext(); ) { Element e = i.next(); ... // Do something with e and i } ... // Compile-time error - cannot find symbol i for (Iterator<Element> i2 = c2.iterator(); i.hasNext(); ) { Element e2 = i2.next(); ... // Do something with e2 and i2 } ``` Moreover, if you use a for loop, it’s much less likely that you’ll make the copy-and-paste error because there’s no incentive to use different variable names in the two loops. The loops are completely independent, so there’s no harm in reusing the element (or iterator) variable name. In fact, it’s often stylish to do so. The for loop has one more advantage over the while loop: it is shorter, which enhances readability. Here is another loop idiom that minimizes the scope of local variables: 此外,如果你使用 for 循環,那么發生復制粘貼錯誤的可能性要小得多,因為這兩種循環中沒有使用不同變量名稱的動機。循環是完全獨立的,所以復用循環(或 iterator)變量名沒有害處。事實上,這樣做通常很流行。for 循環相比 while 循環還有一個優點:它更短,這增強了可讀性。下面是另一個循環習慣用法,它也最小化了局部變量的范圍: ``` for (int i = 0, n = expensiveComputation(); i < n; i++) { ... // Do something with i; } ``` The important thing to notice about this idiom is that it has two loop variables, i and n, both of which have exactly the right scope. The second variable, n, is used to store the limit of the first, thus avoiding the cost of a redundant computation in every iteration. As a rule, you should use this idiom if the loop test involves a method invocation that is guaranteed to return the same result on each iteration. 關于這個用法需要注意的重要一點是,它有兩個循環變量,i 和 n,它們都具有完全正確的作用域。第二個變量 n 用于存儲第一個變量的極限,從而避免了每次迭代中冗余計算的成本。作為一個規則,如果循環測試涉及一個方法調用,并且保證在每次迭代中返回相同的結果,那么應該使用這個習慣用法。 A final technique to minimize the scope of local variables is to keep methods small and focused. If you combine two activities in the same method, local variables relevant to one activity may be in the scope of the code performing the other activity. To prevent this from happening, simply separate the method into two: one for each activity. 最小化局部變量范圍的最后一種技術是保持方法小而集中。如果在同一方法中合并兩個操作,與一個操作相關的局部變量可能位于執行另一個操作的代碼的范圍內。為了防止這種情況發生,只需將方法分成兩個部分:每個操作一個。 --- **[Back to contents of the chapter(返回章節目錄)](/Chapter-9/Chapter-9-Introduction.md)** - **Next Item(下一條目):[Item 58: Prefer for-each loops to traditional for loops(for-each 循環優于傳統的 for 循環)](/Chapter-9/Chapter-9-Item-58-Prefer-for-each-loops-to-traditional-for-loops.md)**
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