<ruby id="bdb3f"></ruby>

    <p id="bdb3f"><cite id="bdb3f"></cite></p>

      <p id="bdb3f"><cite id="bdb3f"><th id="bdb3f"></th></cite></p><p id="bdb3f"></p>
        <p id="bdb3f"><cite id="bdb3f"></cite></p>

          <pre id="bdb3f"></pre>
          <pre id="bdb3f"><del id="bdb3f"><thead id="bdb3f"></thead></del></pre>

          <ruby id="bdb3f"><mark id="bdb3f"></mark></ruby><ruby id="bdb3f"></ruby>
          <pre id="bdb3f"><pre id="bdb3f"><mark id="bdb3f"></mark></pre></pre><output id="bdb3f"></output><p id="bdb3f"></p><p id="bdb3f"></p>

          <pre id="bdb3f"><del id="bdb3f"><progress id="bdb3f"></progress></del></pre>

                <ruby id="bdb3f"></ruby>

                企業??AI智能體構建引擎,智能編排和調試,一鍵部署,支持知識庫和私有化部署方案 廣告
                ## Chapter 9. General Programming(通用程序設計) ### 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) The float and double types are designed primarily for scientific and engineering calculations. They perform binary floating-point arithmetic, which was carefully designed to furnish accurate approximations quickly over a broad range of magnitudes. They do not, however, provide exact results and should not be used where exact results are required. **The float and double types are particularly ill-suited for monetary calculations** because it is impossible to represent 0.1 (or any other negative power of ten) as a float or double exactly. float 和 double 類型主要用于科學計算和工程計算。它們執行二進制浮點運算,該算法經過精心設計,能夠在很大范圍內快速提供精確的近似值。但是,它們不能提供準確的結果,也不應該在需要精確結果的地方使用。**float 和 double 類型特別不適合進行貨幣計算**,因為不可能將 0.1(或 10 的任意負次冪)精確地表示為 float 或 double。 For example, suppose you have $1.03 in your pocket, and you spend 42¢. How much money do you have left? Here’s a naive program fragment that attempts to answer this question: 例如,假設你口袋里有 1.03 美元,你消費了 42 美分。你還剩下多少錢?下面是一個簡單的程序片段,試圖回答這個問題: ``` System.out.println(1.03 - 0.42); ``` Unfortunately, it prints out 0.6100000000000001. This is not an isolated case. Suppose you have a dollar in your pocket, and you buy nine washers priced at ten cents each. How much change do you get? 不幸的是,它輸出了 0.6100000000000001。這不是一個特例。假設你口袋里有一美元,你買了 9 臺洗衣機,每臺 10 美分。你能得到多少零錢? ``` System.out.println(1.00 - 9 * 0.10); ``` According to this program fragment, you get $0.09999999999999998. 根據這個程序片段,可以得到 0.0999999999999999998 美元。 You might think that the problem could be solved merely by rounding results prior to printing, but unfortunately this does not always work. For example, suppose you have a dollar in your pocket, and you see a shelf with a row of delicious candies priced at 10¢, 20¢, 30¢, and so forth, up to a dollar. You buy one of each candy, starting with the one that costs 10¢, until you can’t afford to buy the next candy on the shelf. How many candies do you buy, and how much change do you get? Here’s a naive program designed to solve this problem: 你可能認為,只需在打印之前將結果四舍五入就可以解決這個問題,但不幸的是,這種方法并不總是有效。例如,假設你口袋里有一美元,你看到一個架子上有一排好吃的糖果,它們的價格僅僅是 10 美分,20 美分,30 美分,以此類推,直到 1 美元。你每買一顆糖,從 10 美分的那顆開始,直到你買不起貨架上的下一顆糖。你買了多少糖果,換了多少零錢?這里有一個簡單的程序來解決這個問題: ``` // Broken - uses floating point for monetary calculation! public static void main(String[] args) { double funds = 1.00; int itemsBought = 0; for (double price = 0.10; funds >= price; price += 0.10) { funds -= price; itemsBought++; } System.out.println(itemsBought +"items bought."); System.out.println("Change: $" + funds); } ``` If you run the program, you’ll find that you can afford three pieces of candy, and you have $0.3999999999999999 left. This is the wrong answer! The right way to solve this problem is to **use BigDecimal, int, or long for monetary calculations.** 如果你運行這個程序,你會發現你可以買得起三塊糖,你還有 0.399999999999999999 美元。這是錯誤的答案!解決這個問題的正確方法是 **使用 BigDecimal、int 或 long 進行貨幣計算。** Here’s a straightforward transformation of the previous program to use the BigDecimal type in place of double. Note that BigDecimal’s String constructor is used rather than its double constructor. This is required in order to avoid introducing inaccurate values into the computation [Bloch05, Puzzle 2]: 這里是前一個程序的一個簡單改版,使用 BigDecimal 類型代替 double。注意,使用 BigDecimal 的 String 構造函數而不是它的 double 構造函數。這是為了避免在計算中引入不準確的值 [Bloch05, Puzzle 2]: ``` public static void main(String[] args) { final BigDecimal TEN_CENTS = new BigDecimal(".10"); int itemsBought = 0; BigDecimal funds = new BigDecimal("1.00"); for (BigDecimal price = TEN_CENTS;funds.compareTo(price) >= 0;price = price.add(TEN_CENTS)) { funds = funds.subtract(price); itemsBought++; } System.out.println(itemsBought +"items bought."); System.out.println("Money left over: $" + funds); } ``` If you run the revised program, you’ll find that you can afford four pieces of candy, with $0.00 left over. This is the correct answer. 如果你運行修改后的程序,你會發現你可以買四顆糖,最終剩下 0 美元。這是正確答案。 There are, however, two disadvantages to using BigDecimal: it’s a lot less convenient than using a primitive arithmetic type, and it’s a lot slower. The latter disadvantage is irrelevant if you’re solving a single short problem, but the former may annoy you. 然而,使用 BigDecimal 有兩個缺點:它與原始算術類型相比很不方便,而且速度要慢得多。如果你只解決一個簡單的問題,后一種缺點是無關緊要的,但前者可能會讓你煩惱。 An alternative to using BigDecimal is to use int or long, depending on the amounts involved, and to keep track of the decimal point yourself. In this example, the obvious approach is to do all computation in cents instead of dollars. Here’s a straightforward transformation that takes this approach: 除了使用 BigDecimal,另一種方法是使用 int 或 long,這取決于涉及的數值大小,還要自己處理十進制小數點。在這個例子中,最明顯的方法是用美分而不是美元來計算。下面是一個采用這種方法的簡單改版: ``` public static void main(String[] args) { int itemsBought = 0; int funds = 100; for (int price = 10; funds >= price; price += 10) { funds -= price; itemsBought++; } System.out.println(itemsBought +"items bought."); System.out.println("Cash left over: " + funds + " cents"); } ``` In summary, don’t use float or double for any calculations that require an exact answer. Use BigDecimal if you want the system to keep track of the decimal point and you don’t mind the inconvenience and cost of not using a primitive type. Using BigDecimal has the added advantage that it gives you full control over rounding, letting you select from eight rounding modes whenever an operation that entails rounding is performed. This comes in handy if you’re performing business calculations with legally mandated rounding behavior. If performance is of the essence, you don’t mind keeping track of the decimal point yourself, and the quantities aren’t too big, use int or long. If the quantities don’t exceed nine decimal digits, you can use int; if they don’t exceed eighteen digits, you can use long. If the quantities might exceed eighteen digits, use BigDecimal. 總之,對于任何需要精確答案的計算,不要使用 float 或 double 類型。如果希望系統來處理十進制小數點,并且不介意不使用基本類型帶來的不便和成本,請使用 BigDecimal。使用 BigDecimal 的另一個好處是,它可以完全控制舍入,當執行需要舍入的操作時,可以從八種舍入模式中進行選擇。如果你使用合法的舍入行為執行業務計算,這將非常方便。如果性能是最重要的,那么你不介意自己處理十進制小數點,而且數值不是太大,可以使用 int 或 long。如果數值不超過 9 位小數,可以使用 int;如果不超過 18 位,可以使用 long。如果數量可能超過 18 位,則使用 BigDecimal。 --- **[Back to contents of the chapter(返回章節目錄)](/Chapter-9/Chapter-9-Introduction.md)** - **Previous Item(上一條目):[Item 59: Know and use the libraries(了解并使用庫)](/Chapter-9/Chapter-9-Item-59-Know-and-use-the-libraries.md)** - **Next Item(下一條目):[Item 61: Prefer primitive types to boxed primitives(基本數據類型優于包裝類)](/Chapter-9/Chapter-9-Item-61-Prefer-primitive-types-to-boxed-primitives.md)**
                  <ruby id="bdb3f"></ruby>

                  <p id="bdb3f"><cite id="bdb3f"></cite></p>

                    <p id="bdb3f"><cite id="bdb3f"><th id="bdb3f"></th></cite></p><p id="bdb3f"></p>
                      <p id="bdb3f"><cite id="bdb3f"></cite></p>

                        <pre id="bdb3f"></pre>
                        <pre id="bdb3f"><del id="bdb3f"><thead id="bdb3f"></thead></del></pre>

                        <ruby id="bdb3f"><mark id="bdb3f"></mark></ruby><ruby id="bdb3f"></ruby>
                        <pre id="bdb3f"><pre id="bdb3f"><mark id="bdb3f"></mark></pre></pre><output id="bdb3f"></output><p id="bdb3f"></p><p id="bdb3f"></p>

                        <pre id="bdb3f"><del id="bdb3f"><progress id="bdb3f"></progress></del></pre>

                              <ruby id="bdb3f"></ruby>

                              哎呀哎呀视频在线观看