## 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)**
- 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)