## Chapter 4. Classes and Interfaces(類和接口)
### 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)
Occasionally, you may be tempted to write degenerate classes that serve no purpose other than to group instance fields:
有時候,可能會編寫一些退化類,這些類除了對實例字段進行分組之外,沒有其他用途:
```
// Degenerate classes like this should not be public!
class Point {
public double x;
public double y;
}
```
Because the data fields of such classes are accessed directly, these classes do not offer the benefits of encapsulation (Item 15). You can’t change the representation without changing the API, you can’t enforce invariants, and you can’t take auxiliary action when a field is accessed. Hard-line object-oriented programmers feel that such classes are anathema and should always be replaced by classes with private fields and public accessor methods (getters) and, for mutable classes, mutators (setters):
因為這些類的數據字段是直接訪問的,所以這些類沒有提供封裝的好處([Item-15](/Chapter-4/Chapter-4-Item-15-Minimize-the-accessibility-of-classes-and-members.md))。不改變 API 就不能改變表現形式,不能實現不變量,也不能在訪問字段時采取輔助操作。堅持面向對象思維的程序員會認為這樣的類是令人厭惡的,應該被使用私有字段和公共訪問方法 getter 的類所取代,對于可變類,則是賦值方法 setter:
```
// Encapsulation of data by accessor methods and mutators
class Point {
private double x;
private double y;
public Point(double x, double y) {
this.x = x;
this.y = y;
}
public double getX() { return x; }
public double getY() { return y; }
public void setX(double x) { this.x = x; }
public void setY(double y) { this.y = y; }
}
```
Certainly, the hard-liners are correct when it comes to public classes: if a class is accessible outside its package, provide accessor methods to preserve the flexibility to change the class’s internal representation. If a public class exposes its data fields, all hope of changing its representation is lost because client code can be distributed far and wide.
當然,當涉及到公共類時,強硬派是正確的:如果類可以在包之外訪問,那么提供訪問器方法來保持更改類內部表示的靈活性。如果一個公共類公開其數據字段,那么改變其表示形式的所有希望都將落空,因為客戶端代碼可以廣泛分發。
However, if a class is package-private or is a private nested class, there is nothing inherently wrong with exposing its data fields—assuming they do an adequate job of describing the abstraction provided by the class. This approach generates less visual clutter than the accessor-method approach, both in the class definition and in the client code that uses it. While the client code is tied to the class’s internal representation, this code is confined to the package containing the class. If a change in representation becomes desirable, you can make the change without touching any code outside the package. In the case of a private nested class, the scope of the change is further restricted to the enclosing class.
但是,如果一個類是包級私有的或者是私有嵌套類,那么公開它的數據字段并沒有什么本質上的錯誤(假設它們能夠很好地描述類提供的抽象)。無論是在類定義還是在使用它的客戶端代碼中,這種方法產生的視覺混亂都比訪問方法少。雖然客戶端代碼與類的內部表示綁定在一起,但這段代碼僅限于包含該類的包。如果想要對表示形式進行更改,你可以在不接觸包外部任何代碼的情況下進行更改。對于私有嵌套類,更改的范圍進一步限制在封閉類中。
Several classes in the Java platform libraries violate the advice that public classes should not expose fields directly. Prominent examples include the Point and Dimension classes in the java.awt package. Rather than examples to be emulated, these classes should be regarded as cautionary tales.As described in Item 67, the decision to expose the internals of the Dimension class resulted in a serious performance problem that is still with us today.
Java 庫中的幾個類違反了公共類不應該直接公開字段的建議。突出的例子包括 `java.awt` 包中的 Point 和 Dimension。這些類不應被效仿,而應被視為警示。正如 [Item-67](/Chapter-9/Chapter-9-Item-67-Optimize-judiciously.md) 所述,公開 Dimension 類的內部結構導致了嚴重的性能問題,這種問題至今仍存在。
While it’s never a good idea for a public class to expose fields directly, it is less harmful if the fields are immutable. You can’t change the representation of such a class without changing its API, and you can’t take auxiliary actions when a field is read, but you can enforce invariants. For example, this class guarantees that each instance represents a valid time:
雖然公共類直接公開字段從來都不是一個好主意,但是如果字段是不可變的,那么危害就會小一些。你不能在不更改該類的 API 的情況下更改該類的表現形式,也不能在讀取字段時采取輔助操作,但是你可以實施不變量。例如,這個類保證每個實例代表一個有效的時間:
```
// Public class with exposed immutable fields - questionable
public final class Time {
private static final int HOURS_PER_DAY = 24;
private static final int MINUTES_PER_HOUR = 60;
public final int hour;
public final int minute;
public Time(int hour, int minute) {
if (hour < 0 || hour >= HOURS_PER_DAY)
throw new IllegalArgumentException("Hour: " + hour);
if (minute < 0 || minute >= MINUTES_PER_HOUR)
throw new IllegalArgumentException("Min: " + minute);
this.hour = hour;
this.minute = minute;
} ... // Remainder omitted
}
```
In summary, public classes should never expose mutable fields. It is less harmful, though still questionable, for public classes to expose immutable fields.It is, however, sometimes desirable for package-private or private nested classes to expose fields, whether mutable or immutable.
總之,公共類不應該公開可變字段。對于公共類來說,公開不可變字段的危害要小一些,但仍然存在潛在的問題。然而,有時候包級私有或私有嵌套類需要公開字段,無論這個類是可變的還是不可變的。
---
**[Back to contents of the chapter(返回章節目錄)](/Chapter-4/Chapter-4-Introduction.md)**
- **Previous Item(上一條目):[Item 15: Minimize the accessibility of classes and members(盡量減少類和成員的可訪問性)](/Chapter-4/Chapter-4-Item-15-Minimize-the-accessibility-of-classes-and-members.md)**
- **Next Item(下一條目):[Item 17: Minimize mutability(減少可變性)](/Chapter-4/Chapter-4-Item-17-Minimize-mutability.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)