## Chapter 4. Classes and Interfaces(類和接口)
### Item 23: Prefer class hierarchies to tagged classes(類層次結構優于帶標簽的類)
Occasionally you may run across a class whose instances come in two or more flavors and contain a tag field indicating the flavor of the instance. For example, consider this class, which is capable of representing a circle or a rectangle:
有時候,你可能會遇到這樣一個類,它的實例有兩種或兩種以上的樣式,并且包含一個標簽字段來表示實例的樣式。例如,考慮這個類,它能夠表示一個圓或一個矩形:
```
// Tagged class - vastly inferior to a class hierarchy!
class Figure {
enum Shape {RECTANGLE, CIRCLE};
// Tag field - the shape of this figure
final Shape shape;
// These fields are used only if shape is RECTANGLE
double length;
double width;
// This field is used only if shape is CIRCLE
double radius;
// Constructor for circle
Figure(double radius) {
shape = Shape.CIRCLE;
this.radius = radius;
}
// Constructor for rectangle
Figure(double length, double width) {
shape = Shape.RECTANGLE;
this.length = length;
this.width = width;
}
double area() {
switch (shape) {
case RECTANGLE:
return length * width;
case CIRCLE:
return Math.PI * (radius * radius);
default:
throw new AssertionError(shape);
}
}
}
```
Such tagged classes have numerous shortcomings. They are cluttered with boilerplate, including enum declarations, tag fields, and switch statements. Readability is further harmed because multiple implementations are jumbled together in a single class. Memory footprint is increased because instances are burdened with irrelevant fields belonging to other flavors. Fields can’t be made final unless constructors initialize irrelevant fields, resulting in more boilerplate. Constructors must set the tag field and initialize the right data fields with no help from the compiler: if you initialize the wrong fields, the program will fail at runtime. You can’t add a flavor to a tagged class unless you can modify its source file. If you do add a flavor, you must remember to add a case to every switch statement, or the class will fail at runtime. Finally, the data type of an instance gives no clue as to its flavor. In short, **tagged classes are verbose, error-prone, and inefficient.**
這樣的標簽類有許多缺點。它們充斥著樣板代碼,包括 enum 聲明、標簽字段和 switch 語句。因為多個實現在一個類中混雜,會造成可讀性受損。內存占用也增加了,因為實例被其他類型的不相關字段所拖累。除非構造函數初始化不相關的字段,否則不能將字段設置為 final,但這會導致更多的樣板文件。構造函數必須設置標簽字段并在沒有編譯器幫助的情況下初始化正確的數據字段:如果初始化了錯誤的字段,程序將在運行時失敗。除非你能夠修改它的源文件,否則你不能向標簽類添加樣式。如果你確實添加了一個樣式,那么你必須記住要為每個 switch 語句添加一個 case,否則類將在運行時出錯。最后,實例的數據類型沒有給出它任何關于樣式的提示。簡而言之,**標簽類冗長、容易出錯和低效。**
Luckily, object-oriented languages such as Java offer a far better alternative for defining a single data type capable of representing objects of multiple flavors: subtyping. **A tagged class is just a pallid imitation of a class hierarchy.**
幸運的是,面向對象的語言(如 Java)提供了一個更好的選擇來定義能夠表示多種類型對象的單一數據類型:子類型。**標簽類只是類層次結構的簡易模仿。**
To transform a tagged class into a class hierarchy, first define an abstract class containing an abstract method for each method in the tagged class whose behavior depends on the tag value. In the Figure class, there is only one such method, which is area. This abstract class is the root of the class hierarchy. If there are any methods whose behavior does not depend on the value of the tag, put them in this class. Similarly, if there are any data fields used by all the flavors, put them in this class. There are no such flavor-independent methods or fields in the Figure class.
要將已標簽的類轉換為類層次結構,首先為標簽類中的每個方法定義一個包含抽象方法的抽象類,其行為依賴于標簽值。在 Figure 類中,只有一個這樣的方法,即 area 方法。這個抽象類是類層次結構的根。如果有任何方法的行為不依賴于標簽的值,請將它們放在這個類中。類似地,如果有任何數據字段被所有樣式使用,將它們放在這個類中。在 Figure 類中沒有這樣的獨立于樣式的方法或字段。
Next, define a concrete subclass of the root class for each flavor of the original tagged class. In our example, there are two: circle and rectangle. Include in each subclass the data fields particular to its flavor. In our example, radius is particular to circle, and length and width are particular to rectangle. Also include in each subclass the appropriate implementation of each abstract method in the root class. Here is the class hierarchy corresponding to the original Figure class:
接下來,為原始標簽類的每個類型定義根類的具體子類。在我們的例子中,有兩個:圓形和矩形。在每個子類中包含特定于其樣式的數據字段。在我們的例子中,半徑是特定于圓的,長度和寬度是特定于矩形的。還應在每個子類中包含根類中每個抽象方法的適當實現。下面是原 Figure 類對應的類層次結構:
```
// Class hierarchy replacement for a tagged class
abstract class Figure {
abstract double area();
}
class Circle extends Figure {
final double radius;
Circle(double radius) {
this.radius = radius;
}
@Override
double area() {
return Math.PI * (radius * radius);
}
}
class Rectangle extends Figure {
final double length;
final double width;
Rectangle(double length, double width) {
this.length = length;
this.width = width;
}
@Override
double area() {
return length * width;
}
}
```
This class hierarchy corrects every shortcoming of tagged classes noted previously. The code is simple and clear, containing none of the boilerplate found in the original. The implementation of each flavor is allotted its own class, and none of these classes is encumbered by irrelevant data fields. All fields are final. The compiler ensures that each class’s constructor initializes its data fields and that each class has an implementation for every abstract method declared in the root class. This eliminates the possibility of a runtime failure due to a missing switch case. Multiple programmers can extend the hierarchy independently and interoperably without access to the source for the root class. There is a separate data type associated with each flavor, allowing programmers to indicate the flavor of a variable and to restrict variables and input parameters to a particular flavor.
這個類層次結構糾正了前面提到的標簽類的所有缺點。代碼簡單明了,不包含原始代碼中的樣板代碼。每種樣式的實現都分配有自己的類,這些類沒有被不相關的數據字段拖累。所有字段為 final 字段。編譯器確保每個類的構造函數初始化它的數據字段,并且每個類對于根類中聲明的抽象方法都有一個實現。這消除了由于缺少 switch case 而導致運行時出錯的可能性。多個程序員可以獨立地、可互操作地擴展層次結構,而無需查看根類的源代碼。每種樣式都有一個單獨的數據類型,允許程序員指出變量的樣式,并將變量和輸入參數限制為特定的樣式。
Another advantage of class hierarchies is that they can be made to reflect natural hierarchical relationships among types, allowing for increased flexibility and better compile-time type checking. Suppose the tagged class in the original example also allowed for squares. The class hierarchy could be made to reflect the fact that a square is a special kind of rectangle (assuming both are immutable):
類層次結構的另一個優點是,可以反映類型之間的自然層次關系,從而提高靈活性和更好的編譯時類型檢查。假設原始示例中的標簽類也允許使用正方形。類層次結構可以反映這樣一個事實:正方形是一種特殊的矩形(假設兩者都是不可變的):
```
class Square extends Rectangle {
Square(double side) {
super(side, side);
}
}
```
Note that the fields in the above hierarchy are accessed directly rather than by accessor methods. This was done for brevity and would be a poor design if the hierarchy were public (Item 16).
注意,上面層次結構中的字段是直接訪問的,而不是通過訪問器方法訪問的。這樣做是為了簡潔,如果層次結構是公共的,那么這將是一個糟糕的設計([Item-16](/Chapter-4/Chapter-4-Item-16-In-public-classes-use-accessor-methods-not-public-fields.md))。
In summary, tagged classes are seldom appropriate. If you’re tempted to write a class with an explicit tag field, think about whether the tag could be eliminated and the class replaced by a hierarchy. When you encounter an existing class with a tag field, consider refactoring it into a hierarchy.
總之,標簽類很少有合適的使用場景。如果想編寫一個帶有顯式標簽字段的類,請考慮是否可以刪除標簽并用層次結構替換。當遇到具有標簽字段的現有類時,請考慮將其重構為層次結構。
---
**[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 22: Use interfaces only to define types(接口只用于定義類型)](/Chapter-4/Chapter-4-Item-22-Use-interfaces-only-to-define-types.md)**
- **Next Item(下一條目):[Item 24: Favor static member classes over nonstatic(靜態成員類優于非靜態成員類)](/Chapter-4/Chapter-4-Item-24-Favor-static-member-classes-over-nonstatic.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)