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                ## 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)**
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