第20条：类层次优于标签类

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标签类代码，一看便知何为标签类。

// 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();        }    }}

代码中Shape shape就定义了一个枚举类型的标签，用于表示Figure类属于那种形状。

标签类缺点：对于同一种形状的对象来说，Figure类的代码中就存在冗余的代码。如：构造器的冗余，area()方法的冗余。

很显然类层次就优秀很多，就不在赘述了。。。。

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