標簽類，包含一個標簽屬性（tag field），表示實例的風格

// Tagged class - vastly inferior to a class hierarchy! class Figure {enum Shape { RECTANGLE, CIRCLE };// Tag field - the shape of this figurefinal Shape shape;// These fields are used only if shape is RECTANGLEdouble length;double width;// This field is used only if shape is CIRCLEdouble radius;// Constructor for circleFigure(double radius) {shape = Shape.CIRCLE;this.radius = radius;}// Constructor for rectangleFigure(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);}} }

總之，標簽類是冗長的，容易出錯的，而且效率低下。

子類型化（subtyping）

首先定義一個包含抽象方法的抽象類，該標簽類的行為取決于標簽值。
接下來，為原始標簽類的每種類型定義一個根類的具體子類。

// 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; } }

總結

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