有时候,可能会遇到带有两个甚至更多风格的实例的类,并包含表示实例风格的标签(tag)域。
Demo:
- // 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();
- }
- }
- }
这些标签类(tagged class)有着许多优点,但是破坏了可读性。
标签类过于冗长、容易出错,并且效率低下。
表示多种风格对象的数据类型:子类型化(subtyping)。标签类正是类层次的一种简单的仿效。
Demo:
- // 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; }
- 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;
- }
- double area() { return length * width; }
- }
- class Square extends Rectangle {
- Square(double side) {
- super(side, side);
- }
- }
标签类很少有适用的时候。当你想要编写一个包含显示标签域类时,应该考虑一下,这个标签是否可以被取消,这个类是否可以用类层次来代替。当你遇到一个包含标签与的现有类时,就要考虑将它重构到一个层次结构中去。