如何在工厂方法中初始化具有多个参数构造函数的类

Question

假设我有一个计算形状面积的 Shape 接口。我添加了 2 个实现矩形和正方形。我看到的挑战是两种实现都有自己的多参数构造函数。如何使用工厂模式初始化它们。我想用java来解决这个问题。public class Rectangle implements Shape {int length;int breadth;public Rectangle(List<String> parameters) {    this.length = Integer.parseInt(parameters.get(0));    this.breadth = Integer.parseInt(parameters.get(1));}@Overridepublic int area() {    return length * breadth;}}public class Square implements Shape {int edge;public Square(List<String> parameters) {    this.edge = Integer.parseInt(parameters.get(0));}@Overridepublic int area() {    return edge * edge;}}public interface Shape {int area();}public interface ShapeFactory {public Shape make(String shapeType);public List<String> getParameters(String shapeType);}public class ShapeFactoryImpl implements ShapeFactory {Map<String, List<String>> shapeInitMap = new HashMap<>();public void init(){    shapeInitMap.put("Circle", Arrays.asList(new String[]{"4"}));    shapeInitMap.put("Rectangle", Arrays.asList(new String[]{"2","3"}));}@Overridepublic Shape make(String shapeType) {    switch (shapeType) {    case "Circle":        return new Square(getParameters(shapeType));    case "Square":        return new Rectangle(getParameters(shapeType));    default:        break;    }    return null;}@Overridepublic List<String> getParameters(String shapeType) {    return shapeInitMap.get(shapeType);}}

Answer 1

您的解决方案不是最佳的，因为：1）您必须为具体的构造函数创建专用的构造函数Shape，并且您失去了参数的类型检查（在编译时）。2）init具体工厂的方法容易出错。

这就是我要做的。具体工厂应该携带具体构造函数的参数Shape，但不能作为不确定的字符串（如果从用户输入获取字符串，请在创建具体工厂之前转换它们）：

public interface ShapeFactory {

    public Shape make(String shapeType);

}

public class ShapeFactoryImpl implements ShapeFactory {

    private int circleRadius;

    private int rectangleLength;

    private int rectangleBreadth;

    public ShapeFactoryImpl(int circleRadius, int rectangleLength, int rectangleBreadth){

        this.circleRadius = circleRadius;

        this.rectangleLength = rectangleLength;

        this.rectangleBreadth = rectangleBreadth;

    }

    public Shape make(String shapeType) {

        switch (shapeType) {

            case "Circle": return new Circle(this.circleRadius); 

            case "Rectangle": return new Rectangle(this.rectangleLength, this.rectangleBreadth);

            default: throw new Exception("..."); 

        }

    }

}

客户不需要知道ShapeFactory他正在使用的混凝土，也不必担心Shape他得到的混凝土。依赖关系是相反的：发挥关键作用的是抽象，而不是细节。但如果可能的形状数量增加，您将得到一个具有许多相似参数的构造函数。这是另一个解决方案：

public class ShapeFactoryImpl implements ShapeFactory {

    private Shape circle;

    private Shape rectangle;

    public ShapeFactoryImpl(Circle circle, Rectangle rectangle){

        this.circle = circle;

        this.rectangle = rectangle;

    }

    public Shape make(String shapeType) {

        switch (shapeType) {

            case "Circle": return this.circle.clone(); 

            case "Rectangle": return this.rectangle.clone();

            default: throw new Exception("..."); 

        }

    }

}

这更好，因为您不会混合参数：每种混凝土都Shape包含自己的参数。如果你想让它更灵活，你可以使用 Map 将交换机的责任移出具体工厂：

public class ShapeFactoryImpl implements ShapeFactory {

    private Map<String, Shape> shapeByType;

    public ShapeFactoryImpl(Map<String, Shape> shapeByType){

        this.shapeByType = shapeByType;

    }

    public Shape make(String shapeType) {

        Shape shape = this.shapeByType.get(Type).clone();

        if (shape == null) {

            throw new Exception("...");

        }

        return shape;

    }

}

我什至会使用 anenum来代替形状类型而不是字符串，并使用 anEnumMap来处理开关：

public class ShapeFactoryImpl implements ShapeFactory {

    private EnumMap<ShapeType, Shape> shapeByType;

    public ShapeFactoryImpl(Map<ShapeType, Shape> shapeByType){

        this.shapeByType = shapeByType;

    }

    public Shape make(ShapeType shapeType) {

        return this.shapeByType.get(Type).clone();

    }

}

客户端必须知道Shape接口ShapeFactory和ShapeType枚举。“服务器”提供具体ShapeFactoryImpl实例。