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放宽要求是什么意思？例如，在一个简单的方法中，它接收两个 Integer 并将它们添加到另一个 Integer 中，如何放宽该方法中的要求？

package client
import even.Even
import natural.Natural
    void f(Integer x,Integer y) {
         Integer z = add(x,y);
}

Even 和 Natural 都扩展了 Integer 类，如果相关的话。这三个类都有不同的 add() 方法，Integer 是唯一一个在客户端以方法 f() 的形式拥有它的类。

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1 回答 1

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够有趣的——我在上一篇文章中写过： “
多次调度”行为可以通过使用访问者模式在“单一调度”语言中实现（最初在“设计模式”一书中描述）。

从上面的链接中查看 Java 代码示例：

interface CarElementVisitor {
    void visit(Wheel wheel);
    void visit(Engine engine);
    void visit(Body body);
    void visit(Car car);
}

interface CarElement {
    void accept(CarElementVisitor visitor); // CarElements have to provide accept().
}

class Wheel implements CarElement {
    private String name;

    public Wheel(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }

    public void accept(CarElementVisitor visitor) {
        /*
         * accept(CarElementVisitor) in Wheel implements
         * accept(CarElementVisitor) in CarElement, so the call
         * to accept is bound at run time. This can be considered
         * the first dispatch. However, the decision to call
         * visit(Wheel) (as opposed to visit(Engine) etc.) can be
         * made during compile time since 'this' is known at compile
         * time to be a Wheel. Moreover, each implementation of
         * CarElementVisitor implements the visit(Wheel), which is
         * another decision that is made at run time. This can be
         * considered the second dispatch.
         */ 
        visitor.visit(this);
    }
}

class Engine implements CarElement {
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }
}

class Body implements CarElement {
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }
}

class Car implements CarElement {
    CarElement[] elements;

    public Car() {
        //create new Array of elements
        this.elements = new CarElement[] { new Wheel("front left"), 
            new Wheel("front right"), new Wheel("back left") , 
            new Wheel("back right"), new Body(), new Engine() };
    }

    public void accept(CarElementVisitor visitor) {     
        for(CarElement elem : elements) {
            elem.accept(visitor);
        }
        visitor.visit(this);    
    }
}

class CarElementPrintVisitor implements CarElementVisitor {
    public void visit(Wheel wheel) {      
        System.out.println("Visiting " + wheel.getName() + " wheel");
    }

    public void visit(Engine engine) {
        System.out.println("Visiting engine");
    }

    public void visit(Body body) {
        System.out.println("Visiting body");
    }

    public void visit(Car car) {      
        System.out.println("Visiting car");
    }
}

class CarElementDoVisitor implements CarElementVisitor {
    public void visit(Wheel wheel) {
        System.out.println("Kicking my " + wheel.getName() + " wheel");
    }

    public void visit(Engine engine) {
        System.out.println("Starting my engine");
    }

    public void visit(Body body) {
        System.out.println("Moving my body");
    }

    public void visit(Car car) {
        System.out.println("Starting my car");
    }
}

public class VisitorDemo {
    public static void main(String[] args) {
        CarElement car = new Car();
        car.accept(new CarElementPrintVisitor());
        car.accept(new CarElementDoVisitor());
    }
}
于 2013-10-23T22:11:55.047 回答