使用重载方法动态调度(运行时多态性)而不使用 instanceof [英] Dynamic dispatch (runtime-polymorphism) with overloaded methods without using instanceof

问题描述

我想将Arc 和Line 的对象保存在一个ArrayList 中，然后得到两者的交集.问题是如何将 i 和 j 转换为原始类.我知道 instanceof 有效，但这将是最脏的方法.

I want to save Objects of Arc and Line in one ArrayList, then get the intersection of both. The question is how can I cast i and j to its original class. I know that instanceof works but that would be the dirtiest method.

public class Intersection {
    public static boolean intersect(ArrayList<Curve> list1, ArrayList<Curve> list2) {
        for (Curve i : list1) {
            for (Curve j : list2) {
                if (i.intersection(j).length > 0) 
                    return true;
            }
        }
        return false;
    }
}

public abstract class Curve {
    public Point[] intersection(Curve c) {
        return new Point[] {};
    }
}

public class Line extends Curve {
    public Point[] intersection(Line l) {
        // returns intersection Point of this and l
    }

    public Point[] intersection(Arc a) {
        // returns intersection Point(s)
    }
}

public class Arc extends Curve {
    public Point[] intersection(Line l) {
        // return intersection Point(s) of this and l
    }

    public Point[] intersection(Arc a) {
        // returns intersection Point(s)
    }
}

感谢您的帮助！

推荐答案

由于每个子类已经知道其他子类(例如，Arc 必须知道 Arccode>Line 类来实现Arc 和Line 相交)，使用instanceof 没有错.

Since each sub-class already has to know about the other sub-classes (for example, Arc must be aware of the Line class in order to implement Arc and Line intersection), there is nothing wrong with using instanceof.

在每个子类中，您可以覆盖基类的public Point[]intersection(Curve c)方法并将实现分派给重载方法之一.

In each sub-class you can override the base class's public Point[] intersection(Curve c) method and dispatch the implementation to one of the overloaded methods.

例如:

public class Arc extends Curve {    
    @Override
    public Point[] intersection(Curve c) {
        if (c instanceof Line)
            return instersection ((Line) c);
        else if (c instanceof Arc)
            return intersection ((Arc) c);
        else
            return an empty array or null, or throw some exception
    }

    public Point[] intersection(Line l) {
        // return intersection Point(s) of this and l
    }

    public Point[] intersection(Arc a) {
        // returns intersection Point(s)
    }
}

这样您就不必更改 public static boolean intersect(ArrayList list1, ArrayList list2) 方法中的任何内容.

This way you don't have to change anything in your public static boolean intersect(ArrayList<Curve> list1, ArrayList<Curve> list2) method.

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