java - 抽象类需要访问子类属性

Question

我的代码有问题(编辑:这些类的全部代码)

public abstract class SimplePolygon implements Polygon {

  //protected Vertex2D[] varray; //this is wrong in tests
  public double getWidth(){
    double min = varray[0].getX(), max = varray[0].getX();

    for(int i = 0;i<varray.length;i++){
        max = Math.max(max,varray[i].getX());
        min = Math.min(min,varray[i].getX());
    }
    return max - min;
  }

  public double getHeight(){
    double min = varray[0].getY(), max = varray[0].getY();

    for(int i = 0;i<varray.length;i++){
        max = Math.max(max,varray[i].getY());
        min = Math.min(min,varray[i].getY());
    }
    return max - min;
  }

  public double getLength(){
    double distance = 0;
    for(int i = 0;i<varray.length;i++){
        if((i+1)<varray.length){distance += varray[i].distance(varray[i+1]);}
        else{distance += varray[i].distance(varray[0]);}
    }

    return distance;
  }

  public double getArea(){
    double suma = 0;
    for(int i = 0;i<varray.length-1;i++){
        suma += varray[i].getX()*varray[i+1].getY() - varray[i+1].getX()*varray[i].getY();
    }
    return suma/2;
  }

  public String toString(){
    String str = "Polygon: vertices ="; 
    for(int i = 0;i<varray.length;i++){
        str += " ";
        str += varray[i];
    }
    return str;
  }
}

public class ArrayPolygon extends SimplePolygon {

  public ArrayPolygon(Vertex2D[] array){
    varray = new Vertex2D[array.length];
    if (array == null){}
    for(int i = 0;i<array.length;i++){
        if (array[i] == null){}
        varray[i] = array[i];
    }
  }

  public Vertex2D getVertex(int index) throws IllegalArgumentException{
    return varray[index];
  }

  public int getNumVertices(){
    return varray.length;
  }

}

问题是，我不允许向抽象类 SimplePolygon 添加任何属性或方法，因此我无法正确初始化 varray。它可以简单地用那个类中的 protected attrib 来解决，但是由于某些(愚蠢的)原因我不能那样做。有没有人知道如何在没有它的情况下解决它？感谢所有帮助。

Answer 1

想想:

• 你的 Polygon 接口(interface)作为 java.util.List 接口(interface)
• 您的 SimplePoygon 抽象类作为 java.util.AbstractCollection
• 您的 ArrayPolygon 具体类为 java.util.ArrayList

我认为你的任务的重点是用迭代器实现一个解决方案，这样你就可以在抽象类中实现关于多边形的通用方法，同时在具体类中隐藏包含数据点的实际数据结构；所以:

/* 
 *   This class implements generic methods about polygon, like size, area and 
 *   so on, leveraging on the Vertex2D iterator.
 */
public abstract class SimplePolygon implements Polygon {

    // all concrete subclasses must implement iterator
    public abstract Iterator<Vertex2D> iterator();

    // this generic method prints the vertex list
    // using iterator hides the data structure used in implementation
    public String toString() {
       Iterator<Vertex2D> it = iterator();
       if (! it.hasNext())
           return "[]";

       StringBuilder sb = new StringBuilder();
       sb.append('Polygon: vertices = [');
       for (;;) {
           Vertex2D e = it.next();
           if (! it.hasNext())
               return sb.append("]").toString();
           sb.append(' ');
       }
    }
}

// for instance this concrete classes uses an ArrayList
public ArrayPolygon extends SimplePolygon {

    public Iterator<Vertex2D> iterator() {
       return new .....
    }
}