如何在运行时根据构造函数参数在超类中创建子类对象（在Java中）

Question

I have the following classes and structure (simplified version):

public abstract class AbstractGeoObj {
    private Point position;
    ...
    public abstract calcArea();
    public abstract calcPerimeter();
    ...
    some getter
    ...
    some setter
    ...
}

public class Polygon extends AbstractGeoObj implements InterfaceMove {

    private LinkedList<Point> edges;

    public Polygon(LinkedList<Point> points) {
        //here i want to check the conditions and create the right Object
        //but i think this is the wrong way to do it
    }
    ...

    private boolean isSquare(List<Points> points) { ... }

    private boolean isRectangle(List<Points> points) { ... }

    private boolean isRotated(List<Points> points) { ... }

}

public class Rectangle extends Polygon implements InterfaceMove {

    public Rectangle(Point a, Point b, Point c, Point d) {}
    public Rectangle(Point[] points) { this(...) }
    public Rectangle(LinkedList<Piont> points) { this(...) }

    ...
}

public class Square extends Polygon implements InterfaceMove {

    public Square(Point a, Point b, Point c, Point d) {}
    public Square(Point[] points) { this(...) }
    public Square(LinkedList<Piont> points) { this(...) }

    ...
}

Now the problem is, i need to create either a Polygon-Object, Rectangle-Object or Square-Object, depending on the constructor parameters and the results of the methods isSquare(), isRectangle() and isRotated() at Runtime, the program should choose automatically which Object should be created. For example, if 4 given Points result in isSquare = true and isRotated() = false, i want to create Square Object, if isRotated() = true i will allways create Polygon-Object.

I researched about Builder Pattern and Factory Pattern, but i dont understand it so i could'nt implement it for my problem and i dont know if there is a better Solution for me. Some advice or hints to the right direction and maybe with examples may help me a lot. I hope, you understood my problem.

I know that the constructors of Rectangle and Square are basically the same, but thats not topic here, i will solve it later. ;)

And here is a UML Diagram to show you the current structure, its in German so i translated the important parts for you. (its not final, i may change it): UML Diagram PNG

Thanks for your help in advance.

Answer 1

It is wrong, I think using a Factory would be the best way ( https://en.wikipedia.org/wiki/Factory_method_pattern ) that way you go a more oop approach, since the constructors role is not to determin what kind of object to create, its role is to create the object it is supposed to construct.

Create a class:

class ShapeFactory{
    public static SuperClass getShape(params...){
        if(cond1){return new WhateverSubClass1;}
        else if(cond2){return new WhateverSubClass2;}
        ... (etc for all subclass cases)
    }
}

EDIT: remote reference: http://www.tutorialspoint.com/design_pattern/abstract_factory_pattern.htm

Answer 2

As I understand you have basically always a Polygon Object, so you could use a factory which creates for you a Polygon Object. To check which type you have to use, you could maybe make the methods (isSquare, isRectangle and isRotated) static in the Polygon class. Then the factory uses the concrete implementation of Polygon to create the Polygon Object:

class PolygonFactory(List<Point> points)
{

  public static Polygon createPolygon(List<Point> points)
  {
    if(Polygon.isSquare(points)
      return new Square(points);
    if(Polygon.isRectangle(points)
      return new Rectangle(points);

    return new Polygon(points);
  }
}