小编Sco*_*ott的帖子

我试图简单地在Java中扩展一个抽象类,并调用存储在其中的一些方法.我这样做时不断收到NullPointerException.我在这里遗漏了一些关于抽象的东西吗？

这是父类:

public abstract class Shape {
    public Color color;
    public Point center;
    public double rotation;

    public Shape() {
        Color color = new Color();
        Point center = new Point();
        rotation = 0.0;
        System.out.println("shape created");
    }

    public void setLocation( Point p ) { center.locationX = p.locationX; center.locationY = p.locationY; }
    public void setLocation( double x, double y ) { center.locationX = x; center.locationY = y; }

    public abstract double calcArea();
    public abstract boolean draw();
}

和孩子班:

public class Ellipse extends Shape …