从抽象类调用时的Java NullPointerException

Question

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 {        
    public Ellipse() {
    }

    public double calcArea() {
        return 0.0;
    }

    public boolean draw() {
        return true;
    }
}

你可能想看点:

public class Point {
    public double locationX;
    public double locationY;

    public Point() {
        locationX = 0.0;
        locationY = 0.0;
    }
}

最后主要功能:

public class MakeShapes {
    public static void main(String []args) {
        Ellipse myShapes = new Ellipse();
        myShapes.setLocation( 100.0, 100.0 );
    }
}

一旦我使用setLocation(),我就会获得NPE.有什么想法吗？试图解决这个问题,我的大脑很痛.谢谢!!!

Answer 1

这里的问题是你的Shape构造函数创建了一个Point被调用的本地引用,center并启动了一个而不是初始化字段(并且你有同样的问题color).试试这样:

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

    public Shape() {
        color = new Color(); //changed to intialize the field
        center = new Point(); //changed to intialize the field
        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();
}

Answer 2

这是一个微妙的错误。您正在创建一个局部center变量，但没有将其分配给this.center

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

color将和声明更改center为this.

this.center = new Point()

最终this.center从未真正定义过，因此是 NPE。