Java Jframe / Graphics驱动程序

Question

当我想绘制polygon Pizza对象时，我会继续获得nullpointerexception。我收到此错误消息：

Exception in thread "main" java.lang.NullPointerException
    at Pizza.<init>(Pizza.java:9)
    at PolyDemo$PolyDemoPanel.getRandShape(PolyDemo.java:91)
    at PolyDemo$PolyDemoPanel.<init>(PolyDemo.java:54)
    at PolyDemo.<init>(PolyDemo.java:19)
    at PolyDemo.main(PolyDemo.java:28)

我在圆圈和矩形上没有问题，为什么这不起作用？这是我的披萨课：

    import java.awt.*;
    public class Pizza extends Shape{
        private Polygon P;
        public Pizza(int x, int y) {
            super(x,y);
            P.xpoints = new int[]{x, x+100, x+200};
            P.ypoints = new int[]{y, y+100, y};
            P.npoints = 3;
        }
        @Override
        public void draw(Graphics g){
            g.setColor(Color.RED);
            g.drawPolygon(P);
        }
    }

这是驱动程序：

    import java.util.*;
    import java.awt.*;
    import javax.swing.*;
    class PolyDemo extends JFrame {
        public PolyDemo() {
            getContentPane().add( new PolyDemoPanel());
            setSize( 300,300 );
            setVisible( true );
            setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        }
    public static void main( String args[] ) {
        PolyDemo myApp = new PolyDemo();
    }
    public class PolyDemoPanel extends JPanel {     
        Shape[] myShapes= new Shape[20];
        public PolyDemoPanel() {
            for( int i = 0; i < 20; i++ ) {
                myShapes[i] =  getRandShape();
            }
        }
        public void paint( Graphics g ) {
            super.paint(g); 
            for( int i = 0; i < myShapes.length; i++ ){
                myShapes[i].draw( g );  
            }   
        }
        public int getRandInt() {
            return ( (int) ( Math.random() * 200 ) );   
        }
        public Shape getRandShape() {
            Shape retVal = null;
            final int x = getRandInt();
            final int y = getRandInt();
                retVal = new Pizza(x, y);
                return retVal;
            }
        }   
    }

Answer 1

您声明poligon但不会创建对象。这样，当您在披萨的构造函数中使用它时，它是无效的。在构造函数中使用实例之前，您需要创建一个实例。p也是变量的坏名称

public Pizza(int x, int y) {
        super(x,y);
       //P is null here - add P=new Poligon()
        P.xpoints = new int[]{x, x+100, x+200};
        P.ypoints = new int[]{y, y+100, y};
        P.npoints = 3;
    }

Answer 2

您没有初始化Polygon字段P。尝试以下操作：

public Pizza(int x, int y) {
    super(x,y);
    P = new Polygon();
    P.xpoints = new int[]{x, x+100, x+200};
    P.ypoints = new int[]{y, y+100, y};
    P.npoints = 3;
}