Question

我是Java初学者的初学者。我会做一个应用来计算最大点，最小点，宽度和高度。我在日食中写道。

我成了这个错误：

线程中的异常＆＃34; main＆＃34;显示java.lang.NullPointerException 在BoundingBox.height（BoundingBox.java:50）在App.main（App.java:39）`
我的打印点看起来像

点@ 659e0bfd 点@ 2a139a55 点@ 15db9742

在此代码中，toString方法应该如何？

点

public class Point {
    private double x;
    private double y;

    public Point(double xnew, double ynew) {
        x = xnew;
        y = ynew;
    }

    public double getX() {
        return x;
    }

    public void setX(double xnew) {
        x = xnew;
    }

    public double getY() {
        return y;
    }

    public void setY(double ynew) {
        y = ynew;
    }
}

的BoundingBox

import java.util.List;

public class BoundingBox {

    private static Point minPoint;
    private static Point maxPoint;

    public BoundingBox(List<Point> manyPoints) {
        double minX = manyPoints.get(0).getX();
        double maxX = manyPoints.get(0).getX();
        double minY = manyPoints.get(0).getY();
        double maxY = manyPoints.get(0).getY();

        for (int i = 0; i < manyPoints.size(); i++) {
            Point test = manyPoints.get(i);
            test.getX();

            if (test.getX() < minX) {
                minX = test.getX();

            }

            if (test.getX() > maxX) {
                maxX = test.getX();
            }

            if (test.getY() < minY) {
                minY = test.getY();

            }

            if (test.getY() > maxY) {
                maxY = test.getY();

            }

            minPoint = new Point(minX, minY);
            maxPoint = new Point(maxX, maxY);

        }
    }

    public static double width() {

        double a = (maxPoint.getX() - minPoint.getX());
        return a;
    }

    public static double height() {
        System.out.println("minPoint = " + minPoint);
        System.out.println("maxPoint = " + maxPoint);
        double b = (maxPoint.getY() - minPoint.getY());
        return b;
    }

    public Point getMaxPoint() {
        return maxPoint;
    }

    public Point getMinPoint() {
        return minPoint;
    }
}

应用

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class App {
    public static void main(String[] args) {
        Point p1 = new Point(10.681,-48.857);
        Point p2 = new Point(96.980,20.724);
        Point p3 = new Point(66.647,-66.558);
        Point p4 = new Point(-2.674,-58.571);
        Point p5 = new Point(40.11,-12.342);
        Point p6 = new Point(27.782,46.809);
        Point p7 = new Point(54.759,-46.709);
        Point p8 = new Point(-33.89,-90.787);
        Point p9 = new Point(15.84,-67.553);
        Point p10 = new Point(19.481,51.331);


        List<Point> list1 = new ArrayList<Point>(); // Create ArrayList and add Points
        list1.add(p1);
        list1.add(p2);
        list1.add(p3);
        list1.add(p4);
        list1.add(p5);
        list1.add(p6);
        list1.add(p7);
        list1.add(p8);
        list1.add(p9);
        list1.add(p10);

        Iterator<Point> itr = list1.iterator( );
        while ( itr.hasNext( ) ) {
            System.out.println( itr.next( ) );
        }

        double c = BoundingBox.height();
        double d = BoundingBox.width();

        System.out.println(" The Points: " + list1 );
        System.out.println(" BoundingBox have " + c + "height and" + d + "width." );
        System.out.println(" The max Point is " );
        System.out.println(" The min Point is " );
    }
}

Answer 1

在尝试调用height（）和width（）函数之前，您从未创建过BoundingBox类的实例，因此您在行上接收空指针异常

double a = (maxPoint.getX() - minPoint.getX());

当它试图调用maxPoint.getX（）和minPoint.getX（）时，因为变量中没有存储值

Answer 2

您的变量：

private static Point minPoint;
private static Point maxPoint;

在BoundingBox构造函数中初始化，但在您的App中，您不会调用它，因此它们是null。只需执行以下操作：

new BoundingBox(list1);
double c = BoundingBox.height();
double d = BoundingBox.width();

和NullPointerException将消失。

Java BoundingBox程序

2 个答案: