java - Point[] 数组在尝试设置变量时抛出 NullPointerException

Question

嗯。我什么都试过了，我一直在整个互联网上寻找答案，包括这个网站，但是......一点运气都没有。

这是我的代码:

package com.exercises.algorithms;
import java.awt.*;


public class Points2DN {
    private Point[] points;
    protected int[] closestPoints = new int[2];
    private double distance;

Points2DN(int n) {
    this.points = new Point[n];
    for (int i = 0; i < n; i++) {
        this.points[i].x =  (int) Math.floor(Math.random()*100);
        this.points[i].y =  (int) Math.floor(Math.random()*100);
        System.out.println("Test: " + points[i].x + "," + points[i].y + "\n");
        System.out.println("OK");
    }
    this.distances(this);
}


public static void imprimePuntos(Point[] arrayPuntos) {
    for (int i = 0; i < arrayPuntos.length; i++) {
        System.out.println("Punto " +(i+1) + ": " + "( "
                + arrayPuntos[i].x + " , "
                + arrayPuntos[i].y + " )");
    }
}

public static void printClosest(Points2DN puntos) {
    System.out.println("\nLos puntos más cercanos son: ");
    System.out.println("Puntos: \t" 
            + (puntos.points[puntos.closestPoints[0]]).toString() + " "
            + (puntos.points[puntos.closestPoints[1]]).toString());
    System.out.println("Distancia: " + puntos.distance);


}

private void distances(Points2DN puntos) {
    for (int i = 0; i < puntos.points.length; i++) {
            // Iterative loop for the other points to be check with
        for (int j = (i+1); j < this.points.length; j++) {
            if (i == 0 && j == 1) {
                int p = puntos.closestPoints[0] = i;
                int q = puntos.closestPoints[1] = j;
                puntos.distance = (puntos.points[p]).distance(puntos.points[q]);
                continue;
            }
            double distancePair = puntos.points[i].distance(puntos.points[j]);
            if (distancePair < puntos.distance){
                puntos.closestPoints[0] = i;
                puntos.closestPoints[1] = j;
                puntos.distance = distancePair;
            }
        }
    }
}


public static void main(String[] args) {
    try {
        int numberOfPoints = Integer.parseInt(args[0]);
        if (args.length == 1) {
            System.out.println("Calculando...");
            Points2DN puntos = new Points2DN(numberOfPoints);
            imprimePuntos(puntos.points);
            printClosest(puntos);
        }

        else {
            // Nothing
        }

    } catch (ArrayIndexOutOfBoundsException e) {
        System.out.println("Not a valid number of points");
    } catch (Exception e) {
        System.out.println(e.toString());
    }

}

问题发生在下面片段的最后一行，就在尝试使用数组“points”中的一个 Point 对象之后。它们应在 (x,y) 上填充“int”类型值 (0,0)，并进行初始化。我已经检查了尺寸:

 points.length

结果是我在 IDE 上设置为运行项目的参数的数字。例如，如果“n”是 10，则该值是 10。没关系，所以...为什么这该死的不起作用!？？

 Points2DN(int n) {
    this.points = new Point[n];
    for (int i = 0; i < n; i++) {
        this.points[i].x =  (int) Math.floor(Math.random()*100);

Answer 1

new Point[n] 创建一个 Point 数组，但它不会创建任何实际的 Point 对象放在其中大批。这就像购买一个可以存放书籍的盒子 - 在您将它们放在那里之前，它实际上不会有任何书籍。

您需要为每个要使用的索引元素创建一个Point，例如:

for (int i = 0; i < n; i++) {
    points[i] = new Point();
    points[i].x =  (int) Math.floor(Math.random()*100);
    ...