阅读X，从整数数组

Question

我想从整数数组 读取X，Y值y的值，我的代码：

class Point 
{ 
    int x, y; 
} 

/** 
* 
* @author ADMIN 
*/ 
public class Jarvis { 
    private static Point[] points; 

    private static boolean CCW(Point p, Point q, Point r) 
    { 
     int val = (q.y - p.y) * (r.x - q.x) - (q.x - p.x) * (r.y - q.y); 

     if (val >= 0) 
      return false; 
     return true; 
    } 

    public static void convexHull(Point[] points) 
    { 
     int n = points.length; 
     /** if less than 3 points return **/   
     if (n < 3) 
      return;  
     int[] next = new int[n]; 
     Arrays.fill(next, -1); 

     /** find the leftmost point **/ 
     int leftMost = 0; 
     for (int i = 1; i < n; i++) { 
      if (points[i].x < points[leftMost].x) { 
       leftMost = i; 
      } 
     } 
     int p = leftMost, q; 
     /** iterate till p becomes leftMost **/ 
     do 
     { 
      /** wrapping **/ 
      q = (p + 1) % n; 
      for (int i = 0; i < n; i++) { 
       if (CCW(points[p], points[i], points[q])) 
        q = i; 
      } 

      next[p] = q; 
      p = q; 
     } while (p != leftMost); 

     /** Display result **/ 
     display(points, next); 
    } 

    public static void display(Point[] points, int[] next) 
    { 
     System.out.println("\nJarvis March Convex Hull points : "); 
     for (int i = 0; i < next.length; i++) 
     { 
      if (next[i] != -1) 
      { 
       System.out.println("("+ points[i].x +", "+ points[i].y +")"); 
      } 
     } 
    } 

    public static void main(String[] args) throws IOException { 
     int[] data=readfiles("nani.txt"); 
     System.out.println(Arrays.toString(data)); 
    } 

    public static int[] readfiles(String file) 
    { 

    try { 
     File f = new File(file); 
     Scanner scan = new Scanner(f); 
     int ctr = 0; 

     while (scan.hasNextInt()) { 
      ctr++; 
      System.out.println(scan.nextInt()); 
     } 
     System.out.println("Jarvis Algorithm Test\n"); 
     int[] arr = new int[ctr]; 
     System.out.println(ctr); 
     scan.useDelimiter(",|\\s*"); 

     /** Make an object of Jarvis class **/ 
     int n = ctr; 
     for (int i = 0; i < n; i++) { 

      System.out.println(n); 
      Point[] points = new Point[n]; 

      System.out.println("Reading X,Y Values From File"); 

      points[i] = new Point(); 
      points[i].x = arr[i]; 
      points[i].y = arr[i + 1]; 
      // arr[i]++; 
      i++; 
      System.out.println("(x,y) values are:" + points[i].x + "\t" + points[i].y); 
     } 

O/P部分：这是我得到

输出

运行：

1 
2 
6 
4 
8 
7 
2 
3 
12 
13 
Exception in thread "main" java.lang.NullPointerException 
9 
1 
Jarvis Algorithm Test 
12 

阅读X，Y值从文件//错误我在jarvis.Jarvis.readfiles（Jarvis.java:118）获得 在jarvis.Jarvis.main（Jarvis.java:80） Java结果：1 BUILD SUCCESSFUL（总时间：0秒）

Answer 1

你想从中读取数据的数组未初始化这就是为什么你会得到一个空指针异常。如果您不知道将要保存阵列的元素的数量，则使用ArrayList将会很合适。这是你的readFiles方法应该是什么样子：

public static int[] readfiles(String file) 
{ 

try { 
    File f = new File(file); 
    Scanner scan = new Scanner(f); 
    List<Integer> arr = new ArrayList<>(); 
    int crr = 0; 

    while(scan.hasNextInt()) 
    { 
     arr.add(scan.nextInt()); 
     System.out.println(arr.get(crr++)); 
    } 

    System.out.println("Jarvis Algorithm Test\n"); 

    System.out.println(ctr); 
    scan.useDelimiter(",|\\s*"); 

    /** Make an object of Jarvis class **/ 
    int n = ctr; 
    for (int i = 0; i < n; i++) { 

     System.out.println(n); 
     Point[] points = new Point[n]; 

     System.out.println("Reading X,Y Values From File"); 

     points[i] = new Point(); 
     points[i].x = arr.get(i); 
     points[i].y = arr.get(i + 1); 

     i++; 
     System.out.println("(x,y) values are:" + points[i].x + "\t" + points[i].y); 
    } 

如果你想一个ArrayList转换成数组：

TypeOfTheArray array = new TypeOfTheArray [arrayList.size()]; 

for(int i = 0; i < array.length; i++) 
    array[i] = arrayList.get(i); 

// It's important to clean up the trash :) 
arrayList.clear();