閱讀X，從整數數組

Question

我想從整數數組 讀取X，Y值y的值，我的代碼：

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：這是我得到

輸出

運行：

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

閱讀X，Y值從文件//錯誤我在jarvis.Jarvis.readfiles（Jarvis.java:118）獲得 在jarvis.Jarvis.main（Jarvis.java:80） Java結果：1 BUILD SUCCESSFUL（總時間：0秒）

Answer 1

你想從中讀取數據的數組未初始化這就是爲什麼你會得到一個空指針異常。如果您不知道將要保存陣列的元素的數量，則使用ArrayList將會很合適。這是你的readFiles方法應該是什麼樣子：

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); 
    } 

如果你想一個ArrayList轉換成數組：

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();