java - ArrayList 的 IndexOutOfBoundException

Question

我的代码

public List<GeoPoint> compute_polygon(List<GeoPoint> waypoints, double d)
        {
            int N = waypoints.size();//25
            int T = 2 * N - 1;//49
//          GeoPoint[] reduced_waypoints = waypoints.toArray(new GeoPoint[waypoints.size()]);
            List<GeoPoint> polygon =  new ArrayList<GeoPoint>(T);
            for (int i = 1; i < N - 1; i++)
            {

                GeoPoint M = get_disp(waypoints.get(i - 1), waypoints.get(i), waypoints.get(i + 1), d);
                polygon.add(i, v_add(waypoints.get(i), M));// here it fails and throws exception. 
                polygon.add(T-i, v_sub(waypoints.get(i), M));
            }
}

错误输出

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 1, Size: 0
    at java.util.ArrayList.rangeCheck(ArrayList.java:653)
    at java.util.ArrayList.set(ArrayList.java:444)
    at de.carbook.vendor.search.PathPolygon.compute_polygon(PathPolygon.java:40)
    at de.carbook.vendor.search.Cleansing_the_waypoints.main(Cleansing_the_waypoints.java:41)

我无法找出导致问题的原因。

错误位于 polygon.add(i, v_add(waypoints.get(i), M));这里失败并抛出异常。

我试过polygon.add(index,element)和polygon.set(index,element)两种情况都失败了

在调试时我发现了这一行 List<GeoPoint> polygon = new ArrayList<GeoPoint>(T);初始化为 null(空)。有问题吗？

更新1

public List<GeoPoint> compute_polygon(List<GeoPoint> points, double d)
        {
            int N = points.size();//25
            int T = 2 * N - 1;//49
            GeoPoint[] waypoints = points.toArray(new GeoPoint[N]);
            GeoPoint[] polygon = new GeoPoint[T];
            for (int i = 1; i < N - 1; i++)
            {
                GeoPoint M = get_disp(waypoints[i - 1], waypoints[i], waypoints[i + 1], d);
                polygon[i]     = v_add(waypoints[i], M); 
                polygon[T - i] = v_sub(waypoints[i], M);
            }
            GeoPoint A = v_mul(v_nor(v_prp(v_sub(waypoints[1], waypoints[0]))), d);
            GeoPoint B = v_mul(v_nor(v_prp(v_sub(waypoints[N-1], waypoints[N-2]))), d);
            polygon[0] = v_add(waypoints[0], A); polygon[T] = v_sub(waypoints[0], A);
            polygon[N - 1] = v_add(waypoints[N-1], B); polygon[N] = v_sub(waypoints[N-1], B);

            List<GeoPoint> poly =  new ArrayList<GeoPoint>(Arrays.asList(polygon));
            System.out.println(poly);

            return poly;
        }

更新错误

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 49
    at de.carbook.vendor.search.PathPolygon.compute_polygon(PathPolygon.java:67)
    at de.carbook.vendor.search.Cleansing_the_waypoints.main(Cleansing_the_waypoints.java:41)

Answer 1

这是因为您试图在一个不存在的特定索引处插入。 List<GeoPoint> polygon = new ArrayList<GeoPoint>(T);只是告诉它应该在内存中占据这么大的大小。如果不提及容量，则随着列表的增长，内部数组将不得不重复重新分配。

使用这个:

polygon.add( v_add(waypoints.get(i), M))