java - 为什么我无法使用 (Point2d.Double) 键入强制转换？ - java

Question

我有以下代码片段 - 不起作用。

private void startAddingTrackPointValue()
    {
        Set<Point2D.Double> keySet = this.trackPointList.keySet();
        Point2D.Double[] keys = (Point2D.Double[]) keySet.toArray();
        for(int i = 0; i < keys.length; i++)
        {
            System.out.println(keys[i]);
        }
    }

产生异常的是，这...

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.awt.geom.Point2D$Double;
    at SegmentFactory.startAddingTrackPointValue(SegmentFactory.java:170)
    at SegmentFactory.<init>(SegmentFactory.java:44)
    at TestClass.main(TestClass.java:8)

但这工作正常，如果我这样做，

private void startAddingTrackPointValue()
    {
        Set<Point2D.Double> keySet = this.trackPointList.keySet();
        Object[] keys = (Object[]) keySet.toArray();
        for(int i = 0; i < keys.length; i++)
        {
            System.out.println(keys[i]);
        }
    }

问题是，为什么我不能使用 Point2D.Double[] 进行类型转换？

Answer 1

因为“Object[]”不是“Point2D.Double[]”的实例。

这些都不起作用:

Point2D.Double[] = (Point2D.Double[])new Object[0]; // this is essentially what your code is doing
Point2D.Double = (Point2D.Double)new Object();

改为这样做:

Point2D.Double[] keys = (Point2D.Double[]) keySet.toArray(new Point2D.Double[keySet.size()]);