Question

当我尝试运行程序时，我在第8行得到一个空指针异常，（basePoints [i] .x3D =（int）（R Math.cos（i T））; ）。这在运行时i = 0时立即发生。我不明白我做错了什么我已经初始化了basePoints，（Point3D [] basePoints = new Point3D [50];）

有人可以帮帮我吗？

    Cone newCone = new Cone();
    final int H = newCone.HEIGHT;
    final int R = newCone.RADIUS;
    Point3D[] basePoints = new Point3D[50];         //50 points around the base of the cone
    final double T = 2*Math.PI/basePoints.length;

    for (int i = 0; i < basePoints.length; i++) {
        basePoints[i].x3D = (int) (R*Math.cos(i*T));
        basePoints[i].y3D = (int) (R*Math.sin(i*T));    
    }

Answer 1

我认为你的循环中的第一行应该是basePoints[i] = new Point3D();所以循环是：

for (int i = 0; i < basePoints.length; i++) {
    basePoints[i] = new Point3D();
    basePoints[i].x3D = (int) (R*Math.cos(i*T));
    basePoints[i].y3D = (int) (R*Math.sin(i*T));    
}

行Point3D[] basePoints = new Point3D[50];在内存中为50个Point3D实例保留空间，但它不会初始化它们。

Answer 2

您需要创建一个新的Point3D对象，并在为其赋值之前将其放入数组位置：

...

for (int i = 0; i < basePoints.length; i++) {
    basePoints[i] = new Point3D();
    basePoints[i].x3D = (int) (R*Math.cos(i*T));
    basePoints[i].y3D = (int) (R*Math.sin(i*T));    
}

...

初始化数组上的空指针异常

2 个答案: