Question

我有以下代码：

if(prevPoint != null){
            Log.i("tracking class", "prevPoint:  "+ prevPoint);
          Point pointb = null;
          Projection projection2 = mapView.getProjection();
          projection2.toPixels(prevPoint, pointb);

          Log.i("tracking class", "Pointb:  "+ pointb);
               Paint paint;
                 paint = new Paint();
                 paint.setColor(Color.BLUE);
                 paint.setAntiAlias(true);
                 paint.setStyle(Style.STROKE);
                 paint.setStrokeWidth(2);
    Log.i("tracking class", "Point x:  "+ point.x);
    Log.i("tracking class", "Point y:  "+ point.y);
    Log.i("tracking class", "Pointb x:  "+ pointb.x);
    Log.i("tracking class", "Pointb y:  "+ pointb.y);


            //canvas.drawLine(pointb.x, pointb.y, point.x, point.y, paint);
        }

日志返回：

10-19 08:55:25.505: INFO/tracking class(29970): prevPoint:  41310173,-105558377
10-19 08:55:25.505: INFO/tracking class(29970): Pointb:  null

为什么Pointb会回来null？

Answer 1

您是否希望Java使用pass by reference？它没有。它在任何地方都使用pass by value，尽管引用类型的任何表达式的值都是引用。

调用

projection2.toPixels(prevPoint, pointb);

无法可能更改pointb的值。您需要使用赋值运算符，例如

Point pointb = projection2.toPixels(prevPoint, null);

我相信toPixels通过参数更改现有对象引用的的内容，如果参数为非null ...但它不可能更改用于引用新对象的变量。

Projection.toPixels空指针......帮忙？

1 个答案: