尝试绘制多边形时发生NullPointerException

Question

When I'm trying to draw a polygon I get a NullPointerException . This is how I am trying to do it:

Roi roi = imp.getRoi();
Polygon p = roi.getPolygon();
Graphics g=null;
g.drawPolygon(p.xpoints, p.ypoints, p.npoints);

Please suggest how I can fix this.

Answer 1

You're not using the right Graphics object. This should be done in a JComponent's paintComponent(...) method, and you should use the Graphics object passed in by the JVM. Either that or draw this in a BufferedImage using a Graphics2D object obtained from it via createGraphics() .

Regarding your edit: yikes!

This is guaranteed to throw a NPE every time.

Bar bar = null
bar.someMethod();