MouseInfo mouse = Greenfoot.getMouseInfo(); int x = mouse.getX(); Still Wrong

Greenfoot.getMouseInfo() can return null. When it does, your 'mouse' variable will contain null, and then mouse.getX() fails with a NullPointerException, because you have no 'mouse' object. In general, you should call getMouseInfo after a mouse event, such as mouseClicked.

Thank you! Now I undersdand! Another question about mouse event, how can I find out if the mouse is hold on sth. I tried to figgure it out by getX() and getY(),is there some good iead?Thanks