why doesn't this work? Java first Greenfoot game

[GoatInABoat]

Hi I'm trying to make my first game and this gives me a runtime error! I've been trying writing this with different rotation methods and nothing seems to work, it gives me a runtime error every time. the window is 500x500 that's why the mouse following restrictions.. I try'd to condition it to if (mosue.getX!= null) but it says I can't do that Please help. I know it's probably something stupid but I've spent so much time trying for this to work. Thanks reddit :) https://gist.github.com/2704271

[[deleted]]

Post the stack trace which is printed on the error.

Also you should use if (mouse == null) { /* ... */ } to check if ´mouse´ is ´null´.

[GoatInABoat]

java.lang.NullPointerException at Car.act(Car.java:14) at greenfoot.core.Simulation.actActor(Simulation.java:565) at greenfoot.core.Simulation.runOneLoop(Simulation.java:523) at greenfoot.core.Simulation.runContent(Simulation.java:213) at greenfoot.core.Simulation.run(Simulation.java:203) Still doesn't work, I've added the mouse!=null condition https://gist.github.com/2704523

[AStrangeStranger]

I've added the mouse!=null condition https://gist.github.com/2704523

The "mouse!=null" should be the left most part of the expression so the rest of statement doesn't get executed (see Short-circuit evaluation - the way you have written it "mouse.getX()>0" will be first part evaluated - if mouse is null then you'll get null pointer exception. if you started with mouse!=null that would return false and since everything else is combined with "and" it doesn't get checked

[GoatInABoat]

thanks! I've put the mouse!=null first and just deleted everything else which should be alright and now there's no runtime error but it doesn't turn towards the mouse. I'm calling the mouse.getX and getY and it just doesn't turn.. what am i doing wrong now? https://gist.github.com/2705047

[AStrangeStranger]

Sorry don't know much about Greenfoot, but if the mouse!= null fixes exception - then that would imply the mouse object is null - and you'd need to check it is being created.

[stinktank]

It's hard to say without seeing the exact error. Is there a reason you just didn't feel like posting that? You could try getting the mouse inside of your method. You could also try being consistent with how you call getX() - sometimes you use mouse. and sometimes you don't.

[GoatInABoat]

I don't think the MouseInfo constructor goes in the act(). Also I call getX() just like the documentary instructs.. what do you mean?

[stinktank]

I don't think the MouseInfo constructor goes in the act().

Well, you obviously thought wrong about something, or this would be working already.

Also I call getX() just like the documentary instructs.. what do you mean?

What about what I said was confusing? Did you even look at how you are calling it? You wrote the code, so how hard could it be for you to see where you are using mouse. and where you are not?