Nullpointer accessing public int

[victoryo]

Hi all,

I got a class with a public int.
In my other class i want to acces that public int to lower it.
I already made a new instance of the class with the public int from an other class.
So what i want to do is acces and lower it without making a new instance.

what i do is like this.

private Throw throwDart;

throwDart.dartNumber -= 1;

because the class name is Throw. But i got a null pointer exceptoin! :(
Please help me!

[KevinWorkman]

What is the value of throwDart when you call throwDart.dartNumber()? In other words, when do you initialize that variable?

[victoryo]

throwDart.dartNumber -= 1; happens when you undo a throw.
So in the begin it's 0 but than it't not possible to undo the turn.
So when it's possible to undo a turn the value = 1 or after 2, 3 turns it's 2 or 3.

[KevinWorkman]

You aren't really answering my question. You're talking about dartNumber, which I don't think really matters here.

What is the value of throwDart? If you don't know, use a debugger (or at least some print statements) to figure it out.

[victoryo]

throwDart isnt a value. Its an other name to load the class throw.
if i do throwDart = new Throw(); the nullpointer is gone. But i dont want to make a new Throw...

[KevinWorkman]

throwDart isnt a value. Its an other name to load the class throw.
if i do throwDart = new Throw(); the nullpointer is gone. But i dont want to make a new Throw...

Actually, throwDart is a variable that can hold a reference. If you try to dereference it without initializing it first, you'll get a NPE.

Why do you not want to make a new Throw? It sounds to me like you're pretty confused about the basics of how Objects work. I'd recommend you take another read through the basic tutorials.

[victoryo]

The reason why i dont want to make a new one.
is because an other class already did that. So if i do it again
all values will be reset to the begin state.

[KevinWorkman]

The reason why i dont want to make a new one.
is because an other class already did that. So if i do it again
all values will be reset to the begin state.

I'm really not sure what you mean. Can't you just pass the reference to the original instance then?

[Tolls]

If you already have a throwDart object, and that is the one you want to decrement the dartNumber on, then you'll have to make that one available to this bit of code, either by passing it in or by increasing the scope of the variable.

Java does not magically know that throwDart declared here is the same as the throwDart declared somewhere else. Just because they have the same name (to you).

[victoryo]

Thanks for the feedback. I solved it by giving the constructor of the class a Parameter of throwDart an save it like.
this.throwDart = throwDart.

Now it's also clear to me how it works! =D