The null message is driving me crazy

[Yakg]

Hey , I am writing a code at the moment that represents a polygon.
In the following method: addPolygon I'm trying to set a new point with the x and y cordinates to the polygon and place it in the available place, which hasn't got a point already.
If the placement was done successfully return true and if all places are taken return false.
I am getting a null compiler error.
Does anyone know where I went wrong?

class Polygon{

private Point _vertices [];
private int _noOfVertices = 0;
final int MAX = 10;


public Polygon (){
Point _vertices [] = new Point [MAX];
}

public boolean addVertex (int x,int y){
Point pnt = new Point (x,y);
for (_noOfVertices=0;_vertices [_noOfVertices] !=null && _noOfVertices<MAX; _noOfVertices++);
if (_vertices [_noOfVertices] ==null && _noOfVertices <MAX)
_vertices [_noOfVertices] = pnt;
if (_noOfVertices<MAX)
return true;
return false;
}

Thank you for your help.

[pbrockway2]

The compiler messages are generally very helpful. What is it?: copy and post the entire thing and indicate which line of your code it is referring to.

[Yakg]

java.lang.NullPointerException
at Polygon.addVertex(Polygon.java:14)
at Tester.main(Tester.java:6)

[RockoAndy]

create an instance of private _vertices it will be fine.

private Point _vertices []; is being referenced in the add method and it has no instance to it.

[Yakg]

now I'm trying to get a false return if the number of vertices is not less than the MAX variable, for some reason it still thinks of it as true...

[pbrockway2]

java.lang.NullPointerException
at Polygon.addVertex(Polygon.java:14)
at Tester.main(Tester.java:6)

Look closely at line 14 to try and see what is null when it shouldn't be.

You don't say, but I am guessing that line is

for (_noOfVertices=0;_vertices [_noOfVertices] !=null && _noOfVertices<MAX; _noOfVertices++);

The only candidate for being null here is the array _vertices. It might be a good idea to check (and if there were other things being dereferenced with the "dot" operator, then you would have to check)

System.out.println("About to start the for loop  _vertices=" + _vertices);
for (_noOfVertices=0;_vertices [_noOfVertices] !=null && _noOfVertices<MAX; _noOfVertices++);

If it turns out that _vertices is null then you have to ask yourself why. In particular where did you think you had initialised _vertices? And why did that not happen?