Thread: NullPointerException

Hello.
Why is the NullPointerException generated in the following code snippet at run-time?

  		class Info{			
			public String name;
			public String version;
			public String arch;
			double CPUSpeed;
		};
		Info info =  new Info();
		Info[] queue = new Info[100];
 
		int pos = 0;
 
				queue[pos].name = System.getProperty("os.name");
				System.out.println( queue[pos].name);

(the last two lines are problematic)

Define 'problematic' - in Java terms. Post your errors in their entirety, copied from exactly as they appear at your end.

Java statements apart from variable declarations and initializations must occur within a method. I don't see a method in what you've posted. And there are other syntactic problems with what you've posted.

What are you trying to do?

Originally Posted by PhHein

Info[] queue = new Info[100]; // Array that CAN hold 100 Info references
int pos = 0;
queue[pos].name = System.getProperty("os.name"); // First slot of the array was never filled with an Info reference

What shall I do?

Originally Posted by Ada Lovelace

Just a bit of info in-case your are unsure of what the array-pointer thing is

There is a very close connection between pointers and arrays. In fact they
are more or less one and the same thing. When you declare an array as:



You in fact declaring a pointer a to the first element in the array. That is, a is exactly the same as &a[0].
The only difference between a and a pointer variable is that the array name is a constant pointer
- you cannot change the location it points at. When you write an expression such as a[i]
this is converted into a pointer expression that gives the value of the appropriate element.

Wishes Ada xx

But I wrote

Info[] queue = new Info[100]

madam.
Doesn't that create an anonymous pointer and assign queue to refer to it?

Do I instead write Info queue[] = new Info[100]?

Thanks
I just needed to new each element separately