Thread: NullPointerException Error in Code that Prints Array Elements

I created the ArrayDriver and ArrayClass1 classes to gain some experiences working with arrays and array elements. The code in ArrayDriver is suppose to:
1) Create an array of ArrayClass1 objects.
2) Initialize the two attributes of each array element to 0 and print this information out.
3) Modify each array element's attribute and print out the modified array element's attributes.

I encountered a Nullpointerexception error while compiling the ArrayDriver class with Eclipse. I read that this error occurs when a pointer is dereferenced but has not been instantiated (java - What is a Null Pointer Exception? - Stack Overflow). I created an array and initialized every array element with the first for loop. Still, I cannot correct the problem.

public class ArrayDriver {
	public static void main(String[] args){
 
		int ARRAY_LENGTH = 10;
 
		ArrayClass1 [] alpha = new ArrayClass1[ARRAY_LENGTH];    //Declares and instantiates the ArrayClass1 array called alpha
 
		for(int i = ARRAY_LENGTH-1; i-1>=0; i--){   //Initializes the attributes of each alpha array element (both x and y) to 0
			alpha[i].x = 0;
			alpha[i].y = 0;
		}
 
 
		for(int i = ARRAY_LENGTH-1; i-1 >= 0; i--){   //Print out what is initialized
			System.out.println("Old Array["+i+"]x: "+alpha[i].x);
			System.out.println("Old Array["+i+"]y: "+alpha[i].y);
		}
        for(int i = ARRAY_LENGTH-1; i-1 >= 0; i--){   //Modifies each array element's attributes
        	alpha[i].x = i;
        	alpha[i].y = i+1;
        }
        for(int i = ARRAY_LENGTH-1; i-1 >= 0; i--){   //Prints out new attributes
        	System.out.println("New Array["+i+"]x: "+alpha[i].x);
        	System.out.println("New Array["+i+"]y: "+alpha[i].y);
        }
 
 
 
	}
}

Array Class:

public class ArrayClass1 {
 
	public int x = 0;
	public int y = 0;
 
	public ArrayClass1(int x, int y){
	 this.x = x;
	 this.y = y;
	}
 
}

The NullPointerExpression will occur when you run the program. (Just a nitpick, but it pays to be precise...)

Could you post the stack trace: the long message that you get at the console when an exception occurs? And indicate which line(s) of your code it is referring to?

Originally Posted by Atypically Engineered

I created the ArrayDriver and ArrayClass1 classes to gain some experiences working with arrays and array elements. The code in ArrayDriver is suppose to:
1) Create an array of ArrayClass1 objects.
2) Initialize the two attributes of each array element to 0 and print this information out.
3) Modify each array element's attribute and print out the modified array element's attributes.

I encountered a Nullpointerexception error while compiling the ArrayDriver class with Eclipse. I read that this error occurs when a pointer is dereferenced but has not been instantiated (java - What is a Null Pointer Exception? - Stack Overflow). I created an array and initialized every array element with the first for loop. Still, I cannot correct the problem.

public class ArrayDriver {
	public static void main(String[] args){
 
		int ARRAY_LENGTH = 10;
 
		ArrayClass1 [] alpha = new ArrayClass1[ARRAY_LENGTH];    //Declares and instantiates the ArrayClass1 array called alpha
 
		for(int i = ARRAY_LENGTH-1; i-1>=0; i--){   //Initializes the attributes of each alpha array element (both x and y) to 0
			alpha[i].x = 0;
			alpha[i].y = 0;
		}
 
 
		for(int i = ARRAY_LENGTH-1; i-1 >= 0; i--){   //Print out what is initialized
			System.out.println("Old Array["+i+"]x: "+alpha[i].x);
			System.out.println("Old Array["+i+"]y: "+alpha[i].y);
		}
        for(int i = ARRAY_LENGTH-1; i-1 >= 0; i--){   //Modifies each array element's attributes
        	alpha[i].x = i;
        	alpha[i].y = i+1;
        }
        for(int i = ARRAY_LENGTH-1; i-1 >= 0; i--){   //Prints out new attributes
        	System.out.println("New Array["+i+"]x: "+alpha[i].x);
        	System.out.println("New Array["+i+"]y: "+alpha[i].y);
        }
 
 
 
	}
}

Array Class:

public class ArrayClass1 {
 
	public int x = 0;
	public int y = 0;
 
	public ArrayClass1(int x, int y){
	 this.x = x;
	 this.y = y;
	}
 
}

Hello Atypically Engineered!
Your first for loop does not initialize your alpha array, you assign values to alpha[i].x and alpha[i].y. You should give values to alpha[i].
Here is a typical example for initializing an int array

int[] x = new int [10];
  for (int i = 0; i<x.length; i++ )
  x[i] = i;
  }

Nice link, by the way.

Near the bottom - and garnering zero votes - we read:

"In Java every things is in the form of class.

If you want to use any object then you have two phases

Declare
Initialization
Example:

Declaration: int a;
initialization: a=0;

Same for Array concept

Declaration: Item i[]=new Item[5];
Initialization: i[0]=new Item();

If you not given Initialization section then the NullpointerException arise."

It seems that I confused declaring and initializing the array with initializing the array elements.

Yes, exactly.

But if "alpha[i] = new ArrayClass1(0,0);" initializes each element, then why didn't I get an error when I ran...

Where would you expect the error? The point is you *did* initialise both alpha and each of the alpha[i] elements. Everything initialised == no NullPointerException.

Oddly, you did assign values to alpha[i] twice. We call the first assignment "initialisation", but that's just terminology. You are free to assign as many things as you like to alpha[i] - the old values are simply discarded. (which is why it is odd to make two such assignments).

Don't compilers fail to compile whenever you reinitialize the same declared pointer? Say:
Int x = 0;
Int x = 1;

Would give an error along the lines of "x is already defined. Is that because here "x" is being re-declared whereas

alpha[i] = new ArrayClass1(0,0);
alpha[i] = new ArrayClass1(1,1);

is analogous to an assignment after int x; :
x = 0;
x = 1;

Yes, you can't declare a variable twice.

But in the code you were asking about (#5) only one variable is declared (other than the loop variables): alpha. And it is declared just once.

The expression alpha[i] is not declared at all (expressions never are). And you are free to assign things to it as often as you like.