Array outOf Bound Problem

[amrmb09]

hi,
when i instantiate an object (group) from the class Group and pass the object as a parameter to the method"GenerateRandomQues" implemented in another class, i receive an error says: Null pointer Exception
To Note: Group [][]group = new Group[INDEX][LEVEL];// index=8 and level=2

can u please tell me where my problem is..

public static void main (String[] args)  throws FileNotFoundException, IOException
	  {   			    
	    Group [][]group = new Group[INDEX][LEVEL]; //[#of Question(index)] [level]

public class GenerateRandomQues {
	
	private String [][]packet= new String [5][];
	
    private String randomisedQues[][];
    
	public String [][] randomise(Group [][]group, int level)
	{
		packet[3] = new String[4];
		Random rand = new Random();
		int randIndex = rand.nextInt(3);
		
		this.packet[0][0] = group[randIndex][level].getGroupIndex();
		this.packet[1][0] = group[randIndex][level].getGroupDiff();
		this.packet[2][0] = group[randIndex][level].getGroupQues();
		
		this.packet[3][0] = group[randIndex][level].getGroupOptions()[0];
		this.packet[3][1] = group[randIndex][level].getGroupOptions()[1];
		this.packet[3][2] = group[randIndex][level].getGroupOptions()[2];
		this.packet[3][3] = group[randIndex][level].getGroupOptions()[3];// this method returns array of options options[]
		
		this.packet[4][0] = group[randIndex][level].getGroupAnswer();
		return this.packet;
	}

[pbrockway2]

array out of bound;

Your code will throw an ArrayIndexOutOfBounds exception if you try and access an array using an invalid index value. By "invalid" I mean either negative or greater than or equal to the length of the array. A simple example:

Thing foo[] = new Thing[5];

System.out.println(foo.length); // prints 5

foo[0] = new Thing(); // ok
foo[5] = new Thing(); // bad! the valid indexes only go up to 4
foo[-1] = new Thing(); // bad!

Notice how the length property of an array can be used to print out and check what the length is.

Double arrays can be more tricky. An expression like

arr[a][b] = ...

only makes sense if both:

(1) a is less than arr.length

and

(2) b is less than arr[a].length.

(Further, arr[a] must not be null.) In your code you should check that arrays have the length you are assuming they do.

[amrmb09]

i have test the "packet" array concerning number of rows and columns using
packet.length=5 // ok
packet[0].length=2 //
packet[3].length // generate Null Pinter Exception

[pbrockway2]

packet[3].length // generate Null Pinter Exception

This is telling you that packet[3] has the value null. That is, you have not assigned a value to packet[3].

Perhaps you could post the code that results in this error.

[amrmb09]

the code is mentioned above

[pbrockway2]

I'm sorry, but I can't really tell what's going on. The original post was edited to alter the compiler message, the code it contains is not runnable and does not contain the expression packet[3].length.

As I said if packet[3].length throws a NullPointerException but packet.length does not, that's because packet[3] is null.

Runnable code with the exact and entire runtime message is really required to say any more.

[amrmb09]

thanks alot...i solved, but storing my data in 1D array instead of 2D array
Regards.....