Can someone tell why my program is showing null pointer exception at line 33

[jinnejeevansai]

Hii,I was trying to solve a problem but i don't get why my program is showing null pointer exception on the commented line.Can someone explain the reason.

import java.util.*;
class cod1{
        public static void main(String args[]) throws java.lang.Exception
        {
                int t,i,n,j,k,p,q,r;
                int[] a =new int[100000];
                Scanner s = new Scanner(System.in);
                t=s.nextInt();
                for(i=0;i<t;i++)
                {
                        p=0;
                        n= s.nextInt();
                        ArrayList<Object>[] al = (ArrayList<Object>[])new ArrayList[n+1];
                        for(j=0;j<n;j++)
                        {
                                a[j]=s.nextInt();
                        }
                        int c;
                        for(j=0;j<n;j++)
                        {
                                c=0;
                                for(k=0;k<p;k++)
                                {
                                         if(al[k].size()-1>a[j])
                                        {
                                                al[k].add(a[j]);
                                                c++;
                                                break;
                                        }
                                }
                                if(c==0)
                                {
                                        al[p++].add(a[j]);//null pointer exception 
                                }
                        }
                        for(j=0;j<p;j++)
                        {
                                System.out.println(al[j].size()-1);
                        }
                }
        }
}
 
 
 

[chaarmann]

Because a[j] is null,
and al[k].add(null) then throws this exception:
it is not allowed to add a null to this ArrayList<Object>(). You specified in the generics that it accepts only Object.
Always check for null before trying to add.
Or use ArrayList() without generics, then you can store a null there.

[jinnejeevansai]

but i gave the input to a[j]

[chaarmann]

First of all, you should always first check with s.hasNextInt() before calling s.nextInt().

So you are right, the value of a[j] cannot be null because it's a primitive type, an "int" (sorry, I overlooked that).
Then the reason might be that your array list "al", your "ArrayList<Object>()" can only store Objects and no primitive types.

You can wrap a primitive type "int i" into an object "Integer" before storing it into "al" via "Object object = new Integer(i)". Maybe that helps.

Or much better: declare your "al" as ArrayList<Integer>(), then autoboxing will take over this job.

[jinnejeevansai]

Same error occurs after replacing object with integer

[chaarmann]

And what happens if you also wrap it as proposed? That means, do both?
That means replace the line with the nullPointerException through:

al[p++].add(new Integer(a[j]));

Also change line 13 from ArrayList<Object>() to ArrayList<Integer>(), that means:

ArrayList<Integer>[] al = (ArrayList<Integer>[])new ArrayList[n+1];

By the way, shouldn't the arrayList size be "n" instead of "n+1"? I mean, if you are entering "1", then you create an array of size 2, with a[0] and a[1] as its elements ???

[jinnejeevansai]

I got a doubt after you say,can you tell why the same error appears in the below code.

import java.util.*;
class w1{
        public static void main(String args[]) throws java.lang.Exception
        {
                int n=0;
                ArrayList<Integer>[] al = (ArrayList<Integer>[])new ArrayList[n+1];
                al[0].add(5);
        //      al[1].add(6);
                System.out.println(al[0].get(1));
        //      System.out.println(al[1].get(0));
        }
}
 

[chaarmann]

Sorry for the misinformation. I really should have run your code to see where the nullPointerException occurs.
The nullPointerException occurs, because al[p] is null (and not the value given to the add-method what I assumed wrongly)
You can see that if you add following line right before the error at line 33:

System.out.println("al length=" + al.length + " p=" + p + ((p < al.length) ? " al[p]=" + al[p] : " invalid index"));

The output is:

al length=3 p=0 al[p]=null

That means, you have defined an array of arrayLists.
But you have not initialized its content, that means you have not put anything inside your array of arrayLists. There is everywhere "null" inside. You should put empty arrayLists inside to initialize its content (after line 13):

        for(int index = 0;index < al.length;index++)
        {
                al[index]=new ArrayList<Object>();
        }
 

Try it and see that your null pointer exception is away. (but it can have IndexOutOfBounds-Exception if "p" is equal or bigger than the given array size n+1)


In my understanding that's a design error. You only wanted a single arrayList.
if that assumption is correct, then you should use in line 13:

ArrayList<Object> al = new ArrayList<Object>();

And then in line 33:

al.add(new Integer(a[j]));