I implemented some code involving queues and while running it I get a NullPointerException.Please help me to fix it.I am only writing the shorter form of that code.
import java.util.*;
class ex
{
public static void main(String args[])throws IOException
{
Scanner in=new Scanner(System.in);
int i;
String s;
int n=in.nextInt();
Queue<Integer> q=null;
for(i=0;i<n;i++)
{
q.add(i);//I get the error in this line
}
System.out.println(q.size());
}
}
You have to initialize the queue first:
Queue<Integer> q=null;
Should be:
Queue<Integer> q = new Queue<Integer>();
The reason for the error is that you are trying to add values to q. q is only set to be of the type Queue<Integer> and not to be a reference to an object of that type itself.
You get an NPE because q is null.
You have to create an object before you can use it, for example:
Queue<Integer> q = new LinkedList<Integer>();
Here, I've picked LinkedList as a class implementing the Queue interface. There are lots of others: see the "All Known Implementing Classes" section of the Queue javadoc.
Queue<Integer> q = null;
Well ... that's null and:
q.add(i);
There you're trying to use it. Ergo, Exception.
You have to instantiate the object in order to have one that you can use:
Queue<Integer> q = new Queue<Integer>();
If this wasn't a simple typo / oversight, you may want to start at the beginning of the java tutorials provided by Oracle or get a "Learning java" type book before tackling something more complicated.
Queue<Integer> q=null;
...
q.add(i);//I get the error in this line
Your Queue reference is null, so you get a NullPointerException when trying to access it. Before using it, q has to point to something valid like for example:
Queue<Integer> q = new Queue<Integer>();
That happens because q is null. You need to initialize it with something before you can use it, for example:
Queue<Integer> q = new AbstractQueue<Integer>();
For some details and samples see:
You need to initialize q
Queue<Integer> q = new AbstractQueue<Integer>();
