Iterator - Null Pointer Exception Error

Question

I am having a really frustrating issue:

I am trying to run an iterator, but it keeps on coming up with java.lang.NullPointerException at the hasNext class.

I am not quite sure where it might be trying to use a null value. I am assuming it is something to do with current. I added a if statement to check if current is null. But then it returns and unexpected value.

Help appreciated.

Code below:

private class Iterator implements Iterator
{
    private Link<T> current;

    public boolean hasNext () { 
        if(current.next == null)
            return false;
        return true;
    }

    public T next() throws OutOfBounds
    {
        if (this.hasNext())
        {
            T element = current.element;
            current = current.next;
            return element;
        }
        else 
            throw new OutOfBounds("No next element to call");
    }
}

private class Link<T> 
{
    private T       element;
    private int     priority;
    private Link<T> next;

    public Link(T t, int p, Link<T> n) 
    {
        this.element = t;
        this.priority = p;
        this.next = n;
    }
}

}

Please post a stacktrace.

Ortwin Angermeier
– Ortwin Angermeier

2013-05-17 07:15:13 +00:00
Commented May 17, 2013 at 7:15 — Ortwin Angermeier
– Ortwin Angermeier, Commented May 17, 2013 at 7:15

Habib · Accepted Answer · 2013-05-17 07:19:29Z

5

You are probably not initializing current, so your check in the method hasNext should compare for null against currnet before checking against current.next

Modify your check

if(current.next == null)

to:

if(current == null || current.next == null)

Or modify your method as:

public boolean hasNext () { 
   return (current != null && current.next != null);
}

edited May 17, 2013 at 7:19

answered May 17, 2013 at 7:14

Habib

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2 Comments

heikkim Over a year ago

Even better: return !(current == null || current.next == null)

Habib Over a year ago

@heikkim, yes, simpler and more readable. return (current != null && current.next != null);

Juned Ahsan · Accepted Answer · 2013-05-17 07:20:33Z

1

Try to update your hasNext as below to find the issue:

public boolean hasNext () { 
        if(current == null) {
           System.out.println("current is null");
           return false;
        } else if(current.next == null)
            return false;
        }
        return true;
    }

answered May 17, 2013 at 7:20

Juned Ahsan

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Comments

user12057507 · Accepted Answer · 2019-09-18 08:53:50Z

1

You may use iterator.next() two times inside your while block. Make new object with iterator.next() then use it.

This is the correct way to use it

ArrayList<String> demo = new ArrayList<>();

demo.add("A");
demo.add("B");
demo.add("C");
demo.add("D");

System.out.println(demo);

//Get iterator
Iterator<String> iterator = demo.iterator();

//Iterate over all elements
while(iterator.hasNext()){
/* if you want to use the elemet two times then put in a varialbe and use it.*/
    //Get current element
    String value = iterator.next();
     System.out.println("fist time using"+ value)
    System.out.println( "second time using " + value );

}

answered Sep 18, 2019 at 8:53

user12057507

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