CODEWITHSUNDEEP
javanullpointerexception
Bella Sherman
Bella Sherman

Reputation: 21

what is causing my NoSuchElementException for ArrayList

I know that there are lots of threads on NoSuchElementException in Java here but I still cannot figure out what is going on here

I am trying to come up with a solution for Transitive Dependencies Kata 18 which is posted at http://codekata.pragprog.com/2007/01/kata_eighteen_t.html

dependencies_for method is supposed to take in a char item and compute all dependencies for the item. The exception occurs when I try to add an element to finalDependencies ArrayList

This is the place where my NullPointerException occurs. I have traced all of these data structures and none of them have a Null value. I don't understand what is causing my exception here. Please see my code:

public class Test_Dependencies 
{
    public static void main(String[] args) {
             Dependencies Dep = new Dependencies();
             Dep.add_direct('A', "B C");
             Dep.add_direct('B', "C D");
             Dep.dependencies_for('A');

    }
}


public class Dependencies {
      HashMap dependenciesList;


public Dependencies()
{
    HashMap<Character, ArrayList> dependenciesList = new HashMap<Character, ArrayList>();
}

public void add_direct(char mainItem, String dependentItems)
{
  // code that works here   
}

public String dependencies_for(char item)
{

    ArrayList finalDependencies = new ArrayList<Character>();
    Character key = new Character(item);

    //get initial dependencies for the item and add them
    ArrayList processingDependencies = dependenciesList.get(key);
    Iterator itr = processingDependencies.iterator();

    while(itr.hasNext())
    {
        if(finalDependencies.contains(itr.next()) == false && itr.next() != key)
        {

            // NoSuchElement exception here
            finalDependencies.add(itr.next());

            // look again at each item in dependenciesList.  If it is in the list then add it to processingDependencies
            if(dependenciesList.containsKey(itr.next()) && !processingDependencies.contains(itr.next()))
            {
                processingDependencies.add(itr.next());

            }
        }

    }

    // turn finalDependencies into a string
    itr = finalDependencies.iterator();
    String allDependencies = "";
    while(itr.hasNext())
    {
        allDependencies = allDependencies + " " + itr.next();
    }


    return allDependencies;

}
}

I am a bit perprlexed because processingDependencies and finalDependencies ArrayLists are not null. And processingDependencies arraylist contains an item

Upvotes: 1

Views: 229

Answers (3)

Steve&#39;s a D
Steve&#39;s a D

Reputation: 3821

The problem is here:

HashMap dependenciesList;
public Dependencies()
{
        HashMap<Character, ArrayList> dependenciesList = new HashMap<Character, ArrayList>();
}

You declare a hashmap called dependenciesList. You then try to instantiate that list, but what you actually do is create a local variable named the same thing. They are two separate variables. Then you try to use the one that hasn't been instantiated here:

 ArrayList processingDependencies = dependenciesList.get(key);

What you need to do is instantiate the first dependenciesList instead of creating a new one

(I'm not a pro at java, but something like dependenciesList = new HashMap....() instead of HashMap<..> dependenciesList = new HashMap...() )

Upvotes: 0

Jason Nichols
Jason Nichols

Reputation: 11733

You can't do this: 

while(itr.hasNext())
{
    if(finalDependencies.contains(itr.next()) == false && itr.next() != key)
    {
        // NoSuchElement exception here
        finalDependencies.add(itr.next());

        // stuff removed
    }
}

You must verify that iter.hasNext() is true prior to each call of itr.next(). What happens when you reach the last item in itr, but then call itr.next() three times?

Answer: NoSuchElementException. Check out Iterator

Upvotes: 1

Dirk
Dirk

Reputation: 31053

You are calling twice. The first call is "protected" by a matching hasNext Call. The second is not. Save the result of next into a temporary variable and use that, instead of using the value directly, since every call to next will try to advance the iterator first. In the good case, you get an exception. In the bad case, things seem to work, but your program is dealing with the wrong value.

Upvotes: 1

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