dereferencing null pointer exception

[oll3i]

My variable cp is = 2 when I pass 1 and 2 values but I get a null pointer exception.

Could you please explain?

 
for(int i=0; i<=resLeftL.size(); i++){
  //System.out.println(resLeftL.size());      
  for(int j=0; j<= resRightL.size(); j++)
  {  
    cP=resLeftL.get(i)*resRightL.get(j);
    System.out.println("cp="+cP); // prints 2
    elements.addElements(cP); 
    //dereferecing null
   }   
 }
 return elements;
 }

[r035198x]

Did you check if elements is null at that point?

[oll3i]

elements.addElement(resLeftL.get(i)*resRightL.get(j));

<--is null at that point

[oll3i]

the error can be here

 IBagResult  elements = null;

and the BagResult is

public class BagResult implements IBagResult{
 Collection collection = new HashSet();
 @Override
 public Collection getElements()
 {return collection;} 
 @Override
 public void addElement(Integer i){
 collection.add(i);
 }
}

[Nepomuk]

Do you have something like

elements = new BagResult();

anywhere? If not, add that before you try to access the elements object.

[oll3i]

elements.addElement(resLeftL.get(i)*resRightL.get(j))

returns

thread "main" java.lang.Index OutOfBoundsExce ption: Index: 1, Size: 1
at java.util.Array List.rangeCheck (ArrayList.java :635)
at java.util.Array List.get(ArrayL ist.java:411)
at s4409_s4409_pr2 .QExecUtils.car tesianProduct(Q ExecUtils.java: 60)
at s4409_s4409_pr2 .S4409_s4409_pr 2.main(S4409_s4 409_pr2.java:38 )

[oll3i]

and in main that is the line

IBagResult commaRes = QExecUtils.cartesianProduct(resLeft,resRight

[Nepomuk]

As the loop is dependant on the size of resLeftL, I'd guess that resRightL is smaller than the left list. To be more precise, according to this exception it has one element and you're trying to access the second element.

[Frinavale]

The index problem is because of the loop conditions you have:

for(int i=0; i<=resLeftL.size(); i++)
{
  for(int j=0; j<= resRightL.size(); j++)
  {
  }
}

Arrays etc. in Java start at index 0. This means that if you have something that is 10 units long the valid indices fall between 0 and 9 but in your loop you are going to 10 because of the <= check. It should just be <.



-Frinny

[Nepomuk]

Good catch Frinny, I missed that one!