Question

I trying to find whether the elements of 2 arrayLists are match or not.
But this code give me error Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException since some of the elements are null.

How can I solved this problem?

String level []={"High","High","High","High","High","High"};
ArrayList<Object> n = new ArrayList<Object>(Arrays.asList(level));

String choice []={null,"High","Low","High",null,"Medium"}; 
ArrayList<Object> m = new ArrayList<Object>(Arrays.asList(choice));

//Check if the two arrayList are identical
for(int i=0; i<m.size(); i++){
   if(!(m.get(i).equals(n.get(i)))){   
 result= true;
 break;
   } 
} 
    return  result;
}

Answer 1

The problem is that you are calling the equals method on some elements without first checking for null.

Change to:

for(int i=0; i<m.size(); i++){
   if(m.get(i) != null && !(m.get(i).equals(n.get(i)))){   
     result = true;
     break;
   } 
} 

Or if you want to allow two null values to compare equal:

for(int i=0; i<m.size(); i++){
   if (m.get(i) == null) {
     if (n.get(i) != null) {
       result = true;
     }
   } else if(!(m.get(i).equals(n.get(i)))){   
     result = true;
   } 
   if (result) {
     break;
   }
} 

One thing I don’t get – why are you setting result to true when you find a mismatch? Don’t you want to return true if both lists match and false otherwise?