Trying to debug NullPointerException when using ArrayList

Question

public static ArrayList<Integer> getStudentClasses(Student student, ArrayList<Integer>[] classesXstudents) {
      ArrayList<Integer> classes = new ArrayList<Integer>();
      for (int i = 0; i < classesXstudents.length; i++) {
        for (int j = 0; j < classesXstudents[i].size(); j++) {
          if (classesXstudents[i].get(j) == student.getID()) {
            classes.add(i+1);
          }
        }
      }
      return classes;
  }

throws NullPointerException at the line

  for (int j = 0; j < classesXstudents[i].size(); j++) {

which I'm assuming means classesXstudents[i].size() is null at some point. classesXstudents is an array of ArrayList objects. How should I debug this case?

Did you try to put a break point at line that calls to your method and see what `classesXstudents` contains? — TDG, Mar 04 '16 at 16:51

score 1 · Answer 1 · answered Mar 04 '16 at 16:52

1

How about something like this.

if (classesXstudents[i] != null){
   for (int j = 0; j < classesXstudents[i].size(); j++) {
       //....
   }
} else {
   //
}

Some of your elements are probably null, you should probably check where they're initialized. It might just be that you forgot to initialize one of them.

answered Mar 04 '16 at 16:52

Luka Jacobowitz

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I tried that and it won't compile it says Generate.java:118: error: incomparable types: int and if (classesXstudents[i].size() != null) { – John314 Mar 04 '16 at 17:03
never mind it works :P thanks – John314 Mar 04 '16 at 17:04
Don't compare the size. Look again, I don't compare size, but the array itself. Ints can't be null in java. Only reference types. – Luka Jacobowitz Mar 04 '16 at 17:05
Oh okay, great. If this answers your question please upvote and accept the answer. :) – Luka Jacobowitz Mar 04 '16 at 17:14

score 0 · Answer 2 · answered Mar 04 '16 at 16:50

What that means is that some element of the classesXstudents[i] array is null.

Look at how that array is filled. Search for cases where null might be inserted, or when some element might be left unassigned (default value is null), perhaps at the end of the array.

score 0 · Answer 3 · answered Mar 04 '16 at 16:59

This is happening because some elements of your ArrayList<Integer> classesXstudents[i] are null.

You can fix this problem by checking if the element whose size you are asking for actually isn't null.

 if (classesXstudents[i] != null) {
      // Your for-loops and rest of the code here
 }

Adding the above snippet will check to see if the element is null (say you forgot to initialize it) or not. You can incorporate this in your code as follows:

public static ArrayList<Integer> getStudentClasses(Student student, ArrayList<Integer>[] classesXstudents) 
{
    ArrayList<Integer> classes = new ArrayList<Integer>();
    for (int i = 0; i < classesXstudents.length; i++) {
        if (classesXstudents[i] != null) // This statement is what you need to add

            for (int j = 0; j < classesXstudents[i].size(); j++) {
                if (classesXstudents[i].get(j) == student.getID()) {
                    classes.add(i+1);
                }
            }
   }
   return classes;
}

Trying to debug NullPointerException when using ArrayList

3 Answers3