Bug in Comparator Class

Question

I have implemented a comparator class which sorts an array of objects, or "Persons", in my case. However, there seem to be a few bugs, so when I click the next button of my GUI, I get the following error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at RectangleProgram$CustomComparator.compare(RectangleProgram.java:30)
at RectangleProgram$CustomComparator.compare(RectangleProgram.java:26)
at java.util.TimSort.binarySort(TimSort.java:265)
at java.util.TimSort.sort(TimSort.java:208)

Here are snippets of the code that are relevant:

public class Person//HERE IS THE OBJECT THE COMPARATOR IS REFERRING TO
     {
        String firstname; String lastname; int z; 
        public Person(String l, String m, int e)
        {
           firstname=l; lastname=m; z=e; 
        }
        public String getFirstName()
        {
           return firstname;
        }
     } 
public class CustomComparator implements Comparator<Person> { // COMPARATOR BEGINS HERE
      @Override
      public int compare(Person object1, Person object2)
      {
              return object1.getFirstName().compareTo(object2.getFirstName()); 
      }
} // COMPARATOR ENDS HERE
public static Person [] arr=new Person [100]; // ARRAY TO BE SORTED
// class implementing the sort
public class re implements ActionListener
    {
        public void actionPerformed (ActionEvent e) 
        { 
           if (counter==0)
           {
              getData(); 
              Arrays.sort(arr, new CustomComparator()); 
           }

       }
   }

Could someone tell me what is wrong with the code, and is causing the error? Note, it is not a compilation error, it is just that clicking the next button [of which class re is the action listener] does not make it do anything.

`return object1.getFirstName().compareTo(object2.getFirstName());`: What will happen if `object1` or `object2` is `null`? — BackSlash, Aug 18 '14 at 18:26
Yet another duplicate of http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it — Raedwald, Aug 18 '14 at 18:27
@BackSlash More to the point, `Person [] arr=new Person [100];` creates an array filled with nothing but `null` entries. — Powerlord, Aug 18 '14 at 18:31
getData(); gets all the data... so there are no null entries — user3904846, Aug 18 '14 at 18:34
@Powerlord you are right, and the op did not even use Comparator method at all, so I am sure the issue is from what you said — Kick Buttowski, Aug 18 '14 at 18:34
wait i have a question; if the array is of size 100, but i only get a 5 people input, could that cause a problem? — user3904846, Aug 18 '14 at 18:34
@user3904846 why do not you use dynamic array like array list? — Kick Buttowski, Aug 18 '14 at 18:35
ok thanks :)... so in this case, changing the array size to 5 would work [assuming that the data is of size 5] — user3904846, Aug 18 '14 at 18:35
@kick buttowski because I thought an array would be easier; but i will do that — user3904846, Aug 18 '14 at 18:36
@user3904846 how did you put your data inside your array? how getData does that? can you explain it? — Kick Buttowski, Aug 18 '14 at 18:37
Incidentally, "Prefer lists to arrays" is item 25 in Effective Java, Second Edition. — Powerlord, Aug 18 '14 at 18:37
Don't do what is easy, but rather what is right. Otherwise you will get into worse problems than this down the road. — nmore, Aug 18 '14 at 18:37
@Kick Buttowski it reads from a file. nmore, I'll remember that; but unfortunately I did not know that comparator does not work with null objects... — user3904846, Aug 18 '14 at 18:38
are there any similar errors for arraylist? I'm getting the same thing when i implement with arraylist — user3904846, Aug 18 '14 at 18:52
@user3904846 I posted my answer let me know how it went or you need some explanation — Kick Buttowski, Aug 18 '14 at 20:46

score 2 · Accepted Answer · answered Aug 18 '14 at 20:45

I tried this in Java 8

Code:

        List<Person> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            Person p = new Person(Integer.toString(i), Integer.toString(i), i);
            list.add(p);
        }
        Collections.shuffle(list);
        System.out.println("the shuffled list");
        list.forEach(i -> System.out.println(i.toString() + " "));
        Collections.sort(list, (p1, p2) -> p1.getFirstName().compareTo(p2.getFirstName()));
        System.out.println("------------------------ \n the orderd list");
        list.forEach(i -> System.out.println(i.toString() + " "));

output:

the shuffled list
0 0 0 
9 9 9 
8 8 8 
7 7 7 
3 3 3 
6 6 6 
2 2 2 
4 4 4 
1 1 1 
5 5 5 
------------------------ 
 the orderd list
0 0 0 
1 1 1 
2 2 2 
3 3 3 
4 4 4 
5 5 5 
6 6 6 
7 7 7 
8 8 8 
9 9 9

let me know what happened or you need any explanation

Bug in Comparator Class

1 Answers1