Question

So recently I have been working on “phonebook” project that uses “Binary Search Tree”.

The Phonebook is using BSTree. Each node of the tree is BTNode. In the main class, I replace E with Pair class, which has (String name, String number), when I define the nodes.

I have the following comparator class to compare between 2 E types:

import java.util.Comparator;
public class BTNodeComparator<E extends Comparable<E>> implements Comparator<E>
{
    public int compare(final E a, final E b)
    {
        return (a.compareTo(b));
    }
}

and I use it in BSTree.

Also, I define compareTo() method in Pair class:

@Override
    public int compareTo(Pair pair)
    {
        return name.compareTo(pair.getName());
    }

Now, when I run the program and it comes to the comparator, it gives me errors at the comparator because now it compares between two Pairs

How can I solve this issue? What I want is to compare between the names of the two pairs ?

This error what I got after inserting the second node (Pair) in BST:

java.lang.NullPointerException
    at Pair.compareTo(Pair.java:36) // @ return name.compareTo(pair.getName());
    at Pair.compareTo(Pair.java:2)  // @ public class Pair implements Comparable<Pair>
    at BTNodeComparator.compare(BTNodeComparator.java:24) // @ return (a.compareTo(b));
    at BTNodeComparator.compare(BTNodeComparator.java:20) // @ public class BTNodeComparator<E extends Comparable<E>> implements Comparator<E>
    at BSTree.search(BSTree.java:285)
    at BSTree.insert(BSTree.java:300)
    at PhoneBook.main(PhoneBook.java:25)

BTW, I decleared BTNodeComparator in BSTree as follows:

protected Comparator<E> c = new BTNodeComparator();
if (c.compare(target, cursor.getElement()) < 0) cursor = cursor.getLeft();

Thanks in advance for any help 🙂

Answer 1

Your stacktrace indicates that your problem is that name is null in a Pair.

.. or pair that is an argument to compareTo