Issues with comparing two generic objects

Question

My project is a Phonebook that is using BSTree<E>. Each node of the tree is BTNode<E>. 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> implements Comparator<E>
{
    public int compare(E a, E b) throws ClassCastException
    {
        return ((Comparable<E>) a).compareTo(b);
    }
}

and I use it in BSTree<E>.

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 ?

Sorry for my bad explanation. My English is weak.

=========================

Edit:

@Tim: After trying your solution it gives me this error:

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();

Tim Bender · Accepted Answer · 2011-05-06T01:53:25.067

2

The only possible failure I could see is ClassCastException. To fix your code you will need to specify your Pair class like this:

class Pair implements Comparable<Pair> {
    String name;
    String number;

    //some implementation stuff...

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

In general, if your Comparator implementation relies on the generic type implementing Comparable then you should specify it like this:

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

Doing so just gives you compile time safety without changing the semantics of how the Comparator works.

Based on your comment, I'm guessing you are embedding your BTNodeComparator into the BSTree class as a nested inner-class, which means you probably can't make that change without changing the type definition in the BSTree declaration.

edited May 06 '11 at 01:53

answered May 06 '11 at 01:27

Tim Bender

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How can I declare BTNodeComparator in BSTree class, is it like this: Comparator c = new BTNodeComparator(); ? – Eng.Fouad May 06 '11 at 01:37
@Eng.Fouad, I edited/re-organized my answer. The main take away is that I am assuming that Pair must implement Comparable. If you posted the exception and stack trace you are getting a more definitive answer could be given. – Tim Bender May 06 '11 at 01:55
Take a look on my new post above – Eng.Fouad May 06 '11 at 02:03

Issues with comparing two generic objects

1 Answers1