Using Comparable<T> with Generic class

Question

A newbie question (possibly duplicate too) but can someone point me what's wrong with the below code:

public class Node<T> implements Comparable<Node<T>> {

    protected final Comparable<T> data;
    protected final List<Node<T>> adj;

    public Node(Comparable<T> data) {
        this.data = data;
        adj = new ArrayList<Node<T>>();
    }

    @Override
    public int compareTo(Node<T> other) {
        return data.compareTo(other.data);
    }
}

The compareTo method shows a compilation error: compareTo(T) in Comparable cannot be applied to (java.lang.Comparable).

Alternatively I tried using public class Node<T extends Comparable<? super T>> implements Comparable<Node<T>> but then if I add this Node in, say, an ArrayList> then I am not able to use the contains method of ArrayList. I'd be very thankful if someone points me out the difference.

Community · Accepted Answer · 2017-05-23 11:50:54Z

5

You need to tell Java that T extends Comparable<T>, like this:

public static class Node<T extends Comparable<T>> implements Comparable<Node<T>> {

    protected final T data;
    protected final List<Node<T>> adj;

    public Node(T data) {
        this.data = data;
        adj = new ArrayList<Node<T>>();
    }

    @Override
    public int compareTo(Node<T> other) {
        return data.compareTo(other.data);
    }
}

Comparable<T> says that you can apply compareTo method to an object of type T. When your version declared

Comparable<T> data

and then tried to compare data to Comparable<T> instead of T, the compiler objected, because T and Comparable<T> are incompatible.

NOTE: For greater flexibility, use Node<T extends Comparable<? super T>>. This would let you mix and match nodes based on different subclasses in the same comparison (thanks, JB Nizet, for a great suggestion).

edited May 23, 2017 at 11:50

CommunityBot

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answered Nov 22, 2014 at 10:32

Sergey Kalinichenko

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3 Comments

JB Nizet Over a year ago

Agreed. I would use Node<T extends Comparable<? super T>> though.

gashu Over a year ago

Yes, I tried the suggestion and it works nicely, although I was interested to know if I do Comparable<T> data instead of 'T data' then why does compiler throw error when I do overridden compareTo(<Node<T> other) { data.compareTo(other.data); }

gashu Over a year ago

Ah, got it. Posted my comment before seeing the edit.

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