Using Collections.sort to sort an ArrayList of a specific object

Question

So I have seen multiple questions addressing similar problems to mine, but I was not able to find one that was exactly like my problem.

I have an ArrayList of Contact objects, and I want to sort them using Collections.sort:

public void sortContactArrayList(){
    ArrayList<Contact> newList = new ArrayList<Contact>();
    Collections.sort(newList);
}

In order to do this, I made Contactimplement Comparable<Contact>. And, my compareTo method looks like this:

@Override
public int compareTo(Contact otherContact) {
    return this.getName().compareTo(otherContact.getName());
}

However, I am receiving an error when I am calling Collections.sort(newList);

The error is:

"Bound mismatch: The generic method sort(List<T>) of type Collections is not applicable for the arguments (ArrayList<Contact>). The inferred type Contact is not a valid substitute for the bounded parameter <T extends Comparable<? super T>>"

Does anyone know what the issue is? Like I said, I have seen this similar problem with a customized list of certain objects like "ContactDatabase<Contact>" or something, but I have never seen this problem with just a certain object itself.

Thank you!

That message means that Contact does not implement Comparable<Contact>. Are you sure you've recompiled it and your compiler is looking at the right .class file? — Dawood ibn Kareem
– Dawood ibn Kareem, Commented Apr 23, 2014 at 2:05
It seems right for me. Could you post the code of Contact? — songyuanyao
– songyuanyao, Commented Apr 23, 2014 at 2:12

xi.lin · Accepted Answer · 2014-04-23 02:53:37Z

1

It should be OK if you implemented Comparable<Contact>.

Here is my quick test code:

Contact.java:

public class Contact implements Comparable<Contact> {

    private String name;

    public Contact(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    @Override
    public int compareTo(Contact otherContact) {
        return this.getName().compareTo(otherContact.getName());
    }
}

main method:

public static void main(String[] args) {

    ArrayList<Contact> newList = new ArrayList<Contact>();
    newList.add(new Contact("Midgar"));
    newList.add(new Contact("David"));
    Collections.sort(newList);
    System.out.println(newList);
}

answered Apr 23, 2014 at 2:53

xi.lin

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2 Comments

Midgar77 Over a year ago

Thank you so much! It does work, and I have no idea why it didn't before. For some reason, I didn't have to change anything, I just didn't use Eclipse for a day, and went on it today and it worked. I was almost convinced that the code was correct before I posted my question, so I needed someone to verify that I was right before I started blaming Eclipse acting up. Thank you!

xi.lin Over a year ago

@Midgar77 sometimes you can try clean and rebuild project in eclipse, or close/reopen eclipse to make things right.

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