使用Collections.sort对特定对象的ArrayList进行排序

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 Contact implement 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!

Answer 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);
}