联系人不能转换为java.lang.Comparable

Question

i have been trying hard to spot my error but to no avail. can someone help me? I am sure that I am comparing the correct objects

    Contact[] sortedList = myContactList.sortContactsByName();
    for (Contact c : sortedList) {
        // TODO using this line means you need to implement what in Contact class?
        // Hint: You need to override a method inherited from Object.

        System.out.println(c);
    }
}



public Contact[] sortContactsByName() {
    Contact[] contactArray = contacts.toArray(new Contact[contacts.size()]);
    // TODO Contact class has to implement Comparable
    // interface for this to work!

    Arrays.sort(contactArray);
    // Don't bother how it works now, sorting in Java
    // will be covered later on.
    return contactArray;
}

}

public int compareTo(Contact o1){

    return this.name.compareTo( o1.name);

Answer 1

You have to declare the class Contact to implement Comaprable .

class Contact implements Comparable<Contact> {

Just implementing compareTo is not enough without this declaration. Also it is good practice to put an @Override annotation above compareTo .

Answer 2

The class you want to use the interface Comparable with must implement it.

class Contact implements Comparable

You then implement the method below in the Contact class. You have to remember that this is the first object calling compareTo on the other object of the same type. So you can't access the private variable of name directly in the parameter, use a getter.

public int compareTo(Contact o1){ 
    return this.name.compareTo(o1.getName());
}

Answer 3

@user3327196: You will have to implement Comparable interface to your Contact class. Code is attached below:

public class Contact implements Comparable<Contact> {

    private String name;

    public void setName(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    @Override
    public int compareTo(Contact contact){
        return this.name.compareTo(contact.name);
    }

}

Answer 4

there are two solutions for your problem, first one is described by most people here, make your contact to implement Comparable interface, this method is good, but it has some limits, ie if you want to have two lists with your contacts, one sorted by firstname second by lastname, you cant do this with this approach

fortunatelly there is method sort which takes custom comparator as parameter, with this solution your class doesnt need to be comparable , all what you need to do is create your custom comparator ie

public class Contact{
 private String name;

public static final Comparator<Contact> BY_NAME_ASC = new Comparator<Contact>() {

    @Override
    public int compare(Contact o1, Contact o2) {
        return o1.name.compareTo(o2.name);
    }
};

}

now all what you need to do to sort your array is Arrays.sort(contactArray,Contact.BY_NAME_ASC);