Making your own class 'Comparable'

Question

I followed a tutorial, but failed to make my Country class Comparable for my BST.

Main:

BinarySearchTree A = new BinarySearchTree();
Country a = new Country("Romania", "Bucharest", 1112);
A.insert(a);

Country class:

public int compareTo(Object anotherCountry) throws ClassCastException {
    if (!(anotherCountry instanceof Country))
        throw new ClassCastException("A Country object expected.");
    String anotherCountryName = ((Country) anotherCountry).getName();  
    int i = this.name.compareTo(anotherCountryName);
    if(i < 0){
        return -1;
    } else {
        return 0;
    }
}

error:

@Override
public int compareTo(Object anotherCountry) throws ClassCastException {
    if (!(anotherCountry instanceof Country))
      throw new ClassCastException("A Country object expected.");
    String anotherCountryName = ((Country) anotherCountry).getName();  
    return this.name.compareTo(anotherCountryName);

Description Resource    Path    Location    Type

Name clash: The method compareTo(Object) of type Country has the same erasure as compareTo(T) of type Comparable but does not override it Country.java /Lab2_prob 4/src line 17 Java Problem

Description Resource    Path    Location    Type
The method compareTo(Object) of type Country must override or implement a supertype method  Country.java    /Lab2_prob 4/src    line 17 Java Problem

and class:

public class Country implements Comparable<Country>{
    private String name;
    private String capital;
    private int area;

Description Resource    Path    Location    Type

The type Country must implement the inherited abstract method Comparable.compareTo(Country) Country.java /Lab2_prob 4/src line 2 Java Problem

Does your `Country` class extend `Comparable`? – André Stannek Oct 24 '12 at 14:43 — André Stannek, Oct 24 '12 at 14:43
What problem or error are you actually getting? – DNA Oct 24 '12 at 14:44 — DNA, Oct 24 '12 at 14:44

André Stannek · Accepted Answer · 2020-11-30T09:32:49.507

24

Your Country class should implement Comparable:

public class Country implements Comparable<Country>

Then your compareTo method should look like this:

@Override
public int compareTo(Country anotherCountry) {
    return this.name.compareTo(anotherCountry.getName());
}

Note the signature of compareTo. The parameter can (and must) be of type Country, not Object. This is required because of the generic type parameter on Comparable. The upside is you don't have to check the type anymore. The downside is you can only compare Country to other Country objects (or its subtypes), but in most cases this is what you want anyway. If not you have to change the type parameter, e.g. if you use Comparable<Object> the signature of compareTo can be Object again. You can read more about generics here.

edited Nov 30 '20 at 09:32

answered Oct 24 '12 at 14:50

André Stannek

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1

Just remembered `Comparable` is an interface. Sorry for the error. – André Stannek Oct 24 '12 at 14:56
Can you please check the erros i got after the fixes you suggested me? pretty please – Bogdan M. Oct 24 '12 at 15:23
@mic you are right, my bad. I'll edit it. Thanks! – André Stannek Nov 30 '20 at 09:32

score 5 · Answer 2 · answered Oct 24 '12 at 14:47

a Comparable should return:

a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

However, your code is returning only -1 or 0, which is not correct; this implies that this can be less than the other object, or equal, but not greater!

There is no need to modify the values returned by name.compareTo() - you can just return them directly.

Making your own class 'Comparable'

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