Extending comparable interface and override compareTo

Question

I know this has propably been asked 1000 times, and i have seen 1000 answers, but i didn't see any that actually worked for me.

I have to sort a List of objects which implement my own interface. I decided to extend Comparable interface and implement compareTo method for those objects.

my interface:

public interface Field extends Comparable<Field> {

}

my object class

public class ConcreteField implements Field {

    public int compareTo(Field other){
        return this.getName().compareTo(other.getName());
    }    
}

This doesn't work as i would expect.

Compiler seems to recognize that the ConcreteField must implement compareTo. Thats fine.

With example above i can compare ConcreteField objects. But not when they are represented by their interface (Field).

I can sort a List of ConcreteField's but i cannot sort a List of Field's. Of course this is naturaly, since the implementation of compareTo is in ConcreteField, but i was expecting that java would call compareTo on each concrete object which implements Field. Seems not to be the case.

But then, how do you actually sort objects which are represented by their comparable interface?

Another strange things that i just dont get.

Using @Override over compareTo method makes the compiler yelling: The method compareTo(Field) of type ConcreteField must override or implement a supertype method. Huh? There is no supertype, its just implementing an interface.

When i remove compareTo from ConcreteField class. Compiler complains that it must be implemented. When i use eclipse quickfix, which says "implement missing methods", it fails to do so and tells me "Cannot implement the missing methods, either due to compiler errors or the project build path does not resolve all dependencies".

There is no dependency at all. Its all in the same project and even same package. So what the hell is wrong with this?

Solution

@Masud : Implementation suggested by you (Post deleted)

Create an interface like :

public interface Field<T> extends Comparable<T> {
    public String getName();
}

Create a ConcreteField class like :

import java.util.Arrays;


public class ConcreteField implements Field<Field>{

    private String name;

    @Override
    public int compareTo(Field other) {
        return this.name.compareTo(other.getName());
    }

    @Override
    public String getName() {
        return name;
    }

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

    public static void main(String[] args) {
        ConcreteField[] c = new ConcreteField[3];
        c[0] = new ConcreteField();
        c[1] = new ConcreteField();
        c[2] = new ConcreteField();
        c[0].setName("c");
        c[1].setName("b");
        c[2].setName("a");

        Arrays.sort(c);
        for(int i=0;i<c.length;i++) {
            System.out.println(c[i].getName());
        }

    }
}

The output I am getting is sorted.

a
b
c

Comments

[JB Nizet]

Create an SSCCE. There's something wrong in the code you don't show. As posted, your code wouldn't compile since there is no getName() method in Field.

[Alexej Geldt]

What is SSCCE? I didn't want to bug you with all the methods of Field interface. Just assume there is a name property with a getter on it.

[JB Nizet]

An SSCCE is a short, self-contained compilable example. See sscce.org. Google is your friend, also. The answer from nishu, below, contains an SSCCE and, as you see, it compiles and runs fine.

[Alexej Geldt]

Ah i see. Well, ConcreteField has more then 100 methods :-)

[Alexej Geldt]

You are right though, there must be something wrong something else. I have just created a simple example just like above and everything worked fine. I was able to compare and the compiler didn't complain about @Override. I will have to take a closer look. This seems not to be the problem.

[JB Nizet]

My guess is that you're using some generic type as a raw type somewhere.

[Julien]

To me it would be better with a public interface Field extends Comparable<? extends Field>

[Alexej Geldt]

Arghh. This is actually the same procedure as i was trying to do. And it works indeed when running in a standalone java application. I didn't tell yet, but i am working on a huge Web Application. It does has Fields and i have to sort them. ConcreteFields are Pojos just with properties and getters/setters but the same approach that you described just doesn't work for them. Compiler does not even let me use @Override.

[JB Nizet]

@user2162910 then you're compiling with an old version of Java (pre-1.6)

[cdalxndr]

If the object is not a ConcreteField but some other Field implementation, compareTo will fail with ClassCastException.