I have to convert Hashset to TreeSet, but i have errors

```
// Here, All is ok, Chien is an object.
Set<Chien> animaux = new HashSet<Chien>();
animaux.add(new Chien("non", 10));
animaux.add(new Chien("zoz", 15));
animaux.add(new Chien("non", 10));

// And then i have to convert it to TreeSet 
// IntellJ-Idead says me that's ok...
// But i have an error here
TreeSet<Chien> treseet = new TreeSet<Chien>(animaux);

```

But when I try to compile :

```
Exception in thread "main" java.lang.ClassCastException: fr.univtln.arouani277.vertebre.animal.Chien cannot be cast to java.lang.Comparable
    at java.util.TreeMap.put(TreeMap.java:559)
    at java.util.TreeSet.add(TreeSet.java:255)
    at java.util.AbstractCollection.addAll(AbstractCollection.java:322)
    at java.util.TreeSet.addAll(TreeSet.java:312)
    at java.util.TreeSet.<init>(TreeSet.java:160)
    at fr.univtln.arouani277.App.main(App.java:40)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:622)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

```

Here are my files `Chien` :

```
public class Chien extends Mammifere implements Ibruit {
    public Chien(String pnom, int page) {
        super(pnom, page);
    }

    public void aboyer() {
        System.out.println("ouaf");
    }

    public void crier() {
        System.out.println("ouaf");
    }
}

```

it extends `Mammifere` :

```
public abstract class Mammifere extends Animal {
    public Mammifere(String pnom, int page) {
        super(pnom, page);
    }
}

```

That extends `Animal` :

```
public abstract class Animal extends Vertebre {
    public Animal(String nom, int page) {
        super(nom, page);
    }

    public void aboyer() {
    }

    public String toString() {
        System.out.println("Je suis un animal");
        return super.toString();
    }
}

```

that extends `Vertebre` :
<http://pastebin.com/Aa4Rw8sW>