it's clear now, TreeSet deals with only compareTo and my class was like
that

--------------------------------
/**
*
*/
package test;

/**
* @author Administrator
*
*/
public class Identity implements Comparable{

    private String name;
    private long id;

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

    public String getName(){
        return this.name;
    }
    public double getId(){
        return this.id;
    }

    public boolean equals(Object obj){
        System.out.println("equals says: I'm invoked");
        try{
            Identity otherIdentity = (Identity)obj;
            return (this.name.equals(otherIdentity.name));
        }catch(ClassCastException e){
            return false;
        }
    }

    public int compareTo(Object obj){
        System.out.println("compareTo says: I'm invoked");
        Identity otherIdentity = (Identity)obj;
        if (this.id > otherIdentity.getId())
            return 1;
        else if (this.id < otherIdentity.getId())
            return -1;
        else
            return 0;
    }

    public int hashCode(){
        return name.hashCode();
    }
}
-----------------------------------------

so the compareTo() was not consistent with equals().

thank you,

Bassel