TreeSet comparator example

Hi,

I am looking at below example

http://java2novice.com/java-collections-and-util/treeset/comparator-object/

I wonder why employee class has no default sorting order implementation(not custom one) with method compareTo()

class Empl{
     
    private String name;
    private int salary;
     
    public Empl(String n, int s){
        this.name = n;
        this.salary = s;
    }
     
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getSalary() {
        return salary;
    }
    public void setSalary(int salary) {
        this.salary = salary;
    }
    public String toString(){
        return "Name: "+this.name+"-- Salary: "+this.salary;
    }
}
- See more at: http://java2novice.com/java-collections-and-util/treeset/comparator-object/#sthash.BHVK2GLy.dpuf

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I have expecte above code as below

class Empl implements comparable{
     
    private String name;
    private int salary;
     
    public Empl(String n, int s){
        this.name = n;
        this.salary = s;
    }
     
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getSalary() {
        return salary;
    }
    public void setSalary(int salary) {
        this.salary = salary;
    }
    public String toString(){
        return "Name: "+this.name+"-- Salary: "+this.salary;
    }

public int compareTo(){
//default sorting using some ID etc unless user want custom sorting on first name et then they create classes like //MySalaryComp 
}
}
- See more at: http://java2novice.com/java-collections-and-util/treeset/comparator-object/#sthash.BHVK2GLy.dpuf

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please advise