Working of == in the Comparable interface used in ArrayList

Question

Working of == operator in Comparable Interface

Employee.java

class Employee implements Comparable
{
    int id; String name; int age;

    Employee(int id,String name,int age)
    {
        this.id=id;
        this.name=name;
        this.age=age;
    }

    public int compareTo(Object obj)
    {
        Employee emp = (Employee)obj;
        if(age==emp.age)
        {
            return 0;
        }
        //else if(age>emp.age)
        //return 1;
        else
            return -1;
    }
}

---

display_logic.java

import java.util.*;
class display_logic
{
  public static void main(String args[])
  {
     ArrayList al = new ArrayList();
     al.add(new Employee(1,"Supreeth",21));
     al.add(new Employee(2,"Vijay",31));
     al.add(new Employee(3,"Ganesh",21));
     al.add(new Employee(4,"Aisu",31));
     al.add(new Employee(5,"Aizzz",41));
     Collections.sort(al);
     Iterator it = al.iterator();
     while(it.hasNext())
     {
        Employee emp = (Employee)it.next();
        System.out.println("Employee name" +emp.name+ "," +emp.age);
     }
  }

}

Please let me how does == operator work

• Im not able to get the logic that is being implemented in the output

Output

Employee name   Aizzz,41
Employee name    Aisu,31
Employee name    Ganesh,21
Employee name    Vijay,31
Employee name    Supreeth,21

Thank You In Advance

Answer 1

The contract of compareTo says:

The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y.

So, when using it like this:

Employee emp = (Employee)obj;
if(age==emp.age)
    return 0;
else
    return -1;

it doesn't work at all, because you may have both

emp1.compareTo(emp2) == -1      // "emp1 should come before emp2"

and

emp2.compareTo(emp1) == -1      // "emp2 should come before emp1"

which is a violation of the contract. This means that "all bets are off" and any method taking advantage of compareTo (such as Collections.sort) has undefined behavior.

You can use == but you'll have to take better care of the != case:

Employee emp = (Employee)obj;
if(age==emp.age)
    return 0;
else if (age < emp.age)
    return -1;
else
    return 1;

A better way however is to do

return Integer.compare(age, emp.age);