compareTo and equals methods consistency

[Rashid Mian]

Can any body describe with example the following text taken by
Compareable Interface:
"
It is strongly recommended (though not required) that natural orderings be consistent with equals. This is so because sorted sets (and sorted maps) without explicit comparators behave "strangely" when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract
for set (or map), which is defined in terms of the equals method.
"

[Srinivasan thoyyeti]

Hi,
Fantastic question.
After seeing your question i have gone through API.
I am very to see this question. Thanks.

This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's natural ordering, and the class's compareTo method is referred to as its natural comparison method.

The natural ordering for a class C is said to be consistent with equals if and only if e1.compareTo(e2) == 0 has the same boolean value as e1.equals(e2) for every e1 and e2 of class C. Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

[Anupam Sinha]

Adding a new Float(-0.0F) and new Float(0.0F) to HashSet would result in two values being added. Whereas on a TreeSet only one would be added. This is so because the equals would return false for the two objects whereas the compareTo would return true. Maybe an API reading would help you on this. Effective Java by Joshua Bloch is also a great read.

[Srinivasan thoyyeti]

Hi Rashid,
I have compliled a sample program just for you. because i like your question.
Here in program you will find compateTo inconsistent with equals definition.

import java.util.*;
 
class Employee implements Comparable<Employee>{
 
      String name;
      Double sal;
 
      Employee(String n,double d){
           name = n; sal = d;
     }
 
     public int compareTo(Employee o){
     
         return sal.compareTo(o.sal);
     }
 
     public  boolean equals(Object o){
          if( o instanceof Employee ){ 
      
               Employee e = (Employee)o;
               
               if( name.equals( e.name ) && sal == e.sal ){
                   return true;
               } 
         }         
         return false;
    }
 
    public int hashCode(){
 
    return (name.hashCode() + sal.hashCode()) % 97; 
 
    }
    public String toString(){
          return ("("+name +","+ sal +")"); 
    }
}
 
public class TestInconsistancy{
 
  public static void main(String... a){
    
     Employee e1 = new Employee("Kathy Sierra", 99999.99);
     Employee e2 = new Employee("Bert Bates", 99999.99);
 
     Set s = new HashSet();
     //equals and hascode were coded according to the contract. So no problem in adding them
    // See equals and hasCode considering both the instance variables.
     s.add(e1); 
     s.add(e2);
 
     System.out.println(s);  
 
    Set s1 = new TreeSet();
    s1.add(e1); // Kathy will only added
    s1.add(e2); // Bert can't be added due to compareTo def --> sals equal; So it will say sorry you are duplicate 
 
    System.out.println(s1);           
  
 
  }
 
}  

Khudaa haafiz,

[ February 27, 2007: Message edited by: Srinivasan thoyyeti ]

[Rashid Mian]

Thanks for your help.
You have done it and done it in BIG WAY.
I will be posting such questions in coming days, so please give them special attention.
ALLAH HAFIZ

[Rashid Mian]

Why It is generally a good idea for comparators to implement java.io.Serializable?

[Srinivasan thoyyeti]

Hi Rashid,
Go Ahead. Bring the best out of you.
I will try my level best to support you.
All the best

[Srinivasan thoyyeti]

Hi Rashid,
You are doing the Good thing by going through the Collection API.

Here comes another Biggy for your question. You can easily go through the code. I have taken a TreeSet(Comparator) where comparator is not serializable.
Then point is After deserialization TreeSet Object, if you try to add anyhing it throws "Comparator is not serializble" thats why it has to be Serializable.

import java.util.*;
import java.io.*;
 
public class EmployeeSalReport implements Serializable {
  Set<Employee> s;
 
  EmployeeSalReport (){
        
       s = new TreeSet(new EmployeeSalComparator());
  }
 
  public static void main(String... a){
            new EmployeeSalReport().sendRetriveStuff();
            
  }
  public void sendRetriveStuff(){
 
            s.add(new Employee("Kathy",9999));
            s.add(new Employee("Bert",8888));
            s.add(new Employee("Monica",8500));
            s.add(new Employee("Clinton",10000));
 
           System.out.println("Before Serialyzation"+s);  
           try{
           ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("EmployeeBundle.dat"));
           oos.writeObject(s);
           oos.close();
 
           ObjectInputStream ois = new ObjectInputStream(new FileInputStream("EmployeeBundle.dat"));            
           s = (Set<Employee> :wink: ois.readObject();
           // Here is the BIG one, which will through out an IMP error. See and enjoy.
           s.add(new Employee("Srinivasan",12000));
 
           ois.close();
           }catch(Exception e) { e.printStackTrace(); }
           System.out.println("After De-Serialyzation"+s);
  }
 
}
 
class EmployeeSalComparator implements Comparator<Employee>{
 
public int compare(Employee e1,Employee e2){
   return e2.sal.compareTo(e1.sal);
}
 
}
 
class Employee  {
 
      String name;
      Double sal;
 
      Employee(String n,double d){
           name = n; sal = d;
     }
      public String toString(){
          return ("("+name +","+ sal +")"); 
     }
 
 }      

Lets Rock

[Rashid Mian]

Thanks, Your examples have jardown the in-depth detailed knowledge of "What I was looking for" in my mind.

[Rashid Mian]

I replaced Comparator defination with this:

class EmployeeSalComparator implements Comparator<Employee>,Serializable {

Even then it is giving same old exception. One other thing annoying for me is WHY that print statment, after the catch block is being executed( when exception is thrown?)
Please Answer both of these questions.

[Rashid Mian]

I replaced Comparator defination with this:

class EmployeeSalComparator implements Comparator<Employee>,Serializable {

Even then it is giving same old exception. One other thing annoying for me is WHY that print statment, after the catch block is being executed( when exception is thrown?)
Please Answer both of these questions.

[Nitesh Kant]

In the posted code Employee is not serializable. Since, the Set contains an Employee object, so it needs to be serializable.
Both the comparator and the Employee classes have to be serializable, else you will get the NotSerializableException.
The print statement will always be printed because from the catch block, its not throwing the exception again

[Nitesh Kant]

Moreover, it is not about the comparators having to be serializable, its the knowledge about the objects you are serializing.
Since TreeSet uses a TreeMap and the TreeMap stores the Comparator object, so your comparator object needs to be serializable.
If you notice the exception occurs during serialization and not during the add of the element.
Any object you serialize, all the non-transient and non-static fields should be serializable.

[Srinivasan thoyyeti]

Hi Rashid,

You must be in a position to correct those little errors,like Employee must be serializable. Just i want to show you how comparator exception is thrown.

Practise more.

[Srinivasan thoyyeti]

Originally posted by Srinivasan thoyyeti:
Hi Rashid,

You must be in a position to correct those little errors,like Employee must be serializable. Just i want to show you how comparator exception is thrown.

Nitesh Thanks for your corrections. Now i am also clear why exactly it has to be serializable.

Practise more.