clarification needed for " overriding equals method and hashtable key "

[prathibha ananthapadmanabha]

Hi All ,

could anyone please explain me of what it means by

" If you dont override the equals method , you wont be able to use the object as a key in the hash table " .

Though I have gone through the explanation given in scjp 1.4 ( K&B ) certification book , Iam not quite clear on this one . It will be a great help if anyone explain this more elaborately .

Thanks in Advance.
Regards,
Prathibha.

[Priyam Srivastava]

Originally posted by prathibha ananthapadmanabha:
Hi All ,

" If you dont override the equals method , you wont be able to use the object as a key in the hash table " .

if you don't override the equals(), then the Object class equals() will be called..and Object class equals() only returns true if the two reference variable refer to the same object.

Ex:

class Car {
      int vin;
      public Car(int vin) {
        this.vin = vin;
      }
 
   /*   public boolean equals(Object obj) {  //line 1
        if((obj instanceof Car) && (((Car)obj).vin==this.vin)) {
           return true;
        else
           return false;
      }*/
}
 
class Test {
  public static void main(String[] args) {
    Car obj1 = new Car(989);
    Car obj2 = new Car(989);
    System.out.println(obj2.equals(obj1));
  }
}

If you decide that two car object are equal if they have the same vin then you have to override the equals(), because if you don't then when you will call the equals() then the Object class equals() will only check for references.

Hence when we comment the equals() at line 1 then we get false as the Object class equals() check whether obj1 and obj2 refers to the same object or not.

But on uncommenting we get true because the overriden equals() gets called.

So in case of HashTable if you don't override equals() in your class, then there is no way your two objects created using new operator will be considered meaningfully equal. Hence you cannot use them as key in HashTable i.e on commenting the equals() in the above example the Car object cannot be used as key because they are never meaningfully equal.
[ September 03, 2007: Message edited by: Priyam Srivastava ]