hashtable working & overriding equals()

hi friends,
thanx for the tremendous response for clearing my earlier doubt in string-pool.expecting similar responses from u all,i m posting yet another question and its regarding hashtable working.

we know that hashtable stores values as key/value pairs,where both key and value should b an object.

considering this code,

class car
{
int vehnum;
int yearofmake;

public static void main(String args[])
{
car ob1=new car();
car ob2=new car();

Hashtable hs=new Hashtable();

hs.put(ob1,"Raja");
hs.put(ob2,"Alps");
hs.put(ob1,"Rahi");

String s=(String)hs.get(ob1);
System.out.println(s);
}//end of main

}// end of class car

now the o/p obtained is Rahi.i understand why is it so.as i have reffered the key ob1 for Rahi at last.

i want to know how the hashtable retreives the data,and how can i override equals() method,so that key/value pair is unique.

please give thw way as how i should override the equals() method to make my code more efficient.

Please don't post duplicate threads. It just wastes people's time if they answer a question that's already been answered elsewhere. I'm closing this one.