what It means if you don't override equals()

import java.util.*;
class car
{
String vin;
car()
{

}
car(String v)
 {
vin = v;
}
public boolean equals(Object o)
{

if((o instanceof car) && (((car)o).vin==(vin)))
{

return true;
}
else
{

return false;
}
}
public String toString()
{
return vin;
}
public int hashCode()
{
System.out.println("inside hascode");
System.out.println("vin length()" +vin.length());
return vin.length();
}

}
class person 
{

String ownername;
person(String n)
{
ownername = n;

}
public String toString()
{
return ownername;
}
}
public class carperson 
{
 
 public static void main(String [] args)
 {
 HashMap <car,person> Hm = new HashMap<car,person>();
 car carobj1 = new car("V001Lexus");
 car carobj2 = new car("V002Honda");
 car carobj3 = new car("V003suburau");
 car carobj4 = new car("V004maruti");
 car carobj5 = new car("V005Bmw");

person perobj1= new person("gmu");
          person perobj2 = new person("prabhu");
  person perobj3 = new person("ashwitha");
  person perobj4 = new person("sangi");
  person perobj5 = new person("saritha");

Hm.put(carobj1, perobj1);
 Hm.put(carobj2, perobj1);
 Hm.put(carobj3, perobj3);
 Hm.put(carobj4, perobj4);
 Hm.put(carobj5, perobj5);
 
         car carobj6 = new car("V001Lexus");
 car carobj7 = new car("V002Honda");
 System.out.println(Hm.get(carobj6));
 System.out.println(Hm.get(carobj7));
         System.out.println(Hm.get(carobj1));
System.out.println( Hm.get(carobj2));
System.out.println(Hm.get(carobj3));
System.out.println( Hm.get(carobj4));
System.out.println( Hm.get(carobj5));
 }
 
}

I have a doubt in scjp pg 525(K&B)
The above code is the example for car/owner and equals(),hashcode().

question 1: for search I will be in trouble unless i still have a
reference to the exact object used as the key when added it to the
collection(may i know which object is that from the above code)

question 2: one instance added it to collection, essentially re-create
an identical instance to search based on the object as key.
does it mean like this?
car carobj1 = new car("V001Lexus");
Hm.put(carobj1, perobj1);
car carobj8 = new car("V001Lexus");
Hm.get(carobj8);

question 3: more than one object representing a single unique car can
exist( does this sentence mean this?)

Hm.put(carobj1,perobj1);
Hm.put(carobj1,perobj2);
Hm.put(carobj1,perobj3);
Hm.put(carobj1,perobj4);

which is something like overwriting different owner to the
same car

is my understanding right?

Thanks for your patience and valuable time for replying.

samura

If this link may help you!

Thanks,

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