Overriding hashcode and equals in java -

i unclear use of hashcode , equals method in java.i have following query

first

if override equals method objects of value fred added though hashset implements set interface,which cant take repeated values.

second if override both equals , hashcode 1 object added hashset.why?

third if implement equals in case removing 1 fred object remove all?

class person {     string name;     person(string name) {         this.name=name;     }      @override     public boolean equals(object obj) {         if(!(obj instanceof person))         {        return false;          }     person p = (person)obj;     return p.name.equals(this.name);        }       /*@override     public int hashcode() {         return name.hashcode();     }*/ }  public class hashsetdemo {      /*      * @param args      */     public static void main(string[] args) {         // todo auto-generated method stub         hashset<person> s= new hashset<person>();         s.add(new person("fred"));         s.add(new person("fred"));         s.add(new person("fred"));         s.add(new person("fred1"));         for(person a:s) {             system.out.println(a.name);         }          s.remove(new person("fred"));         system.out.println(s);     } } 

from "effective java" joshua bloch, item 9:

you must override hashcode in every class overrides equals. failure result in violation of general contract object.hashcode, prevent class functioning in conjunction hash-based collections, including hashmap, hashset, , hashtable.

the eclipse ide generate hashcode , equals method you.

most hashcode methods this:

public int hashcode() {     int result = 17;     result = 31 * result + field1;   (int)     result = 31 * result + field2.hashcode();   (object)     ... rest of fields.     return result; }