how to override the hashCode() method ? and What is the advantage of overide the hashcode().

It is not always necessary to override hashcode and equals. But if you think you need to override one, then you need to override both of them.

   @Override
   public int hashCode() {
       final int PRIME = 31;
       int result = 1;
       result = PRIME * result + number;
       return result;
   }

       @Override
       public boolean equals(Object obj) {
       if (this == obj)
           return true;
       if (obj == null)
           return false;
       if (getClass() != obj.getClass())
           return false;
       final Scrap other = (Scrap) obj;
       if (number != other.number)
           return false;
       return true;
   }

If you do not override them you have to use the default implentation in Object.