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.