Overriding hash code and equals in collections

package SDMXDatabaseUploader;

import java.util.HashMap;


public class SDMXSeries {
 public HashMap SeriesKey = new HashMap();

 public HashMap Obs = new HashMap();
 /**
  * Since SeriesKeys have to be unique this is 
  * what defines if an SDMXSeries equals another.
  */
 
 public boolean equals(Object o){
  SDMXSeries comparison = (SDMXSeries)o;
  return SeriesKey.equals(comparison.SeriesKey);

 }
 /**
  * 
  */
 public int hashCode(){
  return SeriesKey.hashCode();

 }

 /**
  * 
  * @param key
  * @param value
  * @throws NullPointerException if key or value is null.
  * @throws DuplicateKeyException if key is repeated.
  */
 public void addSeriesKeyValue(String key, String value)throws NullPointerException, DuplicateKeyException{

  if (key == null || value == null)throw new NullPointerException("Key or Value is null");
  if (SeriesKey.containsKey(key))throw new DuplicateKeyException("Failed trying to add a duplicate key to the series key" + key);
  SeriesKey.put(key, value);

 }
 /**
  * 
  * @param key
  * @param value
  * @throws NullPointerException if key or value is null.
  * @throws DuplicateKeyException if key is repeated.
  */
 public void addObsKeyValue(String key, String value)throws NullPointerException, DuplicateKeyException{

  if (key == null || value == null)throw new NullPointerException("Key or Value is null");
  if (Obs.containsKey(key))throw new DuplicateKeyException("Failed Trying to add a duplicate key to the series key" + key);
  Obs.put(key, value);
 }
 public String toString(){
  return SeriesKey.toString() + " " + Obs.toString();

 }
}

Exception code...

package SDMXDatabaseUploader;

public class DuplicateKeyException extends Exception {

 /**
  * 
  */
 private static final long serialVersionUID = 1L;
public DuplicateKeyException (String message){
 super(message);
}
public DuplicateKeyException (){
super("Failed attempting to add duplicate key."); 

}

}

package SDMXDatabaseUploader;

public class EmptyValueException extends Exception {
 public EmptyValueException (int lineNumber){
  super("Processed Line "+ lineNumber +"has an empty string for one or more entries.");
 }

}