@Entity
@AccessType("property")
@Table(name = "stats")
@IdClass(StatsPK.class)
public class Stats implements Serializable,Comparable<Stats>{

   private String statType;
   private String paperName;
   private Date beginDate;
   private Date endDate;
   private double value;
   private String timePeriod;
   
   @Id
   public String getStatType() {return statType;}
   public void setStatType(String statType) {this.statType = statType;}
   
   @Id
   public String getPaperName() {return paperName;}
   public void setPaperName(String paperName) {this.paperName = paperName;}
   
   @Id
   public Date getBeginDate() {return beginDate;}
   public void setBeginDate(Date beginDate) {this.beginDate = beginDate;}
   
   @Id
   public Date getEndDate() {return endDate;}
   public void setEndDate(Date endDate) {this.endDate = endDate;}
   
   @Column(name="value")
   public double getValue() {return value;}
   public void setValue(double value) {this.value = value;}
   
   @Column(name="timePeriod")
   public String getTimePeriod() {return timePeriod;}
   public void setTimePeriod(String timePeriod) {this.timePeriod = timePeriod;}
   
   @Override
   public String toString() {
      return this.getStatType() + " : " +this.getPaperName()+" : "+this.getBeginDate() + " : " + this.getEndDate() + " : " + this.getValue();
   }
   public int compareTo(Stats stats) {
      return
         this.getBeginDate().hashCode() - stats.getBeginDate().hashCode() +
         this.getEndDate().hashCode() - stats.getEndDate().hashCode() +
         this.getTimePeriod().hashCode() - stats.getTimePeriod().hashCode() +
         this.getStatType().hashCode() - stats.getStatType().hashCode() ;
         
   }
}

@Embeddable
public class StatsPK implements Serializable{
   
   private String statType;
   private String paperName;
   private Date beginDate;
   private Date endDate;

   
   @Column(name="stattype")
   public String getStatType() {return statType;}
   public void setStatType(String statType) {this.statType = statType;}
   
   @Column(name="papername")
   public String getPaperName() {return paperName;}
   public void setPaperName(String paperName) {this.paperName = paperName;}
   
   @Column(name="begindate")
   public Date getBeginDate() {return beginDate;}
   public void setBeginDate(Date beginDate) {this.beginDate = beginDate;}
   
   @Column(name="enddate")
   public Date getEndDate() {return endDate;}
   public void setEndDate(Date endDate) {this.endDate = endDate;}
   
   @Override
   public int hashCode() {
      return this.paperName.hashCode()
               +this.statType.hashCode()
               +this.beginDate.hashCode()
               +this.endDate.hashCode();
   }
   @Override
   public boolean equals(Object other) {
      if ((other==null)||!(other instanceof StatsPK ))return false;
      StatsPK statsPK = (StatsPK)other;
      return (      
               this.paperName.equals(statsPK.paperName) &&
               this.statType.equals(statsPK.statType) &&
               this.beginDate.getTime()==statsPK.beginDate.getTime() &&
               this.endDate.getTime()==statsPK.endDate.getTime()
            );
   }
   

org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session