问不能转换为java.lang.String异常

EN

@Entity
@Table(name = "RateTransmissionDetails")
public class RateTransmissionDetails implements java.io.Serializable {

    private String spaceNum;
    private Integer rateInCent;


    public RateTransmissionDetails() {
    }
    public RateTransmissionDetails(String spaceNum,Integer rateInCent) {

        this.spaceNum=spaceNum;
        this.rateInCent=rateInCent;
    } 


    @Column(name = "SpaceNum", nullable = false)
    public String getSpaceNum() {
        return this.spaceNum;
    }

    public void setSpaceNum(String spaceNum) {
        this.spaceNum = spaceNum;
    }

    @Column(name = "RateInCent", nullable = false)
    public Integer getRateInCent() {
        return this.rateInCent;
    }

    public void setRateInCent(Integer rateInCent) {
        this.rateInCent = rateInCent;
    }

Set<RateTransmissionDetails> rateTrmDtls = new HashSet<RateTransmissionDetails>();
rateTrmDtls.add((new RateTransmissionDetails("A",new Integer(20)));