find the empty time for reserved classes in java

i try to write a program with java and framework hibernate and web for reservation classes of department the time for reserve must not be limit to slots for example we can reserve class at 8:05 until 9:07. in this program we have defined Time for class that save in a table DB and for each class we have table reserved class that save start Time reserve and End time reserve for each person i have problem to find the empty time for new reservation i don't know how to calculate empty time with defined Time and every start time reserved and End Time reserved !!! i put defined_Time_class and reserved_class

@Id@GeneratedValue(strategy = GenerationType.AUTO)//definedTime_class
private Integer Id;

//private Integer classId;
private Date StartTime;
private Date EndTime;

@OneToOne(cascade = CascadeType.ALL)
private class_Info Class_Info;



public Integer getId() {
    return Id;
}
public void setId(Integer id) {
    Id = id;
}


public class_Info getClass_Info() {
    return Class_Info;
}
public void setClass_Info(class_Info cclass_Info) {
    Class_Info = cclass_Info;
}


public Date getEndTime() {
    return EndTime;
}
public void setEndTime(Date endTime) {
    EndTime = endTime;
}



public Date getStartTime() {
    return StartTime;
}
public void setStartTime(Date sstartTime) {
    StartTime = sstartTime;
}

}

@Id @GeneratedValue(strategy = GenerationType.AUTO)
private Integer Id;

//private Integer classId;
//private Integer UserId;
private boolean confirm;
private Date StartTime_reserved;
private Date EndTime_reserved;
private Date date_reserved;

public reserved_class(){
    confirm = false;
}

@ManyToOne(cascade = CascadeType.ALL)
private class_Info Class_Info;

@ManyToOne(cascade = CascadeType.ALL)
private User_Info user_info;


public Integer getId() {
    return Id;
}
public void setId(Integer id) {
    Id = id;
}

public boolean isConfirm() {
    return confirm;
}
public void setConfirm(boolean confirm) {
    this.confirm = confirm;
}

public User_Info getUser_info() {
    return user_info;
}
public void setUser_info(User_Info user_info) {
    this.user_info = user_info;
}


public class_Info getClass_Info() {
    return Class_Info;
}
public void setClass_Info(class_Info class_Info) {
    Class_Info = class_Info;
}

@Column(name = "date_reserved" , unique = true , nullable = false)
public Date getDate_reserved() {
    return date_reserved;
}
public void setDate_reserved(Date date_reserved) {
    this.date_reserved = date_reserved;
}

@Column(name = "EndTime_reserved" , unique = true , nullable = false)
public Date getEndTime_reserved() {
    return EndTime_reserved;
}
public void setEndTime_reserved(Date endTime_reserved) {
    EndTime_reserved = endTime_reserved;
}


@Column(name = "StartTime_reserved" , unique = true , nullable = false)
public Date getStartTime_reserved() {
    return StartTime_reserved;
}
public void setStartTime_reserved(Date startTime_reserved) {
    StartTime_reserved = startTime_reserved;
}

} `