Error 613 - i cant put a list<Athlete> into a Squad object

hey guys

Im a swiss student and my english is not that good and ive never posted in your forum, so thats why im sorry if im doing any mistakes, but i have big trouble, because ive to get this straight till end of week. i uploaded also all classes but if u see the problem in the main problem field the classes are copied after the problem description.

the structure:

the Squad class has a Collection which stores Athlete Objects, the Athlet has another embeddedCollection of Marks and another embedded Association, furthermore the athlete is inherited by a Person. 

the problem:

I tested the hole Athlete structure and it works, but if i want to extend my program with putting the Athletes into a squad it brings up an exception. so the problematic line would u find int the main class line number 19 squad.setAthlets(x);

my Exception:

Exception in thread "main" [ObjectDB 2.2.0] javax.persistence.RollbackException
Failed to commit transaction: Failed to write the value of field test.Squad.athlets using reflection: Attempt to persist a reference to a non managed 'test.Athlet' instance (error 613)
at com.objectdb.jpa.EMImpl.commit(EMImpl.java:259)
at test.Main.main(Main.java:25)
Caused by: com.objectdb.o._PersistenceException: Failed to write the value of field test.Squad.athlets using reflection: Attempt to persist a reference to a non managed 'test.Athlet' instance
at com.objectdb.o.JPE.g(JPE.java:138)
at com.objectdb.o.ERR.f(ERR.java:59)
at com.objectdb.o.OBM.bF(OBM.java:827)
at com.objectdb.o.OBM.bD(OBM.java:711)
at com.objectdb.jpa.EMImpl.commit(EMImpl.java:256)
... 1 more
Caused by: com.objectdb.o.UserException: Failed to write the value of field test.Squad.athlets using reflection: Attempt to persist a reference to a non managed 'test.Athlet' instance
at com.objectdb.o.MSG.d(MSG.java:74)
at com.objectdb.o.UMR.M(UMR.java:834)
at com.objectdb.o.UMR.x(UMR.java:527)
at com.objectdb.o.UML.u(UML.java:517)
at com.objectdb.o.MMM.ae(MMM.java:1006)
at com.objectdb.o.UTY.aF(UTY.java:1181)
at com.objectdb.o.UTY.aE(UTY.java:1170)
at com.objectdb.o.ENH.a(ENH.java:46)
at com.objectdb.o.STA.T(STA.java:512)
at com.objectdb.o.STM.C(STM.java:386)
at com.objectdb.o.OBM.bG(OBM.java:876)
at com.objectdb.jdo.PMImpl.bG(PMImpl.java:2185)
at com.objectdb.o.OBM.bF(OBM.java:792)
... 3 more
Caused by: com.objectdb.o.UserException: Attempt to persist a reference to a non managed 'test.Athlet' instance
at com.objectdb.o.MSG.d(MSG.java:61)
at com.objectdb.o.TYW.writeElement(TYW.java:246)
at com.objectdb.o.ABT.writeArray(ABT.java:527)
at com.objectdb.o.CLT.writeStrictly(CLT.java:187)
at com.objectdb.o.TYW.ar(TYW.java:382)
at com.objectdb.o.TYW.au(TYW.java:431)
at com.objectdb.o.TYW.writeElement(TYW.java:259)
at com.objectdb.o.UMR$P.y(UMR.java:902)
at com.objectdb.o.UMR.x(UMR.java:524)
... 13 more

that are my classes:

first the Squad class:

package test;

import java.util.LinkedList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.OneToMany;


@Entity
public class Squad {
    private int squadId;
    @OneToMany()
    private List<Athlet> athlets;

    public Squad() {
        super();
        // TODO Auto-generated constructor stub
    }

    public Squad(int squadId) {
        super();
        this.squadId = squadId;
        athlets = new LinkedList<Athlet>();
    }

    public void addAthlete(Athlet athlet) {
        athlets.add(athlet);
    }

    public int getSquadId() {
        return squadId;
    }

    public void setSquadId(int squadId) {
        this.squadId = squadId;
    }

    public List<Athlet> getAthlets() {
        return athlets;
    }

    public void setAthlets(Athlet athlet) {
        this.athlets.add(athlet);
    }
}

Now the Athlete Class

package test;

import java.io.Serializable;
import java.util.HashMap;
import java.util.Map;

import javax.persistence.Embedded;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;


@Entity
public class Athlet extends Person implements Serializable {

    /**
      *
      */
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue
    private long id;
    private int squadId;
    private int athletId;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    private String prgClass;
    private int yearOfBirth;
    @OneToMany
    private Map<DeviceType, Mark> marks;
    @Embedded
    private Association association;

    public Athlet() {
        super("a", "b", "c");
    }

    public Athlet(int squadID, int athletID, String prgClass, String vorname,
       String name, String adresse, int yearOfBirth, Association verein) {
        super(name, vorname, adresse);
        this.squadId = squadID;
        this.athletId = athletID;
        this.prgClass = prgClass;
        this.yearOfBirth = yearOfBirth;
        this.association = verein;
        marks = new HashMap<DeviceType, Mark>();
        marks.put(DeviceType.FLOOR_EXCERCISE, new Mark(4, 3, 4, 3, 4, 3, 4));
    }

    Athlet(int x, int y, String c) {
        super("a", "b", c);
        this.squadId = x;
        this.athletId = y;
        marks = new HashMap<DeviceType, Mark>();
        marks.put(DeviceType.FLOOR_EXCERCISE, new Mark(4, 3, 4, 3, 4, 3, 4));
        association = new Association();
    }

    public int getSquadId() {
        return squadId;
    }

    public void setSquadId(int squadId) {
        this.squadId = squadId;
    }

    public int getAthletId() {
        return athletId;
    }

    public void setAthletId(int athletId) {
        this.athletId = athletId;
    }

    public String getPrgClass() {
        return prgClass;
    }

    public void setPrgClass(String prgClass) {
        this.prgClass = prgClass;
    }

    public int getYearOfBirth() {
        return yearOfBirth;
    }

    public void setYearOfBirth(int yearOfBirth) {
    this.yearOfBirth = yearOfBirth;
    }

    @Override
    public String toString() {
        return String.format("(%d, %d)", this.squadId, this.athletId);
    }
}

Thanks for ur help

The exception indicates:

Caused by: com.objectdb.o.UserException:
Attempt to persist a reference to a non managed 'test.Athlet' instance

On commit, an attempt to store the Squad instance fails because it references a non managed Athlet instance.

You must make sure that on commit all the objects that are reachable from objects that you persist are also persistent.

Any of the following actions should solve the problem:

• Persist the referenced Athlet instance explicitly before commit.
• Annotate the athletes collection using @OneToMany(cascade=CascadeType.PERSIST).
• Annotate the athletes collection using @OneToMany(cascade=CascadeType.ALL).
• Set global automatic cascading persist.

See the Storing JPA Entity Objects page in the ObjectDB manual for more details.