JPA many to many Need help

[notionDesign]

I have been reading and watching tons of tutorials and just can't seem to get the many to many relationship to work. I have built a new project to simplify the project down to one many to many relationship. Please take a look at the code below and lend some suggestions as to why this fails. Currently I get a null pointer at the *keywd.getMaterialRecordList().add(record); line. If I comment this out then I get the same null pointer on the next .get....add();. If it matters I have abandoned Derby and moved to an H2 database.

    @Entity
    @Table(name = "MATERIAL_RECORD")
    @XmlRootElement
    @NamedQueries({
    @NamedQuery(name = "MaterialRecord.findAll", query = "SELECT m FROM MaterialRecord m")})
    public class MaterialRecord implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "ID")
    private Integer id;
    @Column(name = "SDS_NUMBER")
    private Integer sdsNumber;
    @Column(name = "PRODUCT_NAME")
    private String productName;
    
    @ManyToMany(mappedBy = "materialRecordList")
    private List<Keywords> keywordsList;

--------------------------------------------------------

    @Entity
    @Table(name = "KEYWORDS")
    @XmlRootElement
    @NamedQueries({
    @NamedQuery(name = "Keywords.findAll", query = "SELECT k FROM Keywords k")})
    public class Keywords implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "KEY_ID")
    private Integer keyId;
    @Column(name = "KEYWORD_NAME")
    private String keywordName;
    @JoinTable(name = "KEYWORD_LOOKUP", joinColumns = {
        @JoinColumn(name = "KEY_ID", referencedColumnName = "KEY_ID")}, inverseJoinColumns = {
        @JoinColumn(name = "SDS_NUMBER", referencedColumnName = "SDS_NUMBER")})
    @ManyToMany
    private List<MaterialRecord> materialRecordList;

------------------------------------------------------------------------------

    public class JpaTest {
    public static void main (String [] args){
       MaterialRecord record = new MaterialRecord();
       record.setProductName("oofbar");
       
       Keywords keywd = new Keywords();
       keywd.setKeywordName("testing");
       
       Keywords keywd2 = new Keywords();
       keywd2.setKeywordName("testing2");
       
       record.getKeywordsList().add(keywd);
       record.getKeywordsList().add(keywd2);
       
       keywd.getMaterialRecordList().add(record);
       keywd2.getMaterialRecordList().add(record);
       
       EntityManagerFactory emf = Persistence.createEntityManagerFactory("JavaApplication26PU");
       
       EntityManager em = emf.createEntityManager();
       EntityTransaction tx = em.getTransaction();
       tx.begin();
       em.persist(record);
       em.persist(keywd);
       em.persist(keywd2);
       tx.commit();

[gimbal2]

But that has nothing to do with JPA, you're simply not initializing the list. Of course you get an NPE then when you try to call add() on a null-reference.

[notionDesign]

gimbal,
Thanks for your reply. I have been struggling trying to implement this many-to-many and thats why I posted here. I thought that when I call "Keywords keywd = new Keywords();
keywd.setKeywordName("testing");" this initialized the variable. Would you be so kind as to give me an example of how I would initialize the list in this situation? It would be greatly appreciated! Thanks in advance

[gimbal2]

Okay, let me make it a little more "I guess this person has very little experience with Java and is biting off more than he can chew":

private List<MaterialRecord> materialRecordList; // <--- this variable is null

Now if you don't know how you can make sure that it is not null, my suggestion is to return to a study of the basics of Java and forget about JPA for now.