Simple generic DAO structure for Java. Note that the interface takes two generic parameters, one for the primary key type, and the second for the bean type, which uses the same primary key. This enables you to use any kind of object as a primary key, making your code very flexible. This is a good behavior especially if you use JPA embedded ids (…

Bean.java

@MappedSuperclass
public abstract class Bean<PK extends Serializable> implements Serializable {

	private static final long serialVersionUID = 1L;

	@Version
	@Column(name = "VERSION", nullable = false)
	private Long version;

    public Bean(PK id, Long version) {
		this.version = version;
	}

	public Bean() {
	}

    public abstract PK getId();

    public abstract void setId(PK id);

	public Long getVersion() {
		return version;
	}

	public void setVersion(Long version) {
		this.version = version;
	}

	@Override
	public String toString() {
        return "Bean [id=" + getId() + ", version=" + version + "]";
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
        result = prime * result + ((getId() == null) ? 0 : getId().hashCode());
		result = prime * result + ((version == null) ? 0 : version.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
        Bean<PK> other = (Bean<PK>) obj;
        if (getId() == null) {
            if (other.getId() != null)
				return false;
        } else if (!getId().equals(other.getId()))
			return false;
		if (version == null) {
			if (other.version != null)
				return false;
		} else if (!version.equals(other.version))
			return false;
		return true;
	}
}

DAO.java

public interface DAO<PK extends Serializable, T extends Bean<PK>> {

    /**
     * Persist the given entity into through EntityManager.
     * 
     * @param t
     *            entity to be saved.
     */
    public T save(T t);

    /**
     * Find all items of this type in the database.
     * 
     * @return a List of T elements from database.
     */
    public List<T> findAll();

    /**
     * Find an item from database based on its ID.
     * 
     * @param id
     *            to look for.
     * @return found entity or null if no entity is found.
     */
    public T find(PK id);

    /**
     * Delete the item from database.
     * 
     * @param t
     *            item to delete.
     */
    public void remove(T t);
}

GenericDAO.java

public class GenericDAO<PK extends Serializable, T extends Bean<PK>> implements DAO<PK, T> {

	protected EntityManager em;
	private final Class<T> clazz;

	public GenericDAO(Class<T> clazz, EntityManager em) {
		super();
		this.clazz = clazz;
		this.em = em;
	}

	@Override
	public T save(T t) {
		if (t.getId() != null) {
			t = update(t);
		} else {
			persist(t);
		}
		return t;
	}

	@Override
	public List<T> findAll() {
		TypedQuery<T> q = em.createQuery(
				"select e from " + clazz.getSimpleName() + " e", clazz);
		return q.getResultList();
	}

	@Override
    public T find(PK id) {
		return em.getReference(clazz, id);
	}

	@Override
	public void remove(T t) {
		em.remove(t);
	}

	/**
	 * Persist new itens in database.
	 * 
	 * @param t
	 * @return
	 */
	protected T persist(T t) {
		em.persist(t);
		return t;
	}

	/**
	 * Update itens on database.
	 * 
	 * @param t
	 * @return
	 */
	protected T update(T t) {
		return em.merge(t);
	}

}