DAO

package com.emp.dao.respository;

import java.util.Collection;

import com.emp.entity.GenericEntity;

public interface GenericRespository {

    public void persist(A e) throws Exception;
    public void remove(A e) throws Exception;
    public void update(A e) throws Exception;
    public A find(String id) throws Exception;
    public Collection findAll() throws Exception;
   
}

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package com.emp.dao.respository;
//All the methods going inside the EmployeeRespository
public interface GenericEmployeeRespository {

}
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package com.emp.dao.respository;

import java.util.Collection;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import javax.persistence.PersistenceException;
import javax.persistence.Query;

import com.emp.entity.GenericEntity;
import com.emp.entity.MyEntityManager;

public class Respository implements GenericRespository
{

    protected EntityManager em;
    private Class persistentClass;
   
    public Respository(EntityManager em, Class persistentClass)
    {
        MyEntityManager e= new MyEntityManager();
        this.em = e.getEm();
        this.persistentClass = persistentClass;
    }
   


   
    @Override
    public final void persist(A e) throws Exception
    {
                em.persist(e);
    }

    @Override
    public final void remove(A e) throws Exception{
        em.remove(e);
    }

    @Override
    public final void update(A e) throws Exception{
       
    }

    @Override
    public final A find(String id) throws Exception{
        em.find(persistentClass, id);
        return null;
    }

    @Override
    public final Collection findAll() throws Exception{
       
        Query query = em.createQuery("SELECT emp FROM "+ persistentClass.getSimpleName() + " AS emp");
        Collection result = query.getResultList();
       
        return result;
    }

    public Class getPersistentClass() {
        return persistentClass;
    }

    public void setPersistentClass(Class persistentClass) {
        this.persistentClass = persistentClass;
    }

}
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package com.emp.dao.respository;

import javax.persistence.EntityManager;

import com.emp.entity.Employee;

public class EmployeeRespository extends Respository
{

    public EmployeeRespository(EntityManager entityManager)
      {
        super(entityManager, Employee.class);
      }
}
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package com.emp.dao.respository;
//DaoRespository types to be created

public enum DaoRespositoryTypes {

    EMPLOYEE,MANAGER,DEPARTMENT;
}
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package com.emp.dao.respository;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import com.emp.entity.GenericEntity;

//this is singleton object to create respositories....So entity manager becomes singleton as well

public class DaoRespositoryFactory {

    private EntityManager em;
   
    public  Respository createDaoImpl(DaoRespositoryTypes type) throws Exception
    {
        Respository  instance = null;
        EntityManagerFactory factoty = Persistence.createEntityManagerFactory("myp");
        this.em = factoty.createEntityManager();
       
        switch (type) {
        case EMPLOYEE:
            instance = new EmployeeRespository(em);
            break;
        case MANAGER:
            instance = new MangerRespository(em);
            break;
           
        case DEPARTMENT:
            instance = new DepartmentRespository(em);
            break;

        default:
            throw new Exception("There is no dao type assign...");
           
        }
       
        return instance;
    }


   
}