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//First load the Person object that needs to be updated     
Person person = em.load(Person.class, 5629499534213120L);

//Make changes to the Person object 
person.setLastName("Smith");
person.setCitizen(false);

//Update the datastore. 
em.update(person);

//Reload the person object
Person updatedPerson = em.load(Person.class, 5629499534213120L);
System.out.printf("First Name: %s; Last Name: %s; Birth Year: %d; Citizen: %s\n",
        updatedPerson.getFirstName(), updatedPerson.getLastName(), updatedPerson.getBirthYear(),
        updatedPerson.isCitizen());