Finding Entities Using the EntityManager

The EntityManager.find method is used to look up entities in the data store by the entity’s primary key.

@PersistenceContext
EntityManager em;
public void enterOrder(int custID, Order newOrder) {
    Customer cust = em.find(Customer.class, custID);
    cust.getOrders().add(newOrder);
    newOrder.setCustomer(cust);
}