How to cast a list in inheritance

For Example:
public class Person {};
public class Student extends Person{};
List<Person> persons=new ArrayList();
List<Student> students=new ArrayList();

persons=students;

how can you assign students to persons? the last line will throw an error.

Example of Inheritance in EJB

table structure

create table "APP".PERSON
(
 ID INTEGER not null primary key,
 NAME VARCHAR(256),
 ADDRESS VARCHAR(256),
 PHONE VARCHAR(256),
 EMAIL VARCHAR(256),
 TYPE VARCHAR(256)
)



person.java

import javax.persistence.*;



@Entity
@Table(name = "PERSON")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "TYPE", discriminatorType = DiscriminatorType.STRING,length=1)
public class Person implements java.io.Serializable {

@Id
@Column(name = "ID")
private int id;
@Column(name = "NAME")
private String name;
@Column(name = "ADDRESS")
private String address;
@Column(name = "PHONE")
private String phone;
@Column(name = "EMAIL")
private String email;
@Column(name = "TYPE")
private String type;

public Person() {
}
public int getId() { return id; }
public void setId(int id) { this.id = id; }
public String getName() { return name; }
public void setName(String name) { this.name = name; }
public String getAddress() { return address; }
public void setAddress(String address) { this.address = address; }
public String getPhone() { return phone; }
public void setPhone(String phone) { this.phone = phone; }
public String getEmail() { return email; }
public void setEmail(String email) { this.email = email; }
public String getType() { return type; }
public void setType(String type) { this.type = type; }
}

student.java

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
@Entity
@DiscriminatorValue(value="S")
public class Student extends Person {
    public Student() {
    }
}

employee.java

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;@Entity
@DiscriminatorValue(value="E")
public class Employee extends Person{
    public Employee() {
    }
} 

How to get max value for identity column in EJB-QL

For example,
I have a table Bookorder  with an Identity column Id.

I can execute the following:
Query query1 = em.createQuery("SELECT MAX(b.id) FROM Bookorder b");       
int bo_max=(Integer)query1.getSingleResult();

Please notice that it's Integer and not int when casting the result of query.

Get Auto Increment Id after persist

If you have entity marked with annotations @Id and @GeneratedValue(strategy = GenerationType.AUTO), you want get the Id after persist(), then use em.flush().

public int addEmployer(String name) {
        Employer e1 = new Employer();       
        e1.setName(name);
        em.persist(e1);
        em.flush();       
        return e1.getId();
    }

How to persist entity with auto increment Id

when create new entity class from database, if the primary key (for example "Id") with auto increment get annotation @NotNull. At this time, if you use entity manager try to persist an entity, neither set the Id value nor automaticly generate the Id.

Foe Example:

Employer e1 = new Employer();       
e1.setName(name);
em.persist(e1);

The above will get error

In the Employer entity class

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@NotNull.-----------------------------------------------------Delete it!!
@Basic(optional = false)
@Column(name = "ID")
private Integer id;

You will  fix the problem.

Id Auto Increment in Java DB

Using Database Table grab structure  can get the following:

create table "APP".EMPLOYER
(
 ID INTEGER default AUTOINCREMENT: start 1 increment 1 not null primary key,
 NAME VARCHAR(256)
);

but  this can never be run in the SQL command window
To fix the problem, change the above to the following:

CREATE TABLE "APP"."EMPLOYER"
("ID" INT not null primary key         GENERATED ALWAYS AS IDENTITY         (START WITH 1, INCREMENT BY 1),  
"NAME" VARCHAR(256));