JPA Inheritance & Cascading

📋 What You'll Build

✅ Pre-flight Checklist

• NetBeans IDE is open
• MySQL Server is running (check Services panel)
• You can connect to MySQL via NetBeans
• Your target database exists (e.g., week3db)
• You reviewed the Week 3 lecture slides
• You know how to create a JPA project (Week 1 Lab Part 2)

Download & Install

java -version
// Expected: openjdk version "21.x.x" ...
javac -version
// Expected: javac 21.x.x

Download & Install

Configure the JDK in NetBeans

Download & Install MySQL

Create the Lab Database

Open MySQL Workbench (installed with MySQL), connect to localhost:3306, and run:

CREATE DATABASE week3db;
CREATE DATABASE week3db2;
-- Verify:
SHOW DATABASES;

Register the driver in NetBeans

Class Hierarchy

Database Target (Joined-Subclass)

Each entity maps to its own table. JPA joins them on primary key to rebuild the full object.

In NetBeans

pom.xml dependencies

<!-- EclipseLink JPA -->
<dependency>
  <groupId>org.eclipse.persistence
  </groupId>
  <artifactId>eclipselink</artifactId>
  <version>4.0.2</version>
</dependency>

<!-- MySQL Connector -->
<dependency>
  <groupId>com.mysql</groupId>
  <artifactId>mysql-connector-j</artifactId>
  <version>8.3.0</version>
</dependency>

Add Item.java inside your existing package au.edu.cqu.week3.

package au.edu.cqu.week3;

import jakarta.persistence.*;

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public class Item {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  protected Long id;
  protected String title;
  protected Float price;
  protected String description;

  public Item() {}

  // Getters
  public Long   getId()          { return id; }
  public String getTitle()       { return title; }
  public Float  getPrice()       { return price; }
  public String getDescription() { return description; }

  // Setters
  public void setId(Long id)                  { this.id = id; }
  public void setTitle(String title)           { this.title = title; }
  public void setPrice(Float price)            { this.price = price; }
  public void setDescription(String desc)      { this.description = desc; }
}

Book.java

package au.edu.cqu.week3;

import jakarta.persistence.*;

@Entity
public class Book extends Item {

  private String  isbn;
  private String  publisher;
  private Integer nbOfPage;
  private Boolean illustrations;

  public Book() {}

  // Getters
  public String  getIsbn()           { return isbn; }
  public String  getPublisher()       { return publisher; }
  public Integer getNbOfPage()        { return nbOfPage; }
  public Boolean getIllustrations()   { return illustrations; }

  // Setters
  public void setIsbn(String isbn)              { this.isbn = isbn; }
  public void setPublisher(String publisher)    { this.publisher = publisher; }
  public void setNbOfPage(Integer nbOfPage)     { this.nbOfPage = nbOfPage; }
  public void setIllustrations(Boolean illus)   { this.illustrations = illus; }
}

CD.java

package au.edu.cqu.week3;

import jakarta.persistence.*;

@Entity
public class CD extends Item {

  private String  musicCompany;
  private Integer numberOfCDs;
  private Float   totalDuration;
  private String  gender;

  public CD() {}

  // Getters
  public String  getMusicCompany()   { return musicCompany; }
  public Integer getNumberOfCDs()    { return numberOfCDs; }
  public Float   getTotalDuration()  { return totalDuration; }
  public String  getGender()         { return gender; }

  // Setters
  public void setMusicCompany(String mc)       { this.musicCompany = mc; }
  public void setNumberOfCDs(Integer n)        { this.numberOfCDs = n; }
  public void setTotalDuration(Float td)       { this.totalDuration = td; }
  public void setGender(String gender)          { this.gender = gender; }
}

Using NetBeans Services Panel

Expected Tables

Verification SQL

-- Check all tables exist
SHOW TABLES FROM week3db;

-- Check ITEM table (root)
SELECT * FROM ITEM;

-- Should show:
-- ID | DTYPE | TITLE      | PRICE
-- 1  | Book  | Clean Code | 49.99
-- 2  | CD    | Kind of... | 12.99

-- Check subclass tables
SELECT * FROM BOOK;
SELECT * FROM CD;

-- Verify the JOIN works
SELECT i.title, b.isbn
FROM ITEM i
JOIN BOOK b ON i.ID = b.ID;

Entities

Your Task List

1. Declare both classes as entities
2. Set up @OneToOne with cascade PERSIST & REMOVE
3. Persist 2 customers (only calling em.persist(customer))
4. Check if a customer is in the persistence context
5. Delete a customer (only calling em.remove(customer))
6. Check if the customer is still in the context

Database Result

Address.java

package au.edu.cqu.week3;

import jakarta.persistence.*;

@Entity
public class Address {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long id;
  private String street1;
  private String city;
  private String zipcode;
  private String country;

  public Address() {}

  public Address(String street1, String city,
                 String zipcode, String country) {
    this.street1 = street1;
    this.city    = city;
    this.zipcode = zipcode;
    this.country = country;
  }

  // Getters
  public Long   getId()       { return id; }
  public String getStreet1()  { return street1; }
  public String getCity()     { return city; }
  public String getZipcode()  { return zipcode; }
  public String getCountry()  { return country; }

  // Setters
  public void setId(Long id)            { this.id = id; }
  public void setStreet1(String s)      { this.street1 = s; }
  public void setCity(String city)      { this.city = city; }
  public void setZipcode(String z)      { this.zipcode = z; }
  public void setCountry(String c)      { this.country = c; }
}

Customer.java

package au.edu.cqu.week3;

import jakarta.persistence.*;

@Entity
public class Customer {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long    id;
  private String  firstName;
  private String  lastName;
  private String  email;

  @OneToOne(cascade = {
    CascadeType.PERSIST,
    CascadeType.REMOVE
  })
  private Address address;

  public Customer() {}

  // Getters
  public Long    getId()         { return id; }
  public String  getFirstName()  { return firstName; }
  public String  getLastName()   { return lastName; }
  public String  getEmail()      { return email; }
  public Address getAddress()    { return address; }

  // Setters
  public void setId(Long id)               { this.id = id; }
  public void setFirstName(String fn)       { this.firstName = fn; }
  public void setLastName(String ln)        { this.lastName = ln; }
  public void setEmail(String email)        { this.email = email; }
  public void setAddress(Address address)   { this.address = address; }
}

Check if managed, then remove

// Start a new transaction
tx.begin();

// Find customer 1
Customer found = em.find(
  Customer.class, c1.getId()
);

// Task 4: Is the entity managed?
boolean isManaged = em.contains(found);
System.out.println(
  "Customer in context: " + isManaged
);
// Expected: Customer in context: true

// Task 5: Remove — cascade fires!
em.remove(found);
// Address will also be removed

tx.commit();

// Task 6: Is it still managed?
boolean afterRemove = em.contains(found);
System.out.println(
  "After remove: " + afterRemove
);
// Expected: After remove: false

Verification SQL

-- After persisting 2 customers:
SELECT * FROM CUSTOMER;
-- Expect: 2 rows (Alice, Bob)

SELECT * FROM ADDRESS;
-- Expect: 2 rows (even though you
-- only called persist on Customer!)

-- After removing Customer 1:
SELECT * FROM CUSTOMER;
-- Expect: 1 row (only Bob)

SELECT * FROM ADDRESS;
-- Expect: 1 row (Alice's address
-- was also deleted via cascade!)

-- Verify the FK link
SELECT c.firstname, a.city
FROM CUSTOMER c
JOIN ADDRESS a
  ON c.ADDRESS_FK = a.ID;

Common Issues

Project 1 Extensions

Project 2 Extensions

✅ Project 1 — Inheritance

✅ Project 2 — Cascading

🗝 Key Concepts to Remember