Set Mapping in JPA

A Set is an interface that contains unique elements. These elements don’t maintain any order. A Set can be used when there is a requirement of retrieving unique elements in an unordered manner.

Set Mapping Example

In this example, we embed an object in an entity class and define it as a collection type List.

1. private Set<Address> address=new HashSet<Address>();  

This example contains the following steps: –

• Create an entity class Employee.java under com.javatpoint.jpa package that contains employee id, name and embedded object (employee Address). The annotation @ElementCollection represents the embedded object.

Employee.java

1. package com.javatpoint.jpa;  
2. import java.util.*;  
3.   
4. import javax.persistence.*;  
5. @Entity  
6.   
7. public class Employee {  
8.   
9.     @Id  
10.     @GeneratedValue(strategy=GenerationType.AUTO)  
11.     private int e_id;  
12.     private String e_name;  
13.       
14.     @ElementCollection  
15.     private Set<Address> address=new HashSet<Address>();  
16.   
17.       
18.     public int getE_id() {  
19.         return e_id;  
20.     }  
21.   
22.     public void setE_id(int e_id) {  
23.         this.e_id = e_id;  
24.     }  
25.   
26.     public String getE_name() {  
27.         return e_name;  
28.     }  
29.   
30.     public void setE_name(String e_name) {  
31.         this.e_name = e_name;  
32.     }  
33.   
34.     public Set<Address> getAddress() {  
35.         return address;  
36.     }  
37.   
38.     public void setAddress(Set<Address> address) {  
39.         this.address = address;  
40.     }  
41.   
42.       
43. }  

• Now, create a class of embedded object Address.java under com.javatpoint.jpa package. The annotation @Embeddable represents the embeddable object.

Address.java

1. package com.javatpoint.jpa;  
2. import javax.persistence.*;  
3.   
4. @Embeddable  
5. public class Address {  
6.   
7.     private int e_pincode;  
8.     private String e_city;  
9.     private String e_state;  
10.     public int getE_pincode() {  
11.         return e_pincode;  
12.     }  
13.     public void setE_pincode(int e_pincode) {  
14.         this.e_pincode = e_pincode;  
15.     }  
16.     public String getE_city() {  
17.         return e_city;  
18.     }  
19.     public void setE_city(String e_city) {  
20.         this.e_city = e_city;  
21.     }  
22.     public String getE_state() {  
23.         return e_state;  
24.     }  
25.     public void setE_state(String e_state) {  
26.         this.e_state = e_state;  
27.     }  
28.       
29.       
30.       
31. }  

• Now, map the entity class and other databases confiuguration in Persistence.xml file.

Persistence.xml

1. <persistence>  
2. <persistence-unit name=”Collection_Type”>  
3.       
4.     <class>com.javatpoint.jpa.Employee</class>  
5. <class>com.javatpoint.jpa.Address</class>  
6.   
7.   
8.   
9. <properties>  
10. <property name=”javax.persistence.jdbc.driver” value=”com.mysql.jdbc.Driver”/>  
11. <property name=”javax.persistence.jdbc.url” value=”jdbc:mysql://localhost:3306/collection_mapping”/>  
12. <property name=”javax.persistence.jdbc.user” value=”root”/>  
13. <property name=”javax.persistence.jdbc.password” value=””/>  
14. <property name=”eclipselink.logging.level” value=”SEVERE”/>  
15. <property name=”eclipselink.ddl-generation” value=”create-or-extend-tables”/>  
16. </properties>  
17.       
18.     </persistence-unit>  
19. </persistence>  

• Create a persistence class ListMapping.java under com.javatpoint.collection package to persist the entity object with data.

SetMapping.java

1. package com.javatpoint.collection;  
2. import javax.persistence.*;  
3.   
4.   
5. import com.javatpoint.jpa.*;  
6. public class SetMapping{  
7.   
8.     public static void main(String[] args) {  
9.           
10.         EntityManagerFactory emf=Persistence.createEntityManagerFactory(“Collection_Type”);  
11.         EntityManager em=emf.createEntityManager();  
12.           
13.         em.getTransaction().begin();  
14.           
15.           
16.   
17.           
18.         Address a1=new Address();  
19.         a1.setE_pincode(201301);  
20.         a1.setE_city(“Noida”);  
21.         a1.setE_state(“Uttar Pradesh”);  
22.           
23.           
24.           
25.           
26.           
27.         Address a2=new Address();  
28.         a2.setE_pincode(302001);  
29.         a2.setE_city(“Jaipur”);  
30.         a2.setE_state(“Rajasthan”);  
31.           
32.         Address a3=new Address();  
33.         a3.setE_pincode(133301);  
34.         a3.setE_city(“Chandigarh”);  
35.         a3.setE_state(“Punjab”);  
36.       
37.         Address a4=new Address();  
38.         a4.setE_pincode(80001);  
39.         a4.setE_city(“Patna”);  
40.         a4.setE_state(“Bihar”);  
41.           
42.           
43.     Employee e1=new Employee();  
44.     e1.setE_id(1);  
45.     e1.setE_name(“Vijay”);  
46.       
47.       
48.     Employee e2=new Employee();  
49.     e2.setE_id(2);  
50.     e2.setE_name(“Vijay”);  
51.       
52.     Employee e3=new Employee();  
53.     e3.setE_id(3);  
54.     e3.setE_name(“William”);  
55.       
56.     Employee e4=new Employee();  
57.     e4.setE_id(4);  
58.     e4.setE_name(“Rahul”);  
59.       
60.     e1.getAddress().add(a1);  
61.     e2.getAddress().add(a2);  
62.     e3.getAddress().add(a3);  
63.     e4.getAddress().add(a4);  
64.       
65.     em.persist(e1);  
66.     em.persist(e2);  
67.     em.persist(e3);  
68.     em.persist(e4);  
69.       
70.     em.getTransaction().commit();  
71.       
72.     em.close();  
73.     emf.close();  
74.           
75.           
76.           
77.     }  
78.       
79. }  

Output:

After the execution of the program, the following tables are generated under MySQL workbench.

• Employee table – This table contains the employee details. To fetch data, run select * from employee query in MySQL.

• Employee_address table – This table represents the mapping between employee and address table. The data in the table is arranged in an unordered manner.To fetch data, run select * from employee_address query in MySQL.