You have a Student data in the database . To return student data, you may write modal class as below.
public class Student {
private int rollNumber;
private String name;
private String address;
public Student(int rollNumber, String name, String address) {
super();
this.rollNumber = rollNumber;
this.name = name;
this.address = address;
}
public int getRollNumber() {
return rollNumber;
}
public String getName() {
return name;
}
public String getAddress() {
return address;
}
@Override
public int hashCode() {
return Objects.hash(address, name, rollNumber);
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof Student)) {
return false;
}
Student other = (Student) obj;
return Objects.equals(address, other.address) && Objects.equals(name, other.name)
&& rollNumber == other.rollNumber;
}
}
private int rollNumber;
private String name;
private String address;
public Student(int rollNumber, String name, String address) {
super();
this.rollNumber = rollNumber;
this.name = name;
this.address = address;
}
public int getRollNumber() {
return rollNumber;
}
public String getName() {
return name;
}
public String getAddress() {
return address;
}
@Override
public int hashCode() {
return Objects.hash(address, name, rollNumber);
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof Student)) {
return false;
}
Student other = (Student) obj;
return Objects.equals(address, other.address) && Objects.equals(name, other.name)
&& rollNumber == other.rollNumber;
}
}
- Here We have created Student modal with three fields .
- It has a constructor to create Student object.
- It has getter methods to read student data.
- It also has overriden hashcode and equals methods to avoid duplicates.
- If you have multiple tables, You need to all this boiler plate code for all the modal classes.
- To avoid boiler plate code , We can use lombok library.
- We can rewrite the above Student modal class as below using lombok library.
@Data
public class Student {
private int rollNumber;
private String name;
private String address;
}
- Instead of using lombok library, We can use Java record.
- Record is special class which is used to store read only data.
- We can create a record using record keyword.
record recordName(variables declaration) {
//optional body
}
- We can write the above Student modal class using record as below
public record Student(int rollNumber, String name, String address) {
}
- We must declare variables within record parenthesis. These are private and final by default.
- Java compiler automatically creates constructor , getter methods, toString, equals , hashCode methods for these variables.
- Java will not generate setter methods. Because record is immutable. Once we have created record object, we can't modify it's data.
- We can use record keyword as variable name also.
- We can create record object using new keyword.
public record Student(int rollNumber, String name, String address) {
}
public class Test {
public static void main(String[] args) {
List<Student> studentList = new ArrayList<>();
var student1 = new Student(1, "Hari", "Nellore");
var student2 = new Student(2, "krishna", "Chennai");
studentList.add(student1);
studentList.add(student2);
studentList.forEach(studentObj->{
System.out.println(studentObj.rollNumber()+" "+studentObj.name()+" "+studentObj.address());
});
} }
Output
1 Hari Nellore
2 krishna Chennai
- Here getter methods are not prefixed by get word. They are same as variable name.
- To test equals, hashcode methods implementation , let's add Student objects to HashSet collection object.
- We can use generics with record.
- Static variables are allowed within record body. Instance variables are not allowed within record body.
- Both Static and instance methods are allowed to write inside record.
- Abstract method is not allowed.
How to write our own constructor in record:
- Normally Java compiler generated constructor is sufficient.
- We may need to implement our own constructor to write validation conditions. For example if the name is empty or null throw an IllegalArgumentException exception.
- We can implement constructor in two ways.
- Canonical constructor
- Non Canonical constructor
Canonical constructor
- We can write canonical constructor in two forms . one is full form, other one is compact form.
- Let's write canonical constructor in full form
- Here constructor parameters should match exactly as the variables defined in record definition . Variables type , name and order must be same.
- You can write same constructor using compact form as follows.
- We don't need to declare constructor parameters.
- We don't need to initialize the record variables.
- At end of constructor , constructor parameters are automatically assigned to corresponding record variable.
- This constructor is useful when we want to add default values for some fields while creating record object.
- We must call canonical constructor as first statement inside this constructor.
- Here we are using this keyword to call canonical constructor. Address field data is default Nellore for all record objects.
